Impact of surface Albedo on Martian photochemistry

Solar energy is the primary driving force behind a planet's climate system, and surface albedo plays a key role in determining the energy budget of the planet. Coupling the Snow, Ice, and Aerosol Radiation (SNICAR) with the Laboratoire de Météorologie Dynamique (LMD) Mars General Circulation Model (MGCM) to create a new coupled model leads to an approximately 4% drop in the net CO2 ice deposition on Mars. Newly simulated surface albedo affects the concentration of gaseous species in the Martian atmosphere (condensation‐sublimation cycle). The new set‐up also impacts the solar energy available in the atmosphere. These two effects together lead to subsequent and significant changes in other chemical species in the Martian atmosphere. Compared with results of the MGCM model alone, in the new coupled model CO2 (gas) and O3 show a drop of about 1.17% and 8.59% in their respective concentrations, while H2O (vapor) and CO show an increase of about 13.63% and 0.56% in their respective concentrations. Among trace species, OH shows a maximum increase of about 29.44%, while the maximum drop of 11.5% is observed in the O concentration. Photochemically neutral species such as Ar and N2 remain unaffected by the albedo changes.


Introduction
Reactions that are initiated by the absorption of a photon of light by a chemical species are known as photochemical reactions. CO 2 is the major component in the Martian atmosphere, with a mixing ratio of 95% (Franz et al., 2017), derived using the most recent data from the Curiosity Mars rover. The primary driver of the photochemistry of the Mars atmosphere is the photo-dissociation of its dominant gas, CO 2 , which is continuously photolyzed producing CO and O, and subsequently O 2 . Other atmospheric components (such as O 3 , H 2 O, CO, and dust) also play important roles in the photochemistry and ion chemistry of the Mars atmosphere. CO is produced mainly by the photolysis of CO 2 but, due to its long lifetime (~ 3.17 Earth years) (González-Galindo et al., 2005), CO acts like a dynamical tracer. The seasonal variation of CO is connected primarily with the condensation of CO 2 and the dynamical transport (Krasnopolsky, 2015). CO was detected for the first time from high-resolution Fourier transform spectra by Kaplan et al. (1969) and the mixing ratio retrieved was 800±300 ppm. Retrievals from recent spacecraft measurements, such as CRISM onboard MRO, PFS onboard MEX, and OMEGA onboard MEX, show an approximate CO mixing ratio of 700 ppm in the Martian atmosphere (Franz et al., 2015(Franz et al., , 2017. The opposite reaction, i.e., CO+O+M→CO 2 +M is very slow and is rotationally forbidden (Atreya and Gu ZG, 1995). McElroy and Donahue (1972) first argued that CO is re-oxidized by OH, suggesting the presence of OH molecules produced by photolysis of water vapor in Mars's atmosphere. Such a low abundance of CO shows the stability of the Martian atmosphere.
Water vapor, the only source of the catalytic species HO x in the Martian atmosphere, is one of the atmosphere's most variable trace species and is also responsible for the atmosphere's stability. The seasonal cycle of water vapor is driven by condensation and sublimation of the polar frost (Barker et al., 1970). The first detailed knowledge of Martian water vapor was obtained from the Mars Atmospheric Water Detector (MAWD) instrument onboard the Viking orbiter (Farmer et al., 1977;Jakosky and Farmer, 1982). Global and seasonal variation of water vapor shows a net transport of water vapor from the southern polar region to the northern polar region during a Martian year (Farmer and Doms, 1979). This extra water vapor corresponds to 1-2 mg/cm 2 of the deposited ice on the surface. Thermal Emission Spectrometer (TES) observations predicted a lower water vapor gradient from south to north as compared to MAWD retrievals (Smith, 2002(Smith, , 2004Pankine et al., 2010).
Also playing an important role in the photochemistry and stability of the Martian atmosphere is ozone. The ultraviolet (UV) spec-trometer onboard the Mariner 7 flyby detected ozone for the firsttime using absorptions of the Hartley band (centered at 255 nm) (Barth and Hord, 1971;Modak et al., 2019). Global distribution of Martian ozone was reported from remote sensing observations by Perrier et al. (2006) and Clancy et al. (2016). Ozone was underestimated in early modeling studies (Nair et al., 1994). However, Lefèvre et al. (2004) achieved better agreement with observations after establishing, and incorporating, a complete water cycle. Various past studies have shown that ozone is anti-correlated with the abundance of water vapor (Barth et al., 1973;Lefèvre et al., 2004;Fedorova et al., 2006;Fast et al., 2006;Clancy et al., 2016;Willame et al., 2017;Modak et al., 2019). Water vapor is abundant on the northern polar cap as compared to the southern polar cap. O 3 is produced primarily by the dissociation of CO 2 and O 2 (which produces the O atom) and combination with O 2 to produce O 3 . The photo-dissociation occurs only in the sunlit areas, and O 3 is transported over dark regions. The degree of anti-correlation between O 3 and water vapor varies from region to region (Lefèvre et al., 2004). In general, at all locations O 3 is at its maximum during winter and minimum during summer, due to its anti-correlation with water vapor (Barth et al., 1973). Table 1 lists the atmospheric mixing ratios of some selected major species in the Martian atmosphere (Mumma et al., 2009;Encrenaz et al., 2011;Franz et al., 2015Franz et al., , 2017McConnochie et al., 2018). The concentration of trace gases such as H 2 O and O 3 has high variability due to seasonal dependence. Table 2 lists four seasons on Mars for both hemispheres with solar longitude (Ls) ranges (Cantor et al., 2001).
Surface albedo controls the amount of solar energy reflected or absorbed by a planet's surface and has a significant impact on any planet's energy budget. Since solar energy is the key to initiate (and continue) photochemical reactions, any change in solar energy flux will have a direct impact on atmospheric photochem-istry. In this work, I have quantified the impact of surface albedo on the mass budget of various photochemical species in the Martian atmosphere by coupling the Laboratoire de Météorologie Dynamique (LMD) Mars General Circulation Model (hereafter MGCM) to the extended Snow, Ice, and Aerosol Radiation (SNICAR) model (Flanner et al., 2007(Flanner et al., , 2009Singh and Flanner, 2016). I assess global and seasonal changes in the mass budget of various photochemical species using a mean solar insolation scenario. This work is focused on understanding the change in the mass budget due to albedo changes, so initial solar insolation would not have any impact on the calculations. All the changes reported in this paper are with respect to the uncoupled MGCM unless stated otherwise.

Model Description & Methodology
The MGCM is a 3-D General Circulation Model that simulates the temporal evolution of the different physical processes that control or describe the meteorology and climate that covers the entire Martian atmosphere. Along with various schemes such as the CO 2 (Forget et al., 1998;1999), water (Navarro et al., 2014), and dust (Madeleine et al., 2011) cycles, the MGCM (Pottier et al., 2017;Singh et al., 2018) also includes a state-of-the-art gas-phase photochemical module (Lefèvre et al., 2004) to provide a realistic description of the concentrations and variations of various species in the Martian atmosphere.
SNICAR utilizes the multiple scattering, multi-layer two-stream radiative approximation described by Toon et al. (1989), with the delta-hemispheric mean approximation. The current version of SNICAR utilizes 480 bands spanning 0.2-5.0 μm at 10 nm spectral resolution to produce a directional-hemispherical albedo (Singh and Flanner, 2016). Singh et al. (2018) prognostically determine snow (both H 2 O and CO 2 ) albedos interactively within the MGCM (integrated with MGCM) using the framework of Singh and Flanner (2016).
First, I determine the total mixing concentration of each species using the MGCM. Next, I convert these concentrations into the total mass of each particular species using its molar mass. After that, I couple SNICAR with MGCM (S-MGCM) using the framework of Singh et al. (2018) to determine new concentrations, and total mass. Finally, I determine the change in the respective chemical species by subtracting S-MGCM values from MGCM values. Singh et al. (2018) showed changes in CO 2 ice deposition, surface pressure, and temperature after the integration of SNICAR with MGCM. Due to a higher simulated surface albedo with SNICAR, the net CO 2 ice deposition decreases by about 4% (Singh et al., 2018). This leads to subsequent changes in other chemical species due to various photochemical interactions within the atmosphere. Figure 1 shows the change in the broadband surface albedo (Band-averaged values are weighted with solar spectral irradiance measurements from Labs and Neckel (1968)) due to the integration of SNICAR with MGCM. The most significant change in albedo is observed in the southern hemisphere from Ls =100° to Ls =200°. A global map (Figure 2) showing changes in surface temperature indicates maximum impact during the southern  (Singh et al., 2018). Surface temperature plays a significant role in determining reaction coefficients. However, due to a very small change in surface temperature, the reaction coefficients are not significantly affected in our analysis.  (Nair et al., 1994). As ozone is anti-correlated with the abundance of water vapor, therefore increase in water vapor content decreases the ozone content with the change in albedo. O 3 is also important in determining the habitability of Mars, as it modulates the surface UV flux.  Figure 4 shows the annual variation of longitudinal-averaged concentrations of four major species in the Mars atmosphere. Surface albedo is a strong positive feedback mechanism (e.g., Bony et al., 2006;Winton, 2006;Randall et al., 2007;Soden et al., 2008;Flato et al., 2013;Singh et al., 2015) that controls the amount of solar energy available to be utilized by the planetary surface and atmosphere. As albedo increases, the surface reflects more solar energy back into space. With lower energy available, both surface and atmosphere become colder and trap more gases via condensation. CO 2 , being the most dominant species, shows signs of changes in its respective concentration over almost the entire globe during the southern spring season. CO is produced primarily by the photolysis of CO 2 from solar energy. Therefore,    CO shows contrasting changes in both hemispheres near late summer (Northern Hemisphere) / early spring (Southern Hemisphere), which is mostly consistent with albedo changes observed in S-MGCM.

Results and Discussion
Since water vapor and ozone are anti-correlated (Lefèvre et al., 2004;Fedorova et al., 2006;Fast et al., 2006;Clancy et al., 2016;Willame et al., 2017;Modak et al., 2019), we observe opposite changes for both species for the most part during the entire year.
Ozone is more abundant in colder regions; water is more abundant in warmer regions (Lefèvre et al., 2004, Fig .1). The latitudinal anomalies are caused primarily by a relative abundance of certain species at a certain latitude. Therefore, the changes are more pronounced when the abundance is relatively high.

Conclusions
Surface albedo plays a significant role in managing a planet's energy budget. The simulated albedo in the new S-MGCM has an impact on the amount of solar energy available in the atmosphere for various gaseous species. The change in surface temperature is too small to cause any significant change in the chemical reaction rates. However, the revised surface albedo changes the concentration of certain atmospheric species by changing the condensation-sublimation cycle. In this case, changes in albedo directly change the amount of CO 2 available in the atmosphere, due to the lower condensation of CO 2 gas into CO 2 ice. Changes in both the sublimation-condensation cycle and in solar energy impact chemical reactions, which further impact the other photochemical reactions occurring in the atmosphere, depending on the availability of other species, and environmental conditions in the vicinity.
Except for Argon and Nitrogen, the new simulated albedo has impacted all other species (either a drop or increase in their net concentrations). Among major species, water vapor observes the maximum change followed by ozone, carbon dioxide, and carbon monoxide. The concentrations of water vapor and ozone are highly affected by seasonal changes on Mars, therefore they react most to the albedo changes caused by the integration of SNICAR into the MGCM. Trace species generally show higher changes as compared to major species, due to their higher dependency on seasons.