# Lethal surface ozone concentrations are possible on habitable zone
exoplanets

G. J. Cooke Institute of Astronomy, University of Cambridge, UK.
School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
D. R. Marsh School of Physics and Astronomy, University of Leeds, Leeds, LS2
9JT, UK
C. Walsh School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT,
UK
F. Sainsbury-Martinez School of Physics and Astronomy, University of Leeds,
Leeds, LS2 9JT, UK

###### Abstract

Ozone (O3) is important for the survival of life on Earth because it shields
the surface from ionising ultraviolet (UV) radiation. However, the existence
of O3 in Earth’s atmosphere is not always beneficial. Resulting from
anthropogenic activity, O3 exists as a biologically harmful pollutant at the
surface when it forms in the presence of sunlight and other pollutants. As a
strong oxidiser, O3 can be lethal to several different organisms; thus, when
assessing the potential habitability of an exoplanet, a key part is
determining whether toxic gases could be present at its surface. Using the
Whole Atmosphere Community Climate Model version 6 (WACCM6; a three-
dimensional chemistry-climate model), twelve atmospheric simulations of the
terrestrial exoplanet TRAPPIST-1 e are performed with a variety of O2
concentrations and assuming two different stellar spectra proposed in the
literature. Four atmospheric simulations of the exoplanet Proxima Centauri b
are also included. Some scenarios for both exoplanets exhibit time-averaged
surface O3 mixing ratios exceeding harmful levels of 40 ppbv, with 2200 ppbv
the maximum concentration found in the cases simulated. These concentrations
are toxic and can be fatal to most life on Earth. In other scenarios O3
remains under harmful limits over a significant fraction of the surface,
despite there being present regions which may prove inhospitable. In the case
that O3 is detected in a terrestrial exoplanet’s atmosphere, determining the
surface concentration is an important step when evaluating a planet’s
habitability.

Exoplanets (498) — Exoplanet atmospheres (487) — Habitable planets (695) —
Exoplanet atmospheric composition (2021)

††thanks: E-mail<EMAIL_ADDRESS>
,

##  1 Introduction

Molecular oxygen (O2) makes up 21% by volume of Earth’s atmospheric
composition and is required for aerobic respiration, provides a fuel for
combustion, and gives rise to the ‘ozone layer’. In an atmosphere with O2 and
sufficient ultraviolet (UV) irradiation at wavelengths shortward of 242.4 nm,
O2 is photodissociated into atomic oxygen (O):

$\displaystyle\ce{O2+}h\nu\ (175.9\ \textrm{nm}<\lambda<242.4\ \textrm{nm})$
$\displaystyle\ce{->O+O},$ (1) $\displaystyle\ce{O2+}h\nu\ (\lambda<175.9\
\textrm{nm})$ $\displaystyle\ce{->O(^{1}D)+O},$ (2)

where $h\nu$ represents a photon of frequency $\nu$, and $h$ is Planck’s
constant111O(^1D) is the first excited state of atomic oxygen, where 1
represents the spin multiplicity and D is the spectroscopic notation for total
orbital angular momentum. Note that the ground state of atomic oxygen can be
written as O(^3P). O2 and O can form ozone (O3) via the following 3 body
reaction:

where M is any third body (usually N2 or O2 on Earth due to their relatively
high abundance). O3 can also be destroyed through photolysis, or by reacting
with atomic oxygen:

$\displaystyle\ce{O3+}h\nu\ (\lambda\geq 320\ \textrm{nm})$
$\displaystyle\ce{->O2(^{3}\Sigma^{-}_{g})+O},$ (3)
$\displaystyle\ce{O3+}h\nu\ (\lambda\leq 320\ \textrm{nm})$
$\displaystyle\ce{->O2(^{1}\Delta_{g})+O(^{1}D)}.$ (4)

On modern-day Earth, the majority of O3 resides in the stratosphere, roughly
15 to 35 km above the surface in the ‘ozone layer’ (Brasseur & Solomon, 2005).
Here, O3 is beneficial for surface-dwelling life, absorbing biologically
harmful UV radiation and providing a partial screen for life exposed to the
Sun’s radiation. Even though the majority of Earth’s O3 is produced in the
equatorial stratosphere, there exists a larger column of O3 at higher
latitudes. This is because O3 is distributed through a seasonal equator-to-
pole circulation driven by atmospheric gravity waves, known as the Brewer-
Dobson circulation (Butchart, 2014). The Brewer-Dobson circulation has been
observed to accelerate and decelerate due to climate change (Garcia & Randel,
2008; Butchart, 2014; Fu et al., 2019), consequently affecting regional
composition and temperatures near the tropopause and lower stratosphere.

The situation in the troposphere (the lowest atmospheric layer where
temperature decreases with altitude) is rather different, because the
photolysis rate of O2 is significantly lower here than in the stratosphere.
Near the surface, volatile organic compounds (VOCs) can also contribute to O3
formation. Hydrocarbon emissions emanate from plants (e.g., isoprene,
$\alpha$-pinene; Chameides et al., 1988; Sharkey et al., 2008) and, on modern
day Earth, from anthropogenic activity (e.g., naphthalene, acetone,
formaldehyde, and many others; Atkinson, 2000). When photooxidation of
hydrocarbons occurs in presence of nitrogen oxides, O3 can eventually be
produced through the ‘smog mechanism’ (Haagen-Smit, 1952). For example, NO2
can be produced when OH reacts with a hydrocarbon (Sillman, 1999), RH (R is an
organic group), producing RO2 and H2O:

RO2 leads to NO2 formation via

Then, NO2 is photolysed in the presence of UV light:

$\displaystyle\ce{NO2+}h\nu\ \ce{&->O+NO}\ (\lambda<\textrm{400 nm}).$ (5)

The O produced can lead to reaction 1, making O3 at low altitudes. On Earth,
surface O3 is low at night when it is removed through reaction with NO,

$\ce{NO+O3->NO2+O2},$ (6)

and O3 increases during the day due to photochemistry (Sillman, 1999).
Reaction 6 is part of a catalytic cycle, where a catalyst (X) leads to the
destruction of O3, but is ultimately not used up in the overall reaction, such
that

$\begin{split}\ce{X+O3&->XO+O2},\\\ \ce{XO+O&->X+O2},\\\ \textrm{Overall:}\
\ce{O3+O&->2O2}.\end{split}$ (7)

At the surface, O3 is considered a pollutant because it causes oxidative
stress to plants, insects, and animals, including humans (Avnery et al., 2011;
Silva et al., 2013; Valavanidis et al., 2013; Squire et al., 2014; Démares et
al., 2022). Oxidative stress is a chemical imbalance between oxidants and
reductants inside an organism that can lead to molecular and biological damage
(Lykkesfeldt & Svendsen, 2007; Sies et al., 2017). It has been demonstrated in
many scenarios that O3 is an antimicrobial agent, capable of microbial
inactivation of fungi, viruses, and bacteria (Kim et al., 1999; Guzel-Seydim
et al., 2004; Najafi & Khodaparast, 2009; Fontes et al., 2012; Epelle et al.,
2023; Rojas-Valencia, 2011). For instance, the removal of microbiota was
demonstrated using ozonation of the air (Epelle et al., 2022), and aqueous O3
is effective at inactivating microorganisms (Premjit et al., 2022).
Additionally, O3 has been found to be toxic across a wide range of organisms,
including guinea pigs, rats, mice (Giese & Christensen, 1954; Stokinger,
1965), terrestrial plants (Rich, 1964; Bytnerowicz et al., 1993; Sandermann
Jr, 1996; Rao & Davis, 2001; Ramya et al., 2023), aquatic life (Jones et al.,
2006), protozoa (Erickson & Ortega, 2006), and algae (Hu et al., 2003;
Gonçalves & Gagnon, 2011).

It is useful to consider some quantities to illustrate ozone’s danger to life.
For example, 40 parts per billion by volume (ppbv) of O3 is defined by the
World Health Organisation (WHO) as a critical limit above which crop yield and
species biomass may be reduced (World Health Organization et al., 2000). The
WHO stated that significant health effects were exhibited by humans at 80 ppbv
(World Health Organization et al., 2000), with O3 damaging lung function at
100 ppbv for 1 – 8 hours of exposure. Indeed, years of evidence has indicated
that long-term exposure to O3 appears to be related to premature human deaths
(Bell et al., 2006; Turner et al., 2016; Sun et al., 2022). For instance, O3
was attributed to 6,000 premature deaths in the EU in 2013 (Nuvolone et al.,
2018), and a modelling study by Malashock et al. (2022) calculated global
O3-attributable mortality in 2019 was 423,100 deaths (95% confidence interval
of 223,200 – 659,400). The majority (77%) of these were estimated to have
occurred in Asia, where ground level O3 concentrations were relatively high
(Malashock et al., 2022). Furthermore, Feng et al. (2022) estimated that in
East Asia, the reduced crop yield to O3 pollution costs US$63 billion
annually. If O3 is detrimental to life on Earth, then the same could be
possible for extraterrestrial life. Due to ozone’s powerful oxidising capacity
(Menzel, 1984; Iriti & Faoro, 2007), it is possible that its toxicity to life
could be ubiquitous. It is highly reactive, ranking amongst the highest
oxidisers222F2 is the strongest oxidiser with a standard electrode potential
of 2.87 eV, whilst O3 usually ranks second and has a standard electrode
potential of 2.075 eV (Kishimoto & Arai, 2022). The standard electrode
potential is “The value of the standard emf of a cell in which molecular
hydrogen under standard pressure is oxidized to solvated protons at the left-
hand electrode.” (McNaught et al., 1997). O3, when internal to an organism,
causes oxidative stress by releasing reactive oxygen species, which can then
cause damage to proteins, DNA, and ultimately result in genetic mutations and
cell growth that potentially turns into cancer (Klaunig et al., 2010).

O3, its spatial distribution on Earth, and its impact on terrestrial
organisms, has been well studied. Less explored have been the implications of
O3 in exoplanet atmospheres. Hundreds of terrestrial exoplanets, rocky planets
orbiting stars other than the Sun, have been detected in our galaxy. Many of
these are in the purported habitable zone (HZ) around their host star (the
region in which liquid water could persist on the surface of a rocky
exoplanet; Kasting et al., 1993), although the potential for exoplanets and
exomoons to be habitable goes beyond the traditional terrestrial-like HZ (see
e.g., Colose et al., 2019; Tjoa et al., 2020; Madhusudhan et al., 2021). If
extraterrestrial life exists, then at some point in its evolution, it is
possible that O2 could be biologically produced just as it is on Earth,
although there are several situations where O2 could be abiotically produced
in high quantities (Des Marais et al., 2002; Wordsworth & Pierrehumbert, 2014;
Domagal-Goldman et al., 2014; Luger & Barnes, 2015; Kleinböhl et al., 2018).
These scenarios include major water loss from photolysis (Wordsworth &
Pierrehumbert, 2014; Luger & Barnes, 2015) and high rates of CO2
photodissociation (Gao et al., 2015; Harman et al., 2015; Schwieterman et al.,
2016). Either way, O3 is a molecule of interest because its detection can
indicate the presence of atmospheric O2 (Leger et al., 1993; Kozakis et al.,
2022). Additionally, O3 has strong spectroscopic signatures in both direct
imaging and transmission spectra observations at relatively small volume
mixing ratios (e.g., between 10-7 and 10-5; Reinhard et al., 2017;
Schwieterman et al., 2018; Kozakis et al., 2022). Due to this property, some
work has shown that in particular scenarios, O3 may be easier to detect than
O2 (Reinhard et al., 2017; Kozakis et al., 2022; Cooke et al., 2023). To date,
O3 has not yet been detected in the atmosphere of a terrestrial exoplanet, so
the only estimates of the full O3 spatial distribution on exoplanets arises
from three-dimensional chemistry-climate simulations.

Tidally locked exoplanets are exoplanets that have a rotational period equal
to their orbital period ($P$), such that they rotate synchronously (Joshi et
al., 1997; Showman & Polvani, 2011; Pierrehumbert & Hammond, 2019). Carone et
al. (2018) simulated tidally locked terrestrial exoplanets with orbital
periods of 1 – 100 days, finding that their atmospheric circulation depends in
part on rotation rate. For $P<25$ days, it was established that stratospheric
transport could occur from the pole to the equator (described as an ‘Anti-
Brewer Dosbon circulation’), or vice versa, depending on stratospheric wind
breaking and the location of the planetary-scale Rossby waves (e.g., tropical
or extratropical). At rotational periods greater than 25 days, the results
from Carone et al. (2018) showed that a thermally driven circulation between
the dayside and nightside could widely distribute air parcels. Yates et al.
(2020) used the Unified Model (UM) to simulate Proxima Centauri b (assuming a
terrestrial exoplanet with a 11.18 day rotation period) in a slab ocean
aquaplanet configuration, and found that the nightside O3 lifetime is much
higher that it is on the dayside. The same conclusion was reached by Proedrou
& Hocke (2016), who simulated a tidally locked Earth with a rotational period
of 365 days (no Brewer-Dobson circulation was present on this simulated
exoplanet). Chen et al. (2019) used WACCM4, and reported that the pole-to-
equator transport predicted by Carone et al. (2018) was present in two of
their chemistry-climate simulations for terrestrial exoplanets with periods of
4.11 days and 7.91 days, and total irradiation of 1.0 $S_{0}$ and 1.1 $S_{0}$,
respectively. Recently, Braam et al. (2023) used the UM and found that O3 is
produced on the dayside and transported to the nightside, with downwelling
motions causing O3 to move into the troposphere at the positions of the
nightside gyres. The use of a slab ocean aquaplanet configuration results in
highly symmetric winds and chemical transport.

Only a few studies have commented upon surface O3 in paleo atmospheres and
exoplanet atmospheres. Grenfell et al. (2013) used a one-dimensional
radiative-convective-photochemical model to investigate the atmospheric
properties of super Earths around M0 to M7 stars and with surface gravity of 1
$g$ and 3 $g$ (where $g=9.81$ m s-2). Whilst the smog mechanism was important
for O3 production around later spectral types, the surface O3 concentrations
did not exceed harmful levels. Grenfell et al. (2006) used a box model and
showed how the smog mechanism could produce ground-level O3 up to 3500 ppbv
during the Proterozoic (2.4 – 0.541 Gyr ago) on Earth at 1% the present
atmospheric level (PAL) of O2. During the Proterozoic, O2 concentrations could
have ranged between $10^{-5}$ – 1 times the PAL of O2 (Large et al., 2019;
Catling & Zahnle, 2020; Steadman et al., 2020; Lyons et al., 2021).

The study by Grenfell et al. (2006) is the only example of a simulated
atmosphere which differs to modern Earth where harmful levels of O3 have been
discussed, although the narrative focused on how O3 would have shielded the
early Earth from UV radiation. No previous work has discussed the hypothetical
dangers from O3 for extraterrestrial life on exoplanets and also used a 3D
chemistry-climate model which accounts for horizontal transport. This work
presents simulations of the exoplanets TRAPPIST-1 e and Proxima Centauri b
using WACCM6, a 3D chemistry-climate model. Both exoplanets are located in the
supposed HZ of their host stars, and TRAPPIST-1 e is a target for JWST
transmission spectra observations. Proxima Centauri b orbits the star Proxima
Centauri (M5.5V spctral type, with a stellar effective temperature of 2,992 K;
Pineda et al., 2021), which is the closest star to the Sun (1.3 pc; Gaia
Collaboration et al., 2016), making it an exciting target for future
observations (Fowler et al., 2023). TRAPPIST-1 e is a roughly Earth-sized
exoplanet orbiting in the HZ around its ultra cool M8V (stellar effective
temperature of 2,566 K; Agol et al., 2021) dwarf host star, TRAPPIST-1. Whilst
faint, TRAPPIST-1 is relatively close at a distance of 12.4 pc (40.5 light
years). As host of 6 other terrestrial exoplanets, the TRAPPIST-1 system is a
prime target to test theories of planetary system formation and evolution, by
confirming whether atmospheres exist on any of the exoplanets, and
characterising their properties if they do. To date, analysis of observations
of the exoplanetary thermal emission with JWST suggests that the two innermost
exoplanets, TRAPPIST-1b and c, have either thin atmospheres, or no atmosphere
at all (Zieba et al., 2023; Greene et al., 2023). Assuming that Earth-like
atmospheres exist on both TRAPPIST-1 e and Proxima Centauri b, we investigate
the abundance and distribution of O3 concentrations in different simulated
scenarios, and discuss the implications for the habitability of oxygenated
worlds.

##  2 Simulations

Table 1: The sixteen simulations used in this study are listed. Twelve for TRAPPIST-1 e: six with the P19 spectrum (Peacock et al., 2019) and six with the W21 spectrum (Wilson et al., 2021b). There are four simulations of Proxima Centauri b, where the MUSCLES spectrum of Proxima Centauri (see text for details) is used as stellar input. Each simulation started with the pre-industrial (PI) WACCM6 simulation composition. Each set of six TRAPPIST-1 e simulations includes three with the present atmospheric level of O2 (0.21 by volume, all denoted as “PI”); one where the substellar point is placed over the Pacific Ocean, one where it is placed over Africa (SPL), and one where it is not tidally locked and the rotation rate is 1 Earth day (noTL). Then, the 10% PAL, 1% PAL, and 0.1% PAL simulations have reduced O2 mixing ratios from the PI simulation by 10, 100, and 1000 times, respectively. Each of the TRAPPIST-1 e simulations receive a total instellation of 900 W m-2, and the Proxima Centauri b simulations receive 884 W m-2 of irradiation. The Proxima Centauri b simulations include the PI, 10% PAL, 1% PAL, and 0.1% PAL cases. The simulated radius and mass of TRAPPIST-1 e are 0.91 $R_{\oplus}$ and 0.772 $M_{\oplus}$, respectively. For Proxima Centauri b, the radius and mass are 1.05 $R_{\oplus}$ and 1.07 $M_{\oplus}$, respectively. The $\oplus$ subscript denotes values relative to the Earth. The orbital parameters assume zero eccentricity and $0^{\circ}$ obliquity, and the table lists the period $P$ and the longitude of the substellar point (SP) relative to Earth’s coordinates (the latitude of the SP is always $0^{\circ}$). Each simulation has been run out for at least 250 model Earth years. Simulation | Planet | Spectrum |  O2 mixing ratio [PAL] | Orbital parameters
---|---|---|---|---
W21 PI | TRAPPIST-1 e | W21 | 1.000 |  $P=6.1$d, SP = $180^{\circ}$ lon
W21 PI noTL | TRAPPIST-1 e | W21 | 1.000 |  $P=1$d
W21 PI SPL | TRAPPIST-1 e | W21 | 1.000 |  $P=6.1$d, SP = $30^{\circ}$ lon
W21 10% PAL | TRAPPIST-1 e | W21 | 0.100 |  $P=6.1$d, SP = $180^{\circ}$ lon
W21 1% PAL | TRAPPIST-1 e | W21 | 0.010 |  $P=6.1$d, SP = $180^{\circ}$ lon
W21 0.1% PAL | TRAPPIST-1 e | W21 | 0.001 |  $P=6.1$d, SP = $180^{\circ}$ lon
P19 PI | TRAPPIST-1 e | P19 | 1.000 |  $P=6.1$d, SP = $180^{\circ}$ lon
P19 PI noTL | TRAPPIST-1 e | P19 | 1.000 |  $P=1$d
P19 PI SPL | TRAPPIST-1 e | P19 | 1.000 |  $P=6.1$d, SP = $30^{\circ}$ lon
P19 10% PAL | TRAPPIST-1 e | P19 | 0.100 |  $P=6.1$d, SP = $180^{\circ}$ lon
P19 1% PAL | TRAPPIST-1 e | P19 | 0.010 |  $P=6.1$d, SP = $180^{\circ}$ lon
P19 0.1% PAL | TRAPPIST-1 e | P19 | 0.001 |  $P=6.1$d, SP = $180^{\circ}$ lon
PCb PI | Proxima Centauri b | PC MUSCLES | 1.000 |  $P=11.18$d, SP = $180^{\circ}$ lon
PCb 10% PAL | Proxima Centauri b | PC MUSCLES | 0.100 |  $P=11.18$d, SP = $180^{\circ}$ lon
PCb 1% PAL | Proxima Centauri b | PC MUSCLES | 0.010 |  $P=11.18$d, SP = $180^{\circ}$ lon
PCb 0.1% PAL | Proxima Centauri b | PC MUSCLES | 0.001 |  $P=11.18$d, SP = $180^{\circ}$ lon
Figure 1: The three stellar input spectra used for the WACCM6 simulations are
the PC MUSCLES spectrum for Proxima Centauri at b (magenta), the Peacock et
al. (2019) (P19; orange) and Wilson et al. (2021b) (W21; blue) spectra for
TRAPPIST-1 at e. The top of atmosphere irradiance per unit wavelength is shown
against the wavelength in nm. In the simulations, TRAPPIST-1 e receives
$900~{}\textrm{W\,m}^{-2}$ of irradiation (0.66 $S_{\oplus}$, where
$S_{\oplus}$ is the total insolation received by the Earth), and Proxima
Centauri b receives $884~{}\textrm{W\,m}^{-2}$ (0.65 $S_{\oplus}$). The UV
range is highlighted in grey between 100 – 400 nm, and the Lyman-$\alpha$ line
is labelled. The average difference between the TRAPPIST-1 spectra in the UV
range is a factor of 500, with a difference of up to 5000 in some wavelength
bins.

We use the three-dimensional Earth System Model WACCM6 (Gettelman et al.,
2019) to perform twelve simulations of possible climates of TRAPPIST-1 e, and
four possible climate simulations of Proxima Centauri b. WACCM6 is a
configuration of the Coupled Earth System Model (CESM), and we use version
2.1.3 (CESM2.1.3). In each simulation the initial conditions represent the
approximate conditions of Earth’s pre-industrial (PI) atmosphere for the year
1850. The simulations have the modern ocean and land configuration, a
horizontal resolution of $1.875^{\circ}$ by $2.5^{\circ}$ (96 latitude points
and 144 longitude points), and 70 vertical atmospheric levels from 1000 hPa to
$4.5\times 10$-6 hPa. Both the atmosphere and ocean models are set up to be
fully interactive so that they respond to physical changes such as
temperature. Because it is likely that Proxima Centauri b and TRAPPIST-1 e
rotate synchronously (they may be tidally locked to their host star, although
spin-orbit resonance states are plausible; Ribas et al., 2016; Renaud et al.,
2021), the substellar point is fixed. This is done by fixing the solar zenith
angle in each grid cell. The exoplanet’s obliquity and orbital eccentricity
are set to zero. We run WACCM6 with middle atmosphere chemistry which is
described in Emmons et al. (2020), where further details can be found. This
chemical mechanism in the WACCM6 simulations has 98 chemical species and 298
chemical reactions, including both the photochemical and heterogeneous
reactions that are necessary to simulate the atmospheric conditions of 1850,
and crucially, O3 chemistry. O3 pollution due to VOCs is not simulated. The
atmospheric time step, $\Delta t$, is 30 minutes. The concentrations of 75
species are computed using the implicit method, which considers the chemical
system at time $t$ and $t+\Delta t$ to evaluate the system at the future time
step $t+\Delta t$ (Sandu et al., 1997). 22 long-lived species are computed
with the explicit method, which calculates the chemical system at a later time
$t+\Delta t$ by considering the current system at time $t$ (Brasseur &
Solomon, 2005). N2 is invariant in each simulation, and its mixing ratio in
each simulation is adapted to ensure that the atmosphere maintains a surface
pressure of 1,000 hPa. Following the work by Ji et al. (2023), we include
absorption by O3, O2, CO2 and H2O in the Schumann-Runge bands (175 – 192 nm).

‘Dry deposition’ is the process through which atmospheric trace gases and
particulate matter are deposited on Earth’s surface and are removed from the
atmosphere, and is an atmospheric sink of O3. Dry deposition in WACCM6 (Emmons
et al., 2020) was updated following Val Martin et al. (2014), and was
originally based on a parameterisation from Wesely (1989). The
parameterisation accounts for variables such as the aerodynamic resistance and
the surface resistance, and is influenced by vegetation, if present.

We assume TRAPPIST-1 e receives $900\ \textrm{W m}^{-2}$ of irradiation and
that Proxima Centauri b receives $884\ \textrm{W m}^{-2}$ (0.66 $S_{0}$ and
0.65 $S_{0}$ respectively, where $S_{0}$ is the total insolation that the
Earth receives). This is consistent with previous work on Proxima Centauri b
(Boutle et al., 2017; Yates et al., 2020; Braam et al., 2022; Ridgway et al.,
2023) and with the TRAPPIST-1 Habitable Atmosphere Intercomparison project
(THAI; Fauchez et al., 2020; Turbet et al., 2022). Proxima Centauri b was
detected using the radial velocity method and has a minimum mass measured of
$M_{P}\sin{i}=1.07\ M_{\oplus}$ (Faria et al., 2022) only, where $M_{P}$ is
the mass of the exoplanet and $i$ is the inclination angle of the planetary
orbit. Therefore, a recently estimated mass-radius relationship from Otegi et
al. (2020), given as $R_{p}=1.03\ M_{p}^{0.29}$, was used to estimate the
planetary radius. Assuming an optimistic mass of $M_{P}=1.07\ M_{\oplus}$,
this places the radius of Proxima Centauri as $1.05\ R_{\oplus}$, and the
surface gravity of Proxima Centauri b at 12.2 m s-2. In our simulations,
TRAPPIST-1 e is set to have a mass of $0.772\ M_{\oplus}$ and a radius of
$0.91\ R_{\oplus}$, consistent with the THAI project and transit timing
variations (TTVs) from Grimm et al. (2018). The surface gravity of TRAPPIST-1
e is therefore set to 9.14 m s-2.

Two semi-empirical stellar spectra were used in the TRAPPIST-1 e simulations.
Peacock et al. (2019), henceforth known as P19, modelled the stellar energy
distribution (SED) of TRAPPIST-1 and produced model 1A, 2A, and 2B, of which
we use model 1A (version 1; Peacock, 2020). More recently, Wilson et al.
(2021b), hereafter known as W21, used further HST observations to produce a
semi-empirical SED of TRAPPIST-1 (version 1; Wilson et al., 2021a) as part of
the Mega-MUSCLES series (Froning et al., 2019; Wilson et al., 2021b). Details
of their spectra can be found in the aforementioned references. Both the
stellar spectra are included here to illustrate how different strengths and
shapes of the incoming UV radiation environment can affect the abundance and
distribution of surface O3. For Proxima Centauri b, we use the GJ 551 MUSCLES
(version 2.2; France et al., 2016; Youngblood et al., 2016; Loyd et al., 2016)
spectrum as input333GJ 551 found at https://archive.stsci.edu/prepds/muscles/.
GJ 551 is the Gliese–Jahreiß catalog name for Proxima Centauri.

Note that the TRAPPIST-1 e simulations were started in the year 2020, before
Agol et al. (2021) published updates to planetary parameters in the TRAPPIST-1
system. For TRAPPIST-1 e, Agol et al. (2021) gave mass and radius values of
$0.69M_{\oplus}$ and $0.92\ R_{\oplus}$, respectively, meaning the surface
gravity would be 8.015 m s-2, instead of 9.14 m s-2 as used here. Using these
updated values, the scale height of the atmosphere would increase, but we
expect that simulations with the parameters from Agol et al. (2021) would
produce similar surface O3 mixing ratios to the ones we present here. Only the
minimum mass has been measured for Proxima Centauri b, so it is conceivable
that it may have a larger mass and radius than the values used here. Brugger
et al. (2016) estimated the radius to be in the range 0.94 – $1.40R_{\oplus}$,
placing it somewhere between a Mercury-like exoplanet and an ocean-like world.
Regardless, with M-dwarf stars being so numerous, it is plausible that
somewhere there exits an exoplanet with similar size and instellation, such
that these simulations remain useful should Proxima Centauri b eventually be
confirmed to have a mass or radius which is significantly larger.

A summary of the simulations is given in Table 1. For TRAPPIST-1 e, six
simulations use the P19 (stronger UV) spectrum, and six simulations use the
W21 (weaker UV) spectrum. For both exoplanets, atmospheric concentrations of
O2 at the present atmospheric level (PI; 0.21 by volume), 10% PAL, 1% PAL, and
0.1% PAL, are simulated. For TRAPPIST-1 e, we move the substellar point for
the 100% PAL simulation between the Pacific Ocean ($180^{\circ}$ longitude; PI
case) and Africa ($30^{\circ}$ longitude; PI SPL case). We run two simulations
which are not tidally locked and have a rotational period of 1 day. Whether a
slab or dynamic ocean is implemented, and whether the land or ocean is at the
substellar point, can modulate the climatology of exoplanets (Hu & Yang, 2014;
Lewis et al., 2018; Del Genio et al., 2019; Salazar et al., 2020; Zhao et al.,
2021; Macdonald et al., 2022; Olson et al., 2022). Salazar et al. (2020) found
that broad climate differences between models (ROCKE-3D and the UM) were
larger than the difference between a slab ocean and a dynamic ocean. Because
of this previous work, and the fact that the PI and PI SPL simulations show
only small globally averaged chemical differences in O3, and due to
computational expense (WACCM6 takes 1,332 core-hours per simulated year to
run), we do not simulate the substellar point over land in any of the reduced
O2 cases. Each simulation has been run out for at least 250 model Earth years,
and then we present the last year of data (365 Earth days). The full details
of the model set-up, alongside simulation scripts, are available via
GitHub444https://github.com/exo-
cesm/CESM2.1.3/tree/main/Tidally_locked_exoplanets.

##  3 Results

Figure 2: The surface mixing ratio of O3, in parts per billion by volume
(ppbv), is displayed for the simulations in this work which have time-averaged
surface O3 mixing ratios exceeding 40 ppbv. This includes all TP-1 e P19
cases, the TP-1 e W21 1% PAL and 0.1% PAL simulations, and only a single
Proxima Cenaturi b case (0.1% PAL). The PI cases start with an initial pre-
industrial atmospheric composition. The PI case has the substellar point
placed over the Pacific Ocean, the SPL case has it placed over Africa, and the
noTL case is not tidally locked, so that the substellar point moves with time.
PAL is the present atmospheric level of O2, which is a mixing ratio of 21% by
volume. The TP-1 e P19 simulations have stronger incident ultraviolet
radiation than the TP-1 e W21 simulations. See Table 1 for a more detailed
description of the simulations. Grey indicates where the O3 mixing ratio is
below 40 ppbv and thus at “safe” levels, whilst the different shades of
yellow-orange-purple indicate places that exceed 40 ppbv, i.e., these
concentrations are known to be harmful to life on Earth. The colour map has a
log scale which extends from 40 ppbv to 2200 ppbv.

###  3.1 Surface ozone concentrations

Fig. 2 shows the time-averaged distribution of O3 at the atmospheric level
closest to surface for the cases which have time-averaged surface O3 mixing
ratios of 40 ppbv or greater. We use 40 ppbv as a lower cut-off for harmful
levels of surface O3 (World Health Organization et al., 2000). Grey indicates
regions below 40 ppbv, whilst the yellow-orange-purple colour map indicates
regions where O3 exceeds 40 ppbv.

The TRAPPIST-1 e (TP-1 e) P19 PI, PI SPL, no TL, and W21 0.1% PAL simulations
everywhere exceed 40 ppbv for surface O3. The P19 PI simulation has a maximum
mixing ratio of 2200 ppbv, which is the largest surface mixing ratio in all of
the simulations presented. In the P19 PI and 0.1% PAL simulations and the W21
0.1% PAL simulation, specific locations (e.g., Antarctica or Greenland) have
extremely high mixing ratios, exceeding 1000 ppbv, which is deadly to some
organisms on Earth. On the other hand, the W21 PI, PI SPL, and noTL
simulations everywhere have O3 mixing ratios below 40 ppbv, and are not shown.
The low O3 surface concentrations are a consequence of the upper atmosphere
efficiently absorbing UV such that insufficient UV reaches altitudes closer to
the planetary surface to synthesise enough O3. The P19 10%, 1%, and 0.1% PAL
simulations, and the W21 1%, simulation, have some areas where O3 exceeds 40
ppbv, whilst maintaining regions below this limit. For the Proxima Centauri b
(PCb) cases, the PI, 10% PAL, and 1% PAL cases have surface O3 levels below 40
ppbv everywhere. For the 0.1% PAL PCb scenario, surface O3 everywhere exceeds
40 ppbv and has an global mean mixing ratio of 203 ppbv.

In terms of time variability, the surface O3 concentrations are not static.
Taking the last year of simulated data and averaging each calendar month, the
fraction of land for each simulation where surface O3 concentrations are under
40 ppbv is given in Table 2. For example, the P19 10%, 1%, and 0.1% PAL
simulations have monthly O3 surface concentrations under harmful levels
varying between 12 – 44%, 75 – 83%, and 4 – 9% of the total surface area.
Considering all these scenarios, the prospect is raised for safe areas on
exoplanets which are sheltered from hazardous O3 concentrations found at other
locations. Meanwhile, some locations will fluctuate between toxic and safe
levels. Only the P19 PI noTL simulation has surface O3 mixing ratios that
everywhere exceed 40 ppbv throughout the final year of data.

Figure 3: (a) Surface O3 mixing ratios are presented for the tidally locked
P19 (orange) and W21 (blue) TP-1 e simulations, and the PCb simulations
(magenta). All of these simulations have the substellar point placed over
ocean. The dots show the mean mixing ratio, whilst the top bar shows the
maximum, and the bottom bar shows the minimum surface O3 mixing ratios. Also
shown in black are the time-averaged O3 mixing ratios of data taken from Cooke
et al. (2022). The horizontal axis indicates the simulations with a fixed O2
mixing ratio at the lower boundary, such that the horizontal ticks are
categories rather than absolute values (the values are offset from each other
for clarity). The grey dotted line indicates the 40 ppbv ‘harmful threshold’,
above which O3 surface mixing ratios are considered dangerous to some forms of
life on Earth. (b) The pre-industrial scenarios are compared in the TP-1 e
simulations. These include the substellar point over ocean (PI), over land (PI
SPL), and the non-tidally locked cases (PI noTL). Figure 4: The production of
O from O2 photolysis (top row) and O3 photolysis (second row) are shown
against pressure, with O3 production (third row) and O3 number density also
shown (bottom row). All profiles are time-averaged global means. The TP-1e PI,
10% PAL, 1% PAL, and 0.1% PAL simulations are shown for both the TP-1 e P19
(orange, brown, yellow, and red, respectively) and TP-1 e W21 (light blue,
blue, lilac, and grey, respectively) stellar spectra in the left and middle
columns, respectively. The right column show the PCb PI (indigo), 10% PAL
(violet), 1% PAL (magenta), and 0.1% PAL (light pink) simulations. Figure 5:
The tropospheric HOx/NOx column ratio is shown against the maximum and mean
surface O3 mixing ratio in each tidally locked simulation in panel a and b,
respectively. The tropospheric NOx and HOx columns are shown against the mean
O3 surface mixing ratio in panel c and d, respectively. The grey vertical
dotted lines show the 40 ppbv cut off for harmful O3 levels. The tropospheric
column is calculated as the column abundance of molecules between 120 hPa and
the surface. Its units are molecules m-2.

The bars in Fig. 3a indicate the full range of surface O3 mixing ratios in
each of the tidally locked simulations where the substellar point is placed
over ocean, with the global mean surface concentrations indicated by the
points. Simulations of Earth (using WACCM6) at atmospheric O2 mixing ratios
between 0.1% – 150% PAL from data from Cooke et al. (2022) are given for
comparison. Fig. 3b presents the same data for the TP-1 e PI scenarios. The
TP-1 e W21 PI and 10% PAL cases have a large range in surface O3
concentrations, spanning 7 and 8 orders of magnitude, respectively. All other
TP-1 e simulations span 5 orders of magnitude or less, with the Earth
simulations spanning only one order of magnitude. The PCb simulations span
between 9 and 2 orders of magnitude. Whilst both the PI and PI SPL simulations
with the P19 spectrum have harmful O3 mixing ratios at the surface, the mean
surface O3 mixing ratio is reduced by 1.7 times when the substellar point is
placed over land (PI SPL case). In contrast, the W21 PI SPL case has 2.0 times
the mean surface O3 mixing ratio of the W21 PI case. Additionally, the noTL
cases have a smaller range than both of the PI tidally locked cases, similar
to the Earth simulations. These results imply that surface topography, the
rotation rate, whether or not a diurnal cycle exists, and the position of the
substeller point, will be important for modulating surface concentrations of
biologically toxic gases such as O3. For the PCb scenarios, the PCb 0.1% PAL
case reaches the largest O3 mixing ratio of 466 ppbv, and everywhere has
mixing ratios exceeding 40 ppbv. None of the WACCM6 Earth simulations from
Cooke et al. (2022) have time-averaged O3 mixing ratios at dangerous levels
which is to be expected because industrial pollutants were not included in the
simulations. If pollutants were included near urban areas, for instance, then
harmful O3 levels would be localised, rather than on a planetary scale. The
simulations which have surface O3 concentrations the most toxic to life are
the PI, PI SPL, and PI noTL simulations with the P19 assumed spectrum.
Quantitatively, this corresponds to the simulations where the global mean
stratospheric O3 number density exceeds $7\times 10\textsuperscript{18}$
molecules m-3. All simulations with O2 concentrations at 0.1% PAL have toxic
mixing ratios exceeding 400 ppbv.

Figure 6: The surface temperature is displayed for the twelve TP-1 e simulations used in this work. The left column shows the P19 simulations and the right column shows the W21 simulations. From top to bottom: PI, PI SPL, PI noTL, and the 0.1% PAL simulations are displayed. White arrows indicate the magnitude and direction of the surface winds. The magenta contours show surface temperatures of 273 K. The 10% PAL and 1% PAL simulations are not shown for brevity, but their surface temperatures are very similar to the PI and 0.1% PAL cases. For scale, a 10 m s-1 arrow is shown in the middle of the figure. Figure 7: The surface temperature is displayed for the four PCb simulations used in this work: PCb PI (top left), PCb 10% PAL (top right), PCb 1% PAL (bottom left), PCb 0.1% PAL (bottom right). White arrows indicate the magnitude and direction of the surface winds. The magenta contours show surface temperatures of 273 K. For scale, a 10 m s-1 arrow is shown in the middle of the figure. Table 2: The time-averaged and global mean dry deposition flux of O3 is given in terms of kg m-2 s-1, for all the exoplanet simulations used in this work, as well as the Earth PI simulation. Dry deposition is a sink for atmospheric surface O3. The time-averaged and global mean O3 surface mixing ratios are given. The fraction of the surface where O3 concentrations are under 40 ppbv and thus considered “safe” is given for each simulated exoplanet scenario. The O3 concentrations vary every calendar month, so the fraction is given as a range over a 1-year period and as a percentage. Additionally, the global mean O3 column is given in Dobson Units (DU), where 1 DU is equal to $2.69\times 10^{20}$ molecules m-2. Simulation |  O3 dry deposition flux [kg m-2 s-1] |  O3 surface mixing ratio [ppbv] |  Fraction of surface with O3 mixing ratios $<40$ ppbv | O3 column [DU]
---|---|---|---|---
Earth PI |  $2.1\times 10$-11 | 12 | N/A | 297
W21 PI |  $1.0\times 10$-12 | 3 | 100 – 100% | 53
W21 PI noTL |  $4.3\times 10$-15 | 5 | 100 – 100% | 46
W21 PI SPL |  $1.3\times 10$-14 | 6 | 100 – 100% | 54
W21 10% PAL |  $6.2\times 10$-13 | 2 | 100 – 100% | 154
W21 1% PAL |  $1.8\times 10$-11 | 82 | 1 – 4% | 901
W21 0.1% PAL |  $4.9\times 10$-11 | 246 | 0 - 0% | 1227
P19 PI |  $9.1\times 10$-11 | 404 | 0 – 0% | 1289
P19 PI noTL |  $4.5\times 10$-12 | 692 | 0 – 0% | 1245
P19 PI SPL |  $4.3\times 10$-11 | 243 | 0 – 0% | 1098
P19 10% PAL |  $1.2\times 10$-11 | 51 | 12 – 44% | 498
P19 1% PAL |  $6.3\times 10$-12 | 31 | 75 – 83% | 260
P19 0.1% PAL |  $1.5\times 10$-11 | 81 | 4 – 9% | 407
PCb PI |  $1.7\times 10$-12 | 5 | 99 – 100% | 179
PCb 10% PAL |  $4.0\times 10$-12 | 11 | 100 – 100% | 134
PCb 1% PAL |  $2.2\times 10$-12 | 7 | 100 – 100% | 256
PCb 0.1% PAL |  $4.6\times 10$-11 | 203 | 0 – 1% | 790

###  3.2 Cause of ozone production at the surface

It is important to state here that initially in the simulations, O3 was
present in Earth-like quantities throughout the troposphere, and no toxic
concentrations existed at the start of the tidally locked simulations. The O3
profile depends on UV radiation, O2 number density, O3 production rates, O3
loss, and the transport of O3. Fig. 4 shows the photolysis rates of O2 leading
to O production (reactions 1 and 2), the photolysis rates of O3 leading to O
production (reactions 3 and 4), the production rates of O3 (reaction 1), and
the O3 number density in each simulation. These quantities are important for
understanding where O3 is produced, and its resulting number density. O2 has
an approximately constant mixing ratio up until the homopause where gases
start to diffusively separate, but in contrast, the mixing ratio of O
increases with altitude until the homopause.

In the W21 simulations, as O2 decreases, the total amount of O3 in the
atmosphere, and at the surface, increases. The opposite is generally true in
the P19 simulations, although there is an increase in surface O3 between 1%
PAL and 0.1% PAL. This difference between the P19 and W21 scenarios occurs due
to the weak UV radiation in the W21 simulations, and the pressure dependency
on the reaction which produces O3 (reaction 1). When the peak of O2 photolysis
occurs at higher altitudes and thus lower pressure, then O does not react with
O2 as quickly to produce O3 compared to the rate lower in the atmosphere where
the density of the third body, M, is higher. In the P19 cases, whilst the UV
can penetrate deeper into the atmosphere when the concentration of O2 is
reduced in the simulations, the availability of O2 becomes the limiting factor
for the production of O3, instead of UV radiation. In the PCb cases, there is
an intermediate amount of UV radiation compared to the P19 and W21 TP-1 e
cases. These PCb cases follow the same trend to the TP-1 e W21 scenarios, with
the surface O3 increasing as O2 is reduced.

The destruction of O3 plays an important role in these atmospheres too.
Photolysis of O3 is not counted as a loss of O3, because the O produced
quickly cycles back to produce O3. The peak in O2 photolysis, O3 production,
and O3 number density, occurs in each simulation at pressures less than 100
hPa (above the troposphere). O3 formation also takes place in the troposphere
because O3 photolysis there is fast due to O3 being present in relatively high
quantities. O3 is being destroyed (by HOx and NOx catalytic cycles) and remade
in the troposphere, but compared to above the troposphere, its production via
O2, CO2, NO2, or H2O photolysis is significantly slower. In the simulations
that have toxic quantities of O3, tropospheric destruction of O3 is dominated
by HOx catalytic cycles rather than NOx catalytic cycles. When O3 is below
harmful levels, NOx catalytic cycles (predominantly NO and NO2) dominate over
HOx catalytic cycle destruction of O3. In other words, when NO and NO2 are
significantly depleted in the troposphere, O3 is able to accumulate to harmful
levels. The tropospheric column amount of NOx exhibits negative correlation
with the mean surface O3 mixing ratio, as shown in Fig. 5. Therefore, the smog
mechanism is not the reason for harmful levels of O3, in contrast to modern
day Earth.

The atmospheric temperatures and dynamics influence the abundance and
distribution of O3. Fig. 6 shows the surface temperatures (colour map) and
surface winds (arrows) in the TP-1 e models, with Fig. 7 showing the same in
the PCb models. Several TP-1 e tidally locked simulations have substellar
points with temperatures above 273 K, and surface winds converging towards
this point (associated with the day-side upwelling at the sub-stellar point),
with some of the lowest surface mixing ratios also found near the substellar
point. Away from the substellar point, temperatures drop below freezing and
can be as low as 170 K on the nightside. The PCb cases have similar surface
temperatures and winds because the total irradiance is quantitatively similar
to the TP-1 e scenarios (0.65 $S_{0}$ versus 0.66 $S_{0}$). The cold
temperatures result in reduced destruction from catalytic cycles (see reaction
7) which proceed slower at lower temperatures (e.g., from HOx and NOx
families), allowing O3 to persist in relatively high quantities.

Fig. 8 shows the dry deposition flux of O3 for some of the TRAPPIST-1 e
simulations and the Earth PI simulation, and Table 2 shows the global mean dry
deposition flux for the Earth PI, TRAPPIST-1 e, and Proxima Centauri b
simulations. Dry deposition over snow and ice is slow compared to that over
other surfaces (Wesely & Hicks, 2000; Helmig et al., 2007; Barten et al.,
2023). Additionally, marine surface deposition is slower than land deposition
when plant stomata are available to take up O3 (Ainsworth et al., 2012). If
surface plants do not exist or have died from prolonged O3 exposure (Rich,
1964; Bytnerowicz et al., 1993; Sandermann Jr, 1996; Rao & Davis, 2001; Ramya
et al., 2023), then dry deposition would occur more slowly, which results in
even more O3 build-up. Even if there are no surface or gaseous molecules for
O3 to interact with, the surface thermal decomposition of O3 can take place
and its importance may vary depending on the surface type, although this
process is currently not sufficiently understood (Fowler et al., 2009; Clifton
et al., 2020). The Earth PI simulation has a global mean dry deposition flux
of $1.6\times 10$-11 kg m-2 s-1. All TRAPPIST-1 e and Proxima Centauri b
simulations have reduced loss rates when compared to this, apart from the P19
PI, W21 1% PAL, W21 0.1% PAL and PCb 0.1% PAL simulations. Despite these cases
having relatively high rates of dry deposition, all retain harmful
concentrations of surface O3. For the Earth PI case, most O3 is deposited over
land (see Fig. 8). For the exoplanet simulations, the majority of O3 is
deposited near the substellar point, regardless of whether it is placed over
land or ocean. The dry deposition flux around the substellar point contributes
to the relatively reduced O3 concentrations at the substellar point (e.g., see
the P19 10% and 0.1% PAL simulations and W21 1% PAL and 0.1% PAL simulations
in Fig. 2). The noTL cases have globally-averaged dry deposition rates that
are slower than their PI tidally locked counterparts by a factor of 236 and 20
for the W21 and P19 scenarios, respectively.

Figure 8: The dry deposition flux of O3, given in terms of kg m-2 s -1, is
plotted for six simulations: The Earth PI simulation, the TRAPPIST-1 e P19 PI,
SPL, noTL, and W21 PI and 0.1% PAL simulations. Yellow indicates relatively
large amounts of dry deposition, whilst red indicates relatively low amounts.
The white areas indicate regions where the dry deposition flux of O3 is below
$10^{-13}$ kg m-2 s -1.

To summarise the O3 distribution in the simulations, O3 is made primarily in
the middle atmosphere on the modelled dayside and the O3 column maximises at
the poles (Cooke et al., 2023). O3 loss on the nightside and at the poles is
slower than the dayside due to relatively low temperatures and a lack of
photolysis producing the molecules that become involved in O3 destroying
catalytic cycles. O3 is lost at the surface due to dry deposition, but the
flux is not large enough to mitigate for the dangerous concentrations of O3.
As an example, in the P19 PI case, the global mean dry deposition flux
increases by $\approx 4.3$ times greater than the Earth PI case, with a
corresponding increase in surface O3 concentrations of $\approx 33.7$. In the
tidally locked cases, the O3 chemical loss rate at the surface is
approximately 2 – 29 times less than the peak stratospheric loss rate. Surface
winds (of order 10 m s-1), which are much stronger than vertical winds (of
order 0.1 m s-1), transport O3 across the surface.

We hypothesise that O3 is transported from where it is produced in the dayside
stratosphere to the nightside and towards the poles as discussed in Braam et
al. (2023), who simulated Proxima Centauri b assuming an initial condition of
a modern-Earth atmosphere. The difference between our work and Braam et al.
(2023) is, in several of our simulations, O3 accumulates to harmful and lethal
quantities. With loss processes in the troposphere less effective than in the
stratosphere, the lifetime of surface O3 increases and enables a build-up of
toxic O3 levels. The atmospheric transport throughout the entire atmosphere
will be explored in a follow-up paper to confirm whether such a scenario is
occurring in the simulations.

##  4 Discussion

###  4.1 Prior results for toxic levels of ozone

As far as can be discerned from the presented data (e.g. globally averaged
vertical profiles), other 3D simulations of oxygenated exoplanets (Proedrou &
Hocke, 2016; Way et al., 2017; Chen et al., 2019; Yates et al., 2020),
excluding Braam et al. (2022), have not produced surface O3 mixing ratios
above 40 ppbv. This is likely due to the investigated scenarios which differ
between each work, although model differences will be important (Ji et al.,
2023, 2024). Figure 5 in Braam et al. (2022) shows dayside mixing ratios of O3
reaching $\approx 45$ ppbv at the surface. It is worth noting here that all of
these studies, including our simulations, assume a surface pressure of 1 bar
(1,000 hPa), so that a mixing ratio of 40 ppbv at 1,000 hPa surface pressure
corresponds to a number density of $1.0\times 10^{18}$ molecules m-3 at 288 K.
Chen et al. (2021) used WACCM4 and Ridgway et al. (2023) used the UM (two 3D
chemistry-climate models) to investigate the impact of flares and coronal mass
ejections on terrestrial exoplanets. The flares significantly perturbed O3
concentrations555The amount of atmospheric O3 decreased by a factor of 3 in
Chen et al. (2021) for active M dwarf stars, whereas total atmospheric O3
increased by up to a factor of 20 in Ridgway et al. (2023)., but the changes
were in the middle and upper atmosphere and the flares did not cause surface
concentrations to exceed 40 ppbv. However, there could be specific cases where
flares act to increase O3 surface concentrations to harmful levels, likely
depending on the atmospheric properties, incoming flare strength, and flare
frequency.

1D photochemical models have also simulated the impact of flares, coronal mass
ejections, and cosmic rays on atmospheric chemistry. The calculations in
Segura et al. (2010), Grenfell et al. (2012), Tabataba-Vakili et al. (2016),
and Tilley et al. (2019), did not demonstrate surface O3 mixing ratios
reaching biologically toxic quantities. The same is true for abiotic O2
production simulations (Segura et al., 2007; Harman et al., 2015, 2018),
although it is unclear in the 100 bar atmosphere from Schwieterman et al.
(2016) because surface O3 mixing ratios are not shown (their figure 1), but O3
is at 1 ppbv by 10 km and its mixing ratio is strongly decreasing with
decreasing height. Other 1D photochemical modelling results for habitable zone
exoplanets have shown that O3 mixing ratios may exceed harmful levels (Kozakis
et al., 2018; Fauchez et al., 2019; Kaltenegger et al., 2020; Kopparapu et
al., 2021). For example, Kozakis et al. (2018) used a 1D photochemical model
(EXO-Prime; Kaltenegger & Sasselov, 2010) to simulate Earth-like exoplanets
with various surface pressures of 0.3, 1.0, 1.5, and 2.0 bar, orbiting white
dwarf stars (stellar effective temperatures of 4000, 5000, and 6000 K). Almost
all simulations had surface O3 below the 40 ppbv threshold, with only a single
simulation (0.3 bar surface pressure and 4000 K stellar effective temperature)
exceeding it. However, if it is the concentration (number of molecules per
unit volume), rather than the mixing ratio (fractional concentration), which
is important, one must take into account the surface density of the
atmosphere. Using this criterion, a few more simulations from Kozakis et al.
(2018) would be close to $1.0\times 10^{18}$ O3 molecules m-3 and therefore
potentially dangerous to any surface life present. This was the case for
simulated atmospheres around white dwarf stars, however, the 1 bar atmospheres
in Kozakis & Kaltenegger (2020) which were simulated around red giant stars
did not surpass dangerous surface O3 concentrations. Regarding smog (see
Section 1 and Reactions 1 – 5), Kopparapu et al. (2021) used the 1D
photochemical model which is part of ‘Atmos’ (see Arney et al., 2016; Arney,
2019) to simulate varying surface-to-atmosphere fluxes of NO2 to test whether
it could be used as a signature that extraterrestrial technology existed on an
exoplanet. In two simulations (using a Sun-like star and a K6V star with
$20\times$ the present Earth flux of NO2), the surface O3 mixing ratios were
$\approx 90$ and $\approx 100$ ppbv, respectively. Alongside modern day
pollution on Earth due to NOx emissions from vehicles, this study by Kopparapu
et al. (2021) demonstrates that the surface fluxes of molecules will be
important for determining whether ground level O3 concentrations reach
concerning levels for life. Fauchez et al. (2019) simulated the habitable zone
exoplanets TRAPPIST-1 e, f, and g, using the 3D model LMD-G and then performed
terminator photochemistry simulations with Atmos. The surface O3
concentrations are $<40$ ppbv for TRAPPIST-1 e, but are $\sim 60$ ppbv and
$\sim 120$ ppbv for TRAPPIST-1 f and g, respectively. In their simulations,
surface O3 increases with decreasing illumination. The simulations presented
here predict, for the P19 PI case, the highest ozone surface concentrations
($7.8\times 10^{19}$ molecules m-3) compared with other results for exoplanets
simulated in the literature.

WACCM6 predicts lower concentrations of O3 for various oxygenation states (0.1
– 150% PAL) for Earth when compared to 1D models (Cooke et al., 2022; Ji et
al., 2023), so there is the question of why it now predicts higher
concentrations of O3 when compared to 1D models for M dwarf stars. There will
likely be many reasons the results differ (e.g., see Ji et al., 2023), but we
suspect that the main causes are differences in atmospheric transport and
temperatures. The atmospheric temperatures in Kozakis et al. (2022) with the
M5V star are warmer below 30 km when compared to the WACCM6 simulations. The
same is true in the M3V and M8V simulations of Rugheimer & Kaltenegger (2018),
the GJ 436 and GJ 876 simulations in Rugheimer et al. (2015), the Gebauer et
al. (2018) simulations around AD Leo (M3.5V), the TRAPPIST-1 e simulations in
Lin et al. (2021), the Proxima Centauri b simulations in Scheucher et al.
(2020), and the 1 bar habitable Proxima Centauri b simulations in Meadows et
al. (2018). Note that the desiccated Proxima Centauri b atmospheres in Meadows
et al. (2018) do have harmful levels of O3, but these would not be considered
habitable due to the lack of H2O.

As an example of the temperature differences, global mean surface temperatures
are approximately 220 – 230 K in the WACCM6 tidally locked simulations,
compared to a surface temperature of $\approx 310$ K in Kozakis et al. (2022),
260 – 280 K in Scheucher et al. (2020), and 273 K in Meadows et al. (2018).
This means that below 30 km, where most of the atmospheric O3 resides, loss
rates will be slower and formation will be faster when compared to the 1D
models. Furthermore, in 1D models, there is constant illumination, but in 3D
models, O3 is transported to the nightside (where there is no direct
starlight) and down to the surface (Braam et al., 2023). The TRAPPIST-1 e 1D
photochemical simulations in Pidhorodetska et al. (2020) used temperature and
pressure profiles from 3D simulations of TRAPPIST-1 e (Fauchez et al., 2019).
Here, the deviations from our results may be due to the synthetic BT-Settl
spectrum (a model of stellar atmospheres; Rajpurohit et al., 2013) used by
Pidhorodetska et al. (2020), and the fact that photochemistry was performed at
the terminator. Between all of these simulations, there will be differences in
photochemical cross sections and chemical schemes, as well as the UV spectra
used and the total instellation, but temperatures and transport may largely
explain the substantial discrepancies both in predicted surface O3, and O3
columns, when compared to similar 1D model simulations.

With regards to why WACCM6 predicts higher surface concentrations of O3
compared to the Unified Model will require a detailed intercomparison. For
now, we speculate at least one difference is the use of a slab ocean with no
ice formation in the Unified Model, such that O3 will be lost in greater
numbers on the nightside than would occur in reality. When simulating Proxima
Centauri b with a slab ocean, Yates et al. (2020) and Braam et al. (2022)
assumed a dry deposition velocity of 0.05 cm s-1 based on previous work
(Ganzeveld & Lelieveld, 1995; Giannakopoulos et al., 1999). The global mean
dry deposition velocities for O3 in our tidally locked ocean substellar point
cases are approximately 4 times slower, and slower still when considering the
“noTL” and “SPL” cases. O3 dry deposition depends on multiple interlinked
parameters which are poorly known (El-Madany et al., 2017), so whether
existing dry deposition parameterisations can be used for exoplanet
simulations is not well known.

###  4.2 Modelling limitations

The model used for the atmospheric simulations is an important factor in these
predictions because varying parameterisations and chemical schemes will impact
the results. WACCM6 is a model that is tuned to Earth’s atmosphere, land, ice
and ocean. WACCM6 accounts for scattering longward of 200 nm, but it doesn’t
account for scattering in the Schumann-Runge bands (175 – 192 nm; these
wavelengths photolyse O2 above $\sim 80$ km in Earth’s atmosphere), becoming
pertinent for Earth-like simulations at O2 mixing ratios of 1% PAL or less (Ji
et al., 2023). In our simulations, WACCM6 accounts for absorption in the
Schumann-Runge bands from O3, O2, CO2, and H2O. The integrated flux in the
Schumann-Runge Bands is 1.15, 656, and 26.6 times lower than Earth, for the
TP-1 e P19, TP-1 e W21, and PCb cases, respectively. Even with the effects of
scattering included, it seems still possible that harmful O3 surface
concentrations could form on the surface of terrestrial exoplanets because all
of the P19 simulations have dangerous concentrations of surface O3. Moreover,
the W21 simulations have a relatively low amount of radiation in the Schumann-
Runge bands, yet toxic O3 concentrations are found in the W21 1%, and 0.1% PAL
cases. We tested these assumptions in the 0.1% PAL cases by reducing the
incoming photon flux in the W21 and P19 spectra by a factor of $10^{5}$. The
total integrated O2 photolysis in the P19 0.1% PAL case reduced by a factor of
1.6, and in the W21 case the decrease was negligible. The decrease in O2
photolysis in the P19 0.1% PAL case primarily takes place between 10 and 0.01
hPa. As seen from the results presented in figure 7 in Ji et al. (2023),
scattering becomes important in the 0.1 – 10% PAL simulations at altitudes
below 10 hPa. Therefore we expect our conclusions to be unaffected by
including scattering in the Schumann-Runge band parameterisation in WACCM6;
however, the test should still be done when that parameterisation is
eventually updated.

###  4.3 Atmospheric evolution

We have assumed planetary conditions based on the last 2.4 billion years of
Earth’s history, where the atmospheric pressure has been $\sim 1$ bar and
atmospheric O2 has varied between $1000\times$ less than the present
atmospheric level to $\sim 1.5\times$ greater (Large et al., 2019; Catling &
Zahnle, 2020; Steadman et al., 2020; Lyons et al., 2021). However, for a
habitable zone tidally locked exoplanet around an M dwarf star, is such a
scenario realistic?

Firstly, atmospheres with high O2, CO2, or H2O mixing ratios and sufficient UV
may develop an O3 layer close to the surface because photodissociation could
produce O2 and O molecules which lead to O3 formation. In previous work
simulating a 1 bar atmosphere around the host star GJ 876, O2 amounts
comparable to the Proterozoic Earth were produced in abiotic scenarios
(Domagal-Goldman et al., 2014; Tian et al., 2014). When CO2 is photolysed it
produces CO and O, but lightning flashes can produce NO, which can catalyse
the recombination of CO and O (instead of O going on to produce O2), meaning
that abiotic O2 may not increase to detectable levels (Harman et al., 2018).
However, on some exoplanets lightning may not be sufficient to prevent O2
build-up (Barth et al., 2024).

Oxygenic photosynthesis on Earth, utilising chlorophyll as a pigment, requires
photosynthetically active radiation (PAR) between $400–700$ nm (Alados et al.,
1996). Due to the luminosity of M dwarf stars peaking at longer wavelengths
when compared to the Sun, the availability of PAR may be lacking on habitable
M dwarf exoplanets. Nevertheless, oxygenic photosynthesis is thought to be
possible on exoplanets orbiting M dwarf stars (Gale & Wandel, 2017; Claudi et
al., 2020; Duffy et al., 2023), although some exoplanets may be limited by a
lack of photons reaching the surface (Lehmer et al., 2018) and this could mean
that anoxygenic photosynthesis (where oxygen is not a product of
photosynthesis) is preferentially selected (Duffy et al., 2023). In
particular, the results from Lingam & Loeb (2019) showed how planets orbiting
stars with masses below $0.13\ M_{\odot}$ may have difficulties in producing
O2 through oxygenic photosynthesis, with Proxima Centauri and TRAPPIST-1
falling under this limit. However, the results of Covone et al. (2021)
contrasted to Lingam & Loeb (2019), and whilst TRAPPIST-1 f and g might be
energy limited (Lehmer et al., 2018; Covone et al., 2021), TRAPPIST-1 e may be
able to host an Earth-like biosphere. Furthermore, cyanobacteria can use
radiation longward of 700 nm to photosynthesize (Gan & Bryant, 2015; Claudi et
al., 2020), and a recent study has also shown this in more complex vegetation
(Zhen et al., 2022). A new metric quantifying the photo-absorption rate of
photosynthetic pigments around different stars, suggests M8V stars could
activate photosynthesis (Marcos-Arenal et al., 2022). Moreover, cyanobacteria
can survive and photosynthesize in caves and under low light conditions
(Hanelt et al., 1997; Behrendt et al., 2020; Jung et al., 2023). Regardless of
the possibility of photosynthesis on M dwarfs, O2 production needs to surpass
O2 sinks in order to produce an oxygenated atmosphere (Lehmer et al., 2018).

It is hypothesized that the oxidation of Earth’s lithosphere (the solid outer
layer of a planet) occurred due to two primary factors: oxygenic
photosynthesis from cyanobacteria over millions of years, and hydrogen escape
to space resulting from water photolysis (Catling et al., 2001; Zahnle et al.,
2013). Modelling an Earth-like planet orbiting the star AD-Leo (M3.5V),
Gebauer et al. (2018) found that it may be possible for a Great Oxidation
Event (see e.g., Lyons et al., 2014; Gumsley et al., 2017; Poulton et al.,
2021) to occur earlier in the history of the modelled planet when compared to
Earth. If O2 increases and results in large amounts of O3 at the surface,
because O3 is a strong oxidiser, it could speed up the oxidation of Earth’s
lithosphere, reducing the sinks of O2 and further enabling the generation of
an oxygenated atmosphere.

###  4.4 Habitability feedback

Continuing with this scenario in mind, there is the potential for biological
feedback. Surface O3 may build-up to toxic concentrations and kill organisms
that produce O2 on the surface, subsequently reducing the production of O2
until O3 reaches safe levels again. Even so, we argue here that O3 could
exceed lethal surface concentrations and a photosynthetic biosphere could
still be safely present in the ocean and under ice.

Firstly, refuge from the dangerous O3 concentrations may be found in any
liquid water ocean present, because O3 has low solubility in water (Egorova et
al., 2015). Although it is possible for O3 to form in water which is
irradiated by the Sun (Lushchak, 2011), O3 then quickly decomposes.
Additionally, for disinfection applications, O3 has to be artificially
inserted into water (e.g., via bubble diffusion; Wert et al., 2017) such that
it seems unlikely that marine life would be adversely affected by high surface
concentrations of O3 in the air. The known effects of O3 exposure on life are
limited to observations of terrestrial organisms, and extraterrestrial
organisms may adapt to survive in an atmosphere with high surface O3 mixing
ratios.

On Earth, phytoplankton blooms have been found under Arctic (Suzuki et al.,
1997; Arrigo et al., 2012; Clement Kinney et al., 2020) and Antarctic sea ice
(McMinn et al., 2007), algae are found at varying depths (Norris & Olsen,
1991; Pritchard et al., 2013; Borlongan et al., 2017), and lichens (Kappen,
1993), aquatic plants (Adams et al., 1974; Campbell et al., 2007), and seaweed
(Ramus et al., 1976; Huovinen & Gómez, 2013), can photosynthesize below the
surface. The long day length during the summers at each pole on Earth can
increase the rate of primary productivity (Henshaw & Laybourn-Parry, 2002).
Given phytoplankton’s contribution to roughly half of Earth’s primary
productivity (Field et al., 1998) and the potential for subsurface
photosynthesis on tidally locked exoplanets with constant day side
illumination, significant oxygen accumulation may occur without surface life.
The amount of ice coverage and available area of open ocean may limit the O2
flux to the atmosphere, although gaseous diffusion through sea ice is possible
(Delille et al., 2007; Loose et al., 2011, 2011; Bortkovskii, 2012), just
slower when compared to open water (Bortkovskii, 2012).

In summary, whether due to biological or abiotic production, O2 could be
produced in quantities large enough to induce toxic concentrations of O3 at
the surface. If life is present on such a planet, there is then the potential
for biogeochemical feedback as organisms are hindered or destroyed by O3. We
encourage simulations of these scenarios using biogeochemical models to
quantitatively determine the possible outcomes we have discussed.

###  4.5 Future work

Future work should aim to determine the parameter space (in UV irradiation,
composition, and atmospheric pressure) for which detrimental levels of surface
level O3 may occur. Ideally, 1D photochemical models would be used for this as
they are less computationally expensive than 3D models, and the use of 1D
photochemical models is indeed a viable investigation for non-tidally locked
exoplanets. However, for tidally locked exoplanets, where O3 is transported to
the nightside and shielded from destruction (Proedrou & Hocke, 2016; Yates et
al., 2020; Braam et al., 2023), 3D chemistry-climate models will be needed to
predict where surface O3 concentrations maximise due to atmospheric dynamics,
and climatology from varied ocean and land configurations (Zhao et al., 2021;
Macdonald et al., 2022). The ocean salinity could be varied to determine the
affect on the sea ice distribution (Olson et al., 2022), which is important
for habitability estimates and O3 dry deposition calculations. Simulations
where only the ocean model is changed between dynamic ocean, slab ocean with
ice formation (e.g., Olson et al., 2022), and slab ocean without ice formation
(e.g., Braam et al., 2022), would be useful to determine how important these
factors are when considering toxic O3 at the surface. Additionally, one could
explore various topographies, gravitational accelerations, and several
different atmospheric compositions (e.g., H2O or CO2 dominated). As the
exoplanets simulated here have limited areas where surface temperatures are
above 273 K (and some have no surface temperatures above 273 K), subsequent
research could investigate the parameter space with warmer exoplanets to
determine how surface O3 concentrations are affected. Examples of other 3D
models that could investigate this chemical phenomenon are LMD-G (Yassin
Jaziri et al., 2022), ROCKE-3D (Way et al., 2017), and the Unified Model
(Boutle et al., 2017). Once a chemical scheme is implemented, the LFRic-
Atmosphere model could also be used (Sergeev et al., 2023).

The classical HZ depends on whether a planet can sustain surface liquid water,
but the notion of a Habitable Zone for Complex Life (Schwieterman et al.,
2019) is influenced by the presence of toxic gases. So far, the toxic gases
that have been suggested are carbon monoxide (CO) and relatively high
concentrations of CO2 (Schwieterman et al., 2019), as well as N2 at high
pressure (e.g., $>2$ bar; Ramirez, 2020). These molecules impede biochemical
processes that may be unique to organisms on Earth. Resulting from its
reactivity as a strong oxidant, O3 could pose a more significant threat to
extraterrestrial life. Therefore, based on our modelling work and the
properties of O3, we recommend that O3 should now be added to the list of
molecules that can influence the Habitable Zone for Complex Life.

##  5 Conclusions

This work used WACCM6 to simulate the climate of two exoplanets: TRAPPIST-1 e
and Proxima Centauri b. For each exoplanet, we considered O2 mixing ratios
between 0.1% PAL and 100% PAL. Additionally, two different stellar spectra
were used for the TRAPPIST-1 e cases to investigate the effect on surface O3
due to their large differences in the strength of incoming UV radiation. In
multiple simulations, surface concentrations of O3 exceed 40 ppbv, with
maximum time-averaged concentrations reaching up to 2200 ppbv in the TP-1 e
P19 PI case. Such concentrations are harmful to life on Earth and may be
potentially fatal through oxidative stress. In these simulated atmospheres, O3
exists not as a pollutant, but as a consequence of the planetary atmospheric
conditions, such as the 1,000 hPa surface pressure, the incoming UV strength
and shape, and the O2 number density vertical profile. Our work suggests the
potential presence of toxic O3 concentrations should be included when
evaluating the habitability of an exoplanet.

The simulations examined in this exploratory work represent a small proportion
of the parameter space in which atmospheres may form relatively high O3
concentrations at the surface. Different planetary rotation rates, topography,
atmospheric pressures, total irradiation and UV irradiation environments, as
well as various chemical fluxes from the surface to the atmosphere, should all
be explored. Upcoming work should consider the potential presence of high
surface concentrations of O3 when simulating oxygenated atmospheres. If O3 is
detected in any future observations of terrestrial exoplanet atmospheres,
ascertaining the O3 surface concentration should be incorporated into
frameworks that aim to determine planetary habitability and decide on the most
promising targets for follow-up observations (see e.g., Truitt et al., 2020;
Méndez et al., 2021; Safonova et al., 2021). In practise, this will require a
combination of planetary modelling, transmission and direct imaging spectra,
as well as precise knowledge of the UV irradiation environment of the
atmosphere. 3D chemistry-climate models are essential for understanding how
transport can create areas with comparatively lower and thus safer O3
concentrations. Just as on Earth, the entire surface does not need to be
hospitable for life to flourish.

## Acknowledgments

We thank the two reviewers for their comments and helpful feedback which
helped to improve the manuscript.

G.J.C. acknowledges the studentship funded by the Science and Technology
Facilities Council of the United Kingdom (STFC; grant number ST/T506230/1).
C.W. acknowledges financial support from the University of Leeds and from the
Science and Technology Facilities Council (grant numbers ST/X001016/1 and
MR/T040726/1). F.S.-M. acknowledges financial support from the University of
Leeds and from the Science and Technology Facilities Council (grant number
MR/T040726/1). This work was undertaken on ARC4, part of the High Performance
Computing facilities at the University of Leeds, UK.

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