Fate of SO2 in the ancient Martian atmosphere: Implications for transient greenhouse warming

Authors


Abstract

[1] There is increasing evidence that sulfur played an important role on early Mars. Sulfur is distributed ubiquitously on the Martian surface, and sulfur in Martian meteorites carries the signature of atmospheric interactions. Recent work suggests that the radiative properties of sulfur volatiles that were degassed into the Martian atmosphere may have caused a greenhouse effect early in the planet's history. It remains unclear, however, over what timescales warming from sulfur volatiles would have persisted, and consequently how significant this warming may have been. While most photochemistry research to date has concentrated on current Martian conditions, the ancient Martian atmosphere was thicker, warmer, and more reducing than the current regime. Here we investigate sulfur photochemistry in a 500 mb ancient Martian atmosphere. After adapting a model used to study sulfur photochemistry on Earth during the Archean, we find a short lifetime for SO2 in the current Martian atmosphere, similar to results of other photochemical studies. However, our simulations suggest that moderate mixing ratios of SO2 (10−8f(SO2) ≤ 10−6) could have persisted in the ancient Martian atmosphere for hundreds of years, generating short but potent warming events following episodes of volcanic activity.

1. Introduction

[2] The element sulfur may be critical to unlocking many of the questions associated with early geologic and climate history of Mars. Because degassed SO2 can act as a powerful greenhouse gas, it has been suggested as an important atmospheric component during periods of enhanced volcanic activity on Mars [Postawko and Kuhn, 1986; Settle, 1979]. While there are no identifiable hot spots and only very weak plumes and gas vents releasing SO2 on current Mars, and while studies suggest that SO2 degassing on Mars today is weaker by 3 orders of magnitude than SO2 degassing on the Earth [Krasnopolsky, 2005], available evidence suggests that the prevalence of SO2 on ancient Mars was significantly greater. Isotopic analyses of Martian meteorites reflect deposition of sulfur species created by atmospheric chemical reactions [Farquhar et al., 2000]. Additionally, a recent investigation has indicated high sulfur solubility in Martian magmas, which, after a volcanic event, can generate a greenhouse warming effect of up to 25 K from SO2 alone, and an even more potent effect if water vapor feedbacks are considered [Johnson et al., 2007; Johnson et al., 2008]. Therefore, despite Mars' greater distance from the Sun than the Earth, and the likelihood of a less luminous Sun early in solar system history [Gough, 1981], a greenhouse “boost” from SO2 appears to have been capable of generating significant warming in the ancient Martian environment.

[3] To estimate how long these warming pulses may have lasted, it is important to understand the fate of sulfur volatiles in the early Martian atmosphere. Three investigations of sulfur photochemistry under the current oxidizing Martian conditions [Settle, 1979; Wong et al., 2003, 2004, 2005; Krasnopolsky, 2005] predict a relatively brief photochemical lifetime for SO2; Wong et al. [2003, 2004, 2005] specifies a lifetime of 600 days at high mixing ratios (f(SO2) = 10−4) while Krasnopolsky [2005] predicts a lifetime of 1.5 to 2.4 years for (f(SO2) = 10−9). Bullock and Moore [2007] address a thicker, oxidizing Martian atmosphere with a set of calculations, but, prima facie, assume copious amounts of odd hydrogen. More detailed modeling [Levine and Summers, 2008] of a Martian atmosphere with 100 times the present atmospheric level of CO2 shows that the lifetime of SO2 is, in fact, strongly dependent on the amount of CO2 in the atmosphere. The atmospheric lifetime of SO2 at low mixing ratios (f(SO2) ≈ 10−9) exceeds 108 s under a CO2 atmosphere of ∼600 mb. Published sulfur photochemistry studies have yet to sufficiently consider the differences between the current Martian atmosphere and significantly denser, warmer atmospheric conditions on ancient Mars. Here we explore a weakly reducing atmospheric regime, as has been suggested for early Earth [Kasting, 1993; Kasting et al., 1993] and which is supported by the reduced state of the Martian mantle in comparison to Earth (near the iron-wüstite buffer as opposed to the quartz-fayalite-magnetite buffer) [Medard and Grove, 2006]. We present the first results that fully integrate sulfur photochemistry into the conditions believed to exist in the ancient Martian atmosphere. We begin by presenting a validation of our photochemical model under the present-day Martian regime, and find results consistent with those of Nair et al. [1994] (see also Yung and DeMore [1999]). Next, we proceed to an investigation of sulfur in the ancient Martian atmosphere, and we demonstrate that sulfur volatile lifetimes could have reached hundreds of years, representing a significantly longer lifetime than currently theorized for Mars. Our results suggest that under these conditions, SO2 may have caused repeated episodes of warming that lasted long enough to allow liquid water to begin generating geologic features.

2. Photochemical Model

[4] To investigate the fate of sulfur in the Martian atmosphere, we use a horizontally averaged one-dimensional model adapted from a previous study of sulfur photochemistry in the terrestrial Archean atmosphere [Pavlov and Kasting, 2002]. The same base code has been used robustly in numerous studies, among them investigations of NOx photochemistry [Kasting and Ackerman, 1985] and chlorine-hydrocarbon photochemistry [Singh and Kasting, 1988] in the present-day Earth atmosphere, as well as NH3 photochemistry [Kasting, 1982; Pavlov et al., 2001], CH4 photochemistry [Kasting et al., 1983; Pavlov et al., 2000], and O2 photochemistry [Pavlov et al., 2001] in the early Earth atmosphere. The one-dimensional model we use adapts this base code, carefully considering sulfur chemistry and incorporating the formation and diffusion of sulfate and elemental sulfur aerosols. In total, the model contains 359 chemical reactions involving 72 chemical species, including 16 sulfur species involved in 74 chemical reactions (see Figure 1 and Table 1). Although the model is complex, the results of the analysis are not sensitive to the complexity of the model. Moreover, this complexity is often necessary to resolve the chemistry of the system. Reactions and rate constants are partly adopted from Pavlov et al. [2001] but reviewed and updated to reflect the latest available measurements and original sources (see Table 1). We assign a gravity value for Mars of 373 cm s−2, an albedo of 0.215, and an altitude for the tropopause of 15 km. We assume the same parameterization for NOx production from lightning as in Pavlov and Kasting [2002]. The solar flux at Mars is scaled to 0.43 of that incident upon Earth. The model atmosphere is divided into 100 layers; at each layer, the continuity equation is solved for long-lived species, including transport by eddy and molecular diffusion. The combined equations are cast in centered, finite difference form. Boundary conditions are applied at the top and bottom of the atmosphere for each chemical species as in Pavlov et al. [2001]. At the upper boundary, zero flux is assumed for most species, though escape of H and H2 is simulated by assuming a diffusion-limited upward flux [Walker, 1977; Kasting and Brown, 1998]. As in Nair et al. [1994] and Wong et al. [2003], a fixed flux of atomic oxygen of 108 cm−2 s−1 is assumed for the upper boundary to balance hydrogen loss.

Figure 1.

A schematic of the primary reactions involving sulfur species in the photochemical model, with oxidation state shown along the upper axis. All reactions are in the gas phase with the exception of SO2 to SO4 aerosols, which occurs in the aqueous phase. AER, aerosols.

Table 1. Photochemical Reaction Ratesa
 ReactionRate ConstantRate Constant ReferenceColumn Reaction Ratesb (Current, Ancient)
  • a

    Units are as follows. Bimolecular reactions: k, cm3 molecule−1 s−1; termolecular reactions: k0, cm6 molecule−2 s−1; kequation image, cm3 molecule−1 s−1.

  • b

    Column reaction rates are in units of cm−2 s−1 and given for all values >1; hyphen indicates otherwise.

  • c

    An effective second-order rate constant for a given temperature and pressure condition is given by: equation image.

  • d

    Effective rate constant is the sum of R15a and R15b.

  • e

    Rate constants for the surface and top of atmosphere, respectively, for the current Mars model (upper line) and the ancient Mars model (lower line).

  • f

    An effective second-order rate constant for a given temperature and pressure condition is given by: equation image.

  • g

    Upper limit.

  • h

    Provided rate constant is for all product pathways, therefore this value should be assumed to be an upper limit for this particular branch.

  • i

    Measured at 298 K.

  • j

    Rate constant for a given temperature and pressure condition is given by: equation image.

  • k

    Products are H2O + NO3. Assumed to decompose to NO2 + O.

  • l

    Reaction rate estimated by first reference based upon values subsequently updated by second reference.

  • m

    The recommended value is 8.5 × 10−41 exp(6540/T)[H2O]2 s−1 ([H2O] in molec cm−3).

  • n

    High-pressure reaction rate assumed equal to kequation image of R180.

  • o

    High-pressure reaction rate assumed equal to 0.3 × kequation image of R140.

R1H2O + O(1D) → 2 OH1.63 × 10−10 exp(60/T)Sander et al. [2006]1.34 × 109, 1.29 × 105
R2H2 + O(1D) → OH + H1.1 × 10−10Sander et al. [2006]2.43 × 108, 8.46 × 107
R3H2 + O → OH + H8.5 × 10−20T2.67 exp(−3163/T)Baulch et al. [1992]1.17 × 107, 1.19 × 104
R4H2 + OH → H2O + H2.8 × 10−12 exp(−1800/T)Sander et al. [2006]2.98 × 108, 3.94 × 105
R5H + O3 → OH + O21.4 × 10−10 exp(−470/T)Sander et al. [2006]1.12 × 1011, 4.15 × 1012
R6cH + O2 + M → HO2 + Mk0: 4.4 × 10−32(T/300)−1.3Sander et al. [2006]1.28 × 1012, 2.65 × 1011
  kequation image: 4.7 × 10−11(T/300)−0.2  
R7H + HO2 → H2 + O26.9 × 10−12Sander et al. [2006]1.32 × 109, 1.02 × 107
R8H + HO2 → H2O + O1.6 × 10−12Sander et al. [2006]3.07 × 108, 2.36 × 106
R9H + HO2 → OH + OH7.2 × 10−11Sander et al. [2006]1.38 × 1010, 1.06 × 108
R10OH + O → H + O22.2 × 10−11 exp(120/T)Sander et al. [2006]2.42 × 1011, 6.92 × 1011
R11OH + HO2 → H2O + O24.8 × 10−11 exp(250/T)Sander et al. [2006]2.46 × 109, 1.34 × 107
R12OH + O3 → HO2 + O21.7 × 10−12 exp(−940/T)Sander et al. [2006]1.20 × 106, 1.01 × 108
R13HO2 + O → OH + O23.0 × 10−11 exp(200/T)Sander et al. [2006]1.06 × 1012, 1.23 × 1011
R14HO2 + O3 → OH + 2O21.0 × 10−14 exp(−490/T)Sander et al. [2006]1.01 × 108, 1.03 × 1010
R15adHO2 + HO2 → H2O2 + O23.5 × 10−13 exp(430/T)Sander et al. [2006]8.15 × 1010, 4.28 × 108
R15bdHO2 + HO2 + M → H2O2 + O2 + M1.7 × 10−33[M] exp(1000/T)Sander et al. [2006]8.15 × 1010, 4.28 × 108
R16H2O2 + OH → HO2 + H2O1.8 × 10−12Sander et al. [2006]2.38 × 108, 1.75 × 106
R17O + O + M → O2 + M5.2 × 10−35 exp(900/T)Tsang and Hampson [1986]2.96 × 1010, 1.75 × 1012
R18cO + O2 + M → O3 + Mk0: 6.0 × 10−34(T/300)−2.4Sander et al. [2006] and DeMore et al. [1992]2.07 × 1012, 5.95 × 1012
  kequation image: 1.0 × 10−10  
R19O + O3 → 2O28.0 × 10−12 exp(−2060/T)Sander et al. [2006]5.73 × 107, 1.17 × 1011
R20OH + OH → H2O + O1.8 × 10−12Sander et al. [2006]6.30 × 105, 1.09 × 104
R21O(1D) + M → O + M7.5 × 10−11 exp(115/T)Sander et al. [2006]1.98 × 1012, 3.07 × 1012
R22O(1D) + O2 → O + O23.3 × 10−11 exp(55/T)Sander et al. [2006]6.68 × 109, 5.50 × 1011
R23O2 + hv → O + O(1D)0, 1.71 × 10−6e1.54 × 1010, 1.30 × 1012
  0, 8.81 × 10−6  
R24O2 + hv → O + O1.84 × 10−10, 2.06 × 10−8e1.20 × 1011, 1.00 × 1012
  8.25 × 10−58, 1.3 × 10−7  
R25H2O + hv → H + OH3.84 × 10−11, 6.57 × 10−6e3.31 × 109, 7.72 × 106
  3.01 × 10−57, 3.35 × 10−5  
R26O3 + hv → O2 + O(1D)1.86 × 10−3, 1.11 × 10−3e1.61 × 1012, 1.27 × 1012
  8.13 × 10−54, 4.22 × 10−6  
R27O3 + hv → O2 + O3.22 × 10−4, 1.14 × 10−3e3.54 × 1011, 1.41 × 1011
  9.04 × 10−55, 4.69 × 10−7  
R28H2O2 + hv → 2OH2.235 × 10−5, 2.62 × 10−5e8.07 × 1010, 4.22 × 108
  1.34 × 10−53, 5.65 × 10−6  
R29CO2 + hv → CO + O(3P)7.86 × 10−13, 5.79 × 10−10e7.98 × 1011, 3.09 × 1012
  6.85 × 10−59, 3.66 × 10−9  
R30CO + OH + M → CO2 + H + Mk0: 1.5 × 10−13(T/300)0.6Sander et al. [2006]f1.16 × 1012, 3.71 × 1012
  kequation image: 2.1 × 109(T/300)6.1  
R31CO + O + M → CO2 + Mk0: 1.7 × 10−33 exp(−1510/T)k0: Tsang and Hampson [1986] kequation image: Simonaitis and Heicklen [1972]c1.76 × 108, 7.60 × 107
  kequation image: 2.66 × 10−14 exp(−1459/T)  
R32H + CO + M → HCO + Mk0: 1.4 × 10−34 exp(−100/T)k0: Wagner and Bowman [1987] kequation image: Arai et al. [1981]c8.53 × 107. 7.18 × 107
  kequation image: 1.96 × 10−13 exp(−1366/T)  
R33H + HCO → H2 + CO3.0 × 10−10Pinto et al. [1980]1.37 × 103, 3.34 × 102
R34HCO + HCO → H2CO + CO6.3 × 10−11Pinto et al. [1980]-, -
R35OH + HCO → H2O + CO1.7 × 10−10Baulch et al. [1992]1.70 × 101, 1.10 × 10°
R36O + HCO → H + CO25.0 × 10−11Baulch et al. [1992]5.07 × 105, 9.60 × 105
R37O + HCO → OH + CO5.0 × 10−11Baulch et al. [1992]5.07 × 105, 9.60 × 105
R38H2CO + hv → H2 + CO3.46 × 10−5, 3.56 × 10−5e-, -
  0,0  
R39H2CO + hv → HCO + H1.87 × 10−5, 1.93 × 10−5e-, -
  0,0  
R40HCO + hv → H + CO5 × 10−3Pinto et al. [1980]e1.28 × 103, 8.22 × 102
R41H2CO + H → H2 + HCO2.1 × 10−16T1.62 exp(−1090/T)Baulch et al. [1994]-, -
R42CO2 + hv → CO + O(1D)0, 1.11 × 10−7e3.63 × 1011, 1.05 × 1012
  0, 5.69 × 10−7  
R43H + H + M → H2 + M2.7 × 10−31T−0.6Baulch et al. [1992]-, -
R44HCO + O2 → HO2 + CO5.2 × 10−12Sander et al. [2006]8.43 × 107, 6.99 × 107
R45H2CO + OH → H2O + HCO5.5 × 10−12 exp(125/T)Sander et al. [2006]-, -
R46H + OH + M → H2O + M9.36 × 10−26T−2.0Baulch et al. [1992] and Black and Porter [1962]4.31 × 105, 3.21 × 103
R47OH + OH + M → H2O2 + Mk0: 6.9 × 10−31(T/300)−1.0Sander et al. [2006]c3.20 × 103, 3.10 × 101
  kequation image: 2.6 × 10−11  
R48H2CO + O → HCO + OH3.4 × 10−11 exp(−1600/T)Sander et al. [2006]-, -
R49H2O2 + O → OH + HO21.4 × 10−12 exp(−2000/T)Sander et al. [2006]2.23 × 107, 8.70 × 105
R50HO2 + hv → OH + O1.42 × 10−4, 1.65 × 10−4e3.38 × 1010, 6.66 × 107
  1.31 × 10−52, 4.05 × 10−5  
R51CH4 + hv1CH2 + H20, 1.42 × 10−6e-, -
  0, 7.12 × 10−6  
R52C2H6 + hv → 23CH2 + H20, 6.72 × 10−7e-, -
  0, 3.36 × 10−6  
R53C2H6 + hv → CH4 + 1CH20, 6.72 × 10−7e-, -
  0, 3.36 × 10−6  
R54HNO2 + hv → NO + OH1.7 × 10−3Cox [1974]5.00 × 104, 2.09 × 105
R55HNO3 + hv → NO2 + OH1.95 × 10−5, 4.16 × 10−5e9.21 × 104, 8.01 × 104
  1.97 × 10−52, 1.16 × 10−4  
R56NO + hv → N + O7.2 × 10−14, 1.85 × 10−6e2.50 × 105, 2.92 × 106
  0, 1.85 × 10−6  
R57NO2 + hv → NO + O2.05 × 10−3, 2.08 × 10−3e7.08 × 109, -
  0, 0  
R58CH4 + OH → CH3 + H2O2.45 × 10−12 exp(−1775/T)Sander et al. [2006]-, -
R59CH4 + O(1D) → CH3 + OH(0.75) × 1.5 × 10−10Sander et al. [2006]-, -
R60CH4 + O(1D) → H2CO + H2(0.05) × 1.5 × 10−10Sander et al. [2006]-, -
R611CH2 + CH4 → 2 CH35.9 × 10−11Bohland et al. [1985]-, -
R621CH2 + O2 → HCO + OH3.0 × 10−11Ashfold et al. [1981]-, -
R631CH2 + M → 3CH2 + M8.8 × 10−12Ashfold et al. [1981]-, -
R643CH2 + H2 → CH3 + H5.0 × 10−14Braun et al. [1970]g-, -
R653CH2 + CH4 → 2 CH37.1 × 10−12 exp(−5051/T)Bohland et al. [1985]-, -
R663CH2 + O2 → HCO + OH1.5 × 10−12Laufer and Bass [1974]-, -
R67CH3 + O2 → H2CO + OH5.5 × 10−13 exp(−4500/T)Baulch et al. [1992]-, -
R68CH3 + OH → H2CO + H23.76 × 10−14T−0.12 exp(209/T)DeAvillez Pereira et al. [1997]-, -
R69CH3 + O → H2CO + H1.1 × 10−10Sander et al. [2006]h-, -
R70CH3 + O3 → H2CO + HO25.4 × 10−12 exp(−220/T)Sander et al. [2006]h-, -
R71CH3 + CH3 + M → C2H6 + Mk0: 8.76 × 10−7T−7.03 exp(−1390/T)Slagle et al. [1988]c-, -
  kequation image: 1.5 × 10−7T−1.18 exp(−329/T)  
R72CH3 + hv1CH2 + H1.16 × 10−4, 1.3 × 10−4e-, -
  1.16 × 10−18, 1.01 × 10−4  
R73CH3 + H + M → CH4 + MFC: 0.63 exp(−T/3315) + 0.37 exp(−T/61)Baulch et al. [1994]c-, -
  k0: 1.7 × 10−24T−1.8  
  kequation image: 3.5 × 10−10  
R74CH3 + HCO → CH4 + CO8.2 × 10−11Hochanadel et al. [1980]-, -
R75CH3 + HNO → CH4 + NO3.0 × 10−14Zahnle [1986]-, -
R76CH3 + H2CO → CH4 + HCO1.3 × 10−31T6.1 exp(−990/T)Baulch et al. [1994]-, -
R77H + NO + M → HNO + M2.1 × 10−32 exp(300/T)Hampson and Garvin [1977]1.94 × 104, -
R78N + N + M → N2 + M8.27 × 10−34 exp(490/T)Campbell and Thrush [1967]-, -
R79N + O2 → NO + O1.5 × 10−11 exp(−3600/T)Sander et al. [2006]7.96 × 106, 4.77 × 103
R80N + O3 → NO + O22.0 × 10−16Sander et al. [2006]g,i-, -
R81N + OH → NO + H3.8 × 10−11 exp(85/T)Baulch et al. [1982]1.45 × 105, 9.69 × 103
R82N + NO → N2 + O2.1 × 10−11 exp(100/T)Sander et al. [2006]5.25 × 105, 2.90 × 106
R83NO + O3 → NO2 + O23.0 × 10−12 exp(−1500/T)Sander et al. [2006]1.30 × 107, 2.56 × 1011
R84NO + O + M → NO2 + Mk0: 9.0 × 10−32(T/300)−1.5Sander et al. [2006]c3.61 × 106, 8.08 × 107
  kequation image: 3.0 × 10−11  
R85NO + HO2 → NO2 + OH3.5 × 10−12 exp(250/T)Sander et al. [2006]8.46 × 109, 3.91 × 1010
R86NO + OH + M → HNO2 + Mk0: 7.0 × 10−31(T/300)−2.6Sander et al. [2006]b5.01 × 104, 2.09 × 105
  kequation image: 3.6 × 10−11(T/300)−0.1  
R87NO2 + O → NO + O25.1 × 10−12 exp(210/T)Sander et al. [2006]1.34 × 109, 1.27 × 1011
R88NO2 + OH + M → HNO3 + Mk0: 9.1 × 10−32(T/300)−3.0Sander et al. [2006]b921. × 104, 8.02 × 104
  kequation image: 4.2 × 10−11(T/300)−0.5  
R89NO2 + H → NO + OH4.0 × 10−10 exp(−340/T)Sander et al. [2006]5.67 × 107, 6.45 × 108
R90HNO3 + OH → H2O + NO2 + Ok0: 2.4 × 10−14 exp(460/T)Sander et al. [2006]j,k4.30 × 101, 1.22 × 102
  k2: 2.7 × 10−17 exp(2199/T)  
  k3: 6.5 × 10−34 exp(1335/T)  
R91HCO + NO → HNO + CO1.2 × 10−10T−0.4Veyret and Lesclaux [1981]3.53 × 10°, 4.96 × 101
R92HNO + hv → NO + H(=JHNO2)See (R54)2.53 × 107, 6.48 × 109
R93H + HNO → H2 + NO5.0 × 10−13T0.5 exp(−1200/T)Nicolet [1965]6.28 × 101, 1.46 × 103
R94O + HNO → OH + NO5.0 × 10−13T0.5 exp(−1200/T)Nicolet [1965]1.45 × 104, 8.21 × 105
R95OH + HNO → H2O + NO6.0 × 10−11Baulch et al. [1973]1.89 × 104, 1.70 × 106
R96HNO2 + OH → H2O + NO21.8 × 10−11 exp(−390/T)Sander et al. [2006]5.86 × 10°, 1.16 × 10°
R97CH4 + O → CH3 + OH1.15 × 10−15T1.56 exp(−4270/T)Baulch et al. [1992]-, -
R981CH2 + H2 → CH3 + H1.2 × 10−10Baulch et al. [1992]-, -
R991CH2 + CO2 → H2CO + CO1.0 × 10−12Zahnle [1986]-, -
R1003CH2 + O → HCO + H1.0 × 10−11Huebner and Giguere [1980]-, -
R1013CH2 + CO2 → H2CO + CO3.9 × 10−14Laufer [1981]-, -
R102C2H6 + OH → C2H5 + H2O8.7 × 10−12 exp(−1070/T)Sander et al. [2006]-, -
R103C2H6 + O → C2H5 + OH1.66 × 10−15T1.5 exp(−2920/T)Baulch et al. [1992]-, -
R104C2H6 + O(1D) → C2H5 + OH1.5 × 10−10Kasting et al. [1983] and Sander et al. [2006]l-, -
R105C2H5 + H → CH3 + CH31.25 × 10−10Sillesen et al. [1993]-, -
R106C2H5 + O → CH3 + HCO + H1.7 × 10−11Baulch et al. [1992]-, -
R107C2H5 + OH → CH3 + HCO + H21.1 × 10−10Pavlov et al. [2001] and Sander et al. [2006]l-, -
R108C2H5 + HCO → C2H6 + CO2.0 × 10−10Tsang and Hampson [1986]-, -
R109C2H5 + HNO → C2H6 + NO3.0 × 10−14Zahnle [1986]-, -
R110C2H5 + O2 + M → CH3 + HCO + OH + Mk0: 1.5 × 10−28(T/300)−3.0Sander et al. [2006]c-, -
  kequation image: 8.0 × 10−12  
R111SO + hv → S + O0, 2.83 × 10−5e-, -
  0, 1.42 × 10−4  
R112SO2 + hv → SO + O2.83 × 10−5, 5.08 × 10−5e9.25 × 107, 3.01 × 1011
  1.62 × 10−52, 9.76 × 10−5  
R113H2S + hv → HS + H4.98 × 10−5, 7.18 × 10−5e1.87 × 106, 9.28 × 103
  2.82 × 10−52, 8.91 × 10−5  
R114SO + O2 → O + SO21.25 × 10−13 exp(−2190/T)Sander et al. [2006]2.38 × 107, 6.14 × 1010
R115SO + HO2 → SO2 + OH2.8 × 10−11Yung and DeMore [1982] and Sander et al. [2006]l6.53 × 107, 4.60 × 1010
R116SO + O + M → SO2 + M6.0 × 10−31[M]Kasting [1990]2.99 × 105, 5.29 × 109
R117SO + OH → SO2 + H2.7 × 10−11 exp(335/T)Sander et al. [2006]2.70 × 106, 1.13 × 1010
R118SO2 + OH + M → HSO3 + Mk0: 3.3 × 10−31(T/300)−4.3Sander et al. [2006]c9.23 × 103, 1.70 × 107
  kequation image: 1.6 × 10−12  
R119SO2 + O + M → SO3 + Mk0: 1.8 × 10−33(T/300)2.0Sander et al. [2006]c7.85 × 103, 2.56 × 107
  kequation image: 4.2 × 10−14(T/300)1.8  
R120SO3 + H2O + H2O → H2SO4 + H2OmSander et al. [2006]m1.74 × 104, 2.86 × 107
R121HSO3 + O2 → HO2 + SO31.3 × 10−12 exp(−330/T)Sander et al. [2006]9.23 × 103, 1.70 × 107
R122HSO3 + OH → H2O + SO31.0 × 10−11Kasting [1990]-, -
R123HSO3 + H → H2 + SO31.0 × 10−11Kasting [1990]-, -
R124HSO3 + O → OH + SO31.0 × 10−11Kasting [1990]6.32 × 10°, 1.34 × 103
R125H2S + OH → H2O + HS6.1 × 10−12 exp(−75/T)Sander et al. [2006]7.43 × 103, 1.32 × 101
R126H2S + H → H2 + HS1.96 × 10−17T2.1 exp(−352/T)Yoshimura et al. [1992]-, -
R127H2S + O → OH + HS9.2 × 10−12 exp(−1800/T)Sander et al. [2006]3.17 × 103, 4.36 × 10°
R128HS + O → H + SO1.6 × 10−10Sander et al. [2006]i3.15 × 106, 6.38 × 106
R129HS + O2 → OH + SO4.0 × 10−19Sander et al. [2006]g,i5.39 × 104, 7.24 × 103
R130HS + HO2 → H2S + O23.0 × 10−11McElroy et al. [1980]1.88 × 106, 4.51 × 103
R131HS + HS → H2S + S4.0 × 10−11Stachnik and Molina, [1987]8.01 × 10°, -
R132HS + HCO → H2S + CO5.0 × 10−11Kasting [1990]-, -
R133HS + H → H2 + S2.16 × 10−11Nicholas et al. [1979]3.74 × 103, 1.13 × 103
R134HS + S → H + S22.2 × 10−11 exp(117/T)Kasting [1990]-, -
R135S + O2 → SO + O2.3 × 10−12Sander et al. [2006]3.76 × 103, 9.33 × 109
R136S + OH → SO + H6.6 × 10−11Sander et al. [2006]i-, 7.91 × 101
R137S + HCO → HS + CO5.0 × 10−11Kasting [1990]-, -
R138S + HO2 → HS + O21.5 × 10−11Kasting [1990]-, 3.93 × 102
R139S + HO2 → SO + OH3.0 × 10−11 exp(200/T)Yung and DeMore [1982] and Sander et al. [2006]l-, 2.59 × 103
R140S + S + M → S2 + Mk0: 1.18 × 10−29k0: Nicholas et al. [1979] kequation image: Mills [1998]c,n-, -
  kequation image: 1.0 × 10−10  
R141S2 + OH → HSO + S0 -, -
R142S2 + O → S + SO1.12 × 10−11Hills et al. [1987]-, -
R143HS + H2CO → H2S + HCO1.7 × 10−11 exp(−800/T)Kasting [1990]-, -
R144SO2 + hv1SO24.86 × 10−4, 5.08 × 10−4e1.47 × 109, -
  0,0  
R145SO2 + hv3SO23.08 × 10−7, 3.12 × 10−7e9.23 × 105, -
  0,0  
R146S2 + hv → S + S3.05 × 10−4, 3.15 × 10−4e-, -
  1.79 × 10−54, 6.0 × 10−7  
R147S2 + hv → S*20 -, -
R148H2SO4 + hv → SO2 + 2OH4.57 × 10−9, 3.58 × 10−7e1.53 × 102, 1.41 × 105
  8.49 × 10−55, 1.85 × 10−6  
R149SO3 + hv → SO2 + O0 -, -
R1501SO2 + M → 3SO2 + M1.0 × 10−12Turco et al. [1982]1.28 × 108, -
R1511SO2 + M → SO2 + M1.0 × 10−11Turco et al. [1982]1.28 × 109, -
R1521SO23SO2 + hv1.5 × 103Turco et al. [1982]4.37 × 106, -
R1531SO2 → SO2 + hv2.2 × 104Turco et al. [1982]6.41 × 107, -
R1541SO2 + O2 → SO3 + O1.0 × 10−16Turco et al. [1982]2.99 × 101, -
R1551SO2 + SO2 → SO3 + SO4.0 × 10−12Chung et al. [1975]-, -
R1563SO2 + M → SO2 + M1.89 × 10−13Sidebottom et al. [1972]1.19 × 108, -
R1573SO2 → SO2 + hv1.13 × 103Turco et al. [1982]1.39 × 107, -
R1583SO2 + SO2 → SO3 + SO7.0 × 10−14Chung et al. [1975]-, -
R159SO + NO2 → SO2 + NO1.4 × 10−11Sander et al. [2006]4.10 × 105, 1.68 × 1011
R160SO + O3 → SO2 + O21.25 × 10−13 exp(−2190/T)Sander et al. [2006]3.53 × 101, 1.38 × 108
R161SO2 + HO2 → SO3 + OH1.0 × 10−18Sander et al. [2006]h,i2.89 × 102, 4.29 × 105
R162HS + O3 → HSO + O29.0 × 10−12 exp(−280/T)Sander et al. [2006]5.20 × 105, 7.74 × 107
R163HS + NO2 → HSO + NO2.9 × 10−11 exp(240/T)Sander et al. [2006]9.11 × 104, 9,00 × 104
R164S + O3 → SO + O21.2 × 10−11Sander et al. [2006]i-, 9.93 × 107
R165SO + SO → SO2 + S8.3 × 10−15Herron and Huie [1980]7.89 × 10°, 9.43 × 109
R166SO3 + SO → SO2 + SO22.0 × 10−15Chung et al. [1975]-, 1.44 × 107
R167S + CO2 → SO + CO1.0 × 10−20Yung and DeMore [1982]-, 3.64 × 103
R168SO + HO2 → HSO + O22.8 × 10−11Yung and DeMore [1982] and Sander et al. [2006]l6.53 × 107, 4.60 × 1010
R169SO + HCO → HSO + CO5.5 × 10−11T−0.4Kasting [1990]-, 2.70 × 102
R170H + SO + M → HSO + Mk0: 5.7 × 10−32(T/300)−1.6Kasting [1990]c7.53 × 102, 1.01 × 106
  kequation image: 7.5 × 10−11  
R171HSO + hv → HS + O1.42 × 10−4, 1.65 × 10−4e3.30 × 106, 4.11 × 107
  1.31 × 10−52, 4.05 × 10−5  
R172HSO + NO → HNO + SO1.0 × 10−15Sander et al. [2006]h,i8.37 × 105, 6.49 × 109
R173HSO + OH → H2O + SO4.8 × 10−11 exp(250/T)Kasting [1990] and Sander et al. [2006]l2.00 × 105, 1.01 × 107
R174HSO + H → HS + OH7.2 × 10−11Kasting [1990] and Sander et al. [2006]l5.21 × 105, 4.29 × 107
R175HSO + H → H2 + SO6.9 × 10−12Kasting [1990] and Sander et al. [2006]l5.00 × 104, 4.11 × 106
R176HSO + HS → H2S + SO1.0 × 10−12Kasting [1990]7.31 × 10°, 1.98 × 102
R177HSO + O → OH + SO3.0 × 10−11 exp(200/T)Kasting [1990] and Sander et al. [2006]l6.10 × 107, 3.95 × 1010
R178HSO + S → HS + SO1.0 × 10−11Kasting [1990]-, 2.93 × 102
R179S + S2 + M → S3 + Mk0: 1.0 × 10−25T−2.0k0: Moses et al. [2002] kequation image: Mills [1998]c,o-, -
  kequation image: 3.0 × 10−11  
R180S2 + S2 + M → S4 + Mk0: 4.0 × 10−31 exp(900/T)k0: Moses et al. [2002] kequation image: Fowles et al. [1967]c-, -
  kequation image: 1.0 × 10−10  
R181S + S3 + M → S4 + Mk0: 1.0 × 10−25T−2.0k0: Moses et al. [2002] kequation image: Mills [1998]c,o-, -
  kequation image: 3.0 × 10−11  
R182S4 + S4 + M → S8(AER) + Mk0: 4.0 × 10−31 exp(900/T)k0: Moses et al. [2002] kequation image: Mills [1998]c,o-, -
  kequation image: 3.0 × 10−11  
R183S4 + hv → S2 + S23.05 × 10−4, 3.15 × 10−5e-, -
  1.79 × 10−54, 6.00 × 10−7  
R184S3 + hv → S2 + S3.05 × 10−4, 3.15 × 10−5e-, -
  1.79 × 10−54, 6.00 × 10−7  
R185NH3 + hv → NH2 + H1.75 × 10−5, 2.78 × 10−5e5.55 × 107, 1.13 × 105
  1.07 × 10−52, 4.43 × 10−5  
R186NH3 + OH → NH2 + H2O1.7 × 10−12 exp(−710/T)Sander et al. [2006]1.23 × 104, -
R187NH3 + O(1D) → NH2 + OH2.5 × 10−10Sander et al. [2006]2.05 × 102, -
R188NH2 + H + M → NH3 + M(6.0 × 10−30[M])/(1 + 3 × 10−20[M])Gordon et al. [1971]1.09 × 104, -
R189NH2 + NO → N2 + H2O4.0 × 10−12 exp(450/T)Sander et al. [2006]1.46 × 107, 1.14 × 105
R190NH2 + NH2 + M → N2H4 + M1.0 × 10−10Gordon et al. [1971]4.78 × 106, 1.74 × 102
R191NH2 + O → NH + OH5.0 × 10−12Albers et al. [1969]1.45 × 107, 1.96 × 101
R192NH2 + O → HNO + H5.0 × 10−12Albers et al. [1969]1.45 × 107, 1.96 × 101
R193NH + NO → N2 + O + H4.9 × 10−11Sander et al. [2006]h8.10 × 106, 3.11 × 101
R194NH + O → N + OH1.0 × 10−11Kasting [1982]7.85 × 106, -
R195N2H4 + hv → N2H3 + H3.46 × 10−5, 4.91 × 10−5e8.55 × 106, 1.66 × 102
  1.59 × 10−52, 5.77 × 10−5  
R196N2H4 + H → N2H3 + H29.9 × 10−12 exp(−1198/T)Stief and Payne, [1976]2.36 × 103, -
R197N2H3 + H → 2NH22.7 × 10−12Pavlov et al. [2001]7.44 × 104, 1.40 × 10°
R198N2H3 + N2H3 → N2H4 + N2 + H26.0 × 10−11Kuhn and Atreya, [1979]3.99 × 106, 8.27 × 101
R199NH + H + M → NH2 + M(6.0 × 10−30[M])/(1 + 3 × 10−20[M])Gordon et al. [1971] and Kasting [1982]l2.94 × 10−3, -
R200NH + hv → N + H1.75 × 10−5, 2.78 × 10−5e5.33 × 105, -
  1.07 × 10−52, 4.43 × 10−5  
R201NH2 + hv → NH + H1.75 × 10−5, 2.78 × 10−5e1.97 × 106, 1.15 × 101
  1.07 × 10−52, 4.43 × 10−5  
R202NH2 + hv → NH2*3.8 × 10−3Kasting [1982]4.76 × 108, 4.55 × 106
R203NH2* → NH2 + hv1.25 × 105Lenzi et al. [1972]-, -
R204NH2* + M → NH2 + M3.0 × 10−11Kasting [1982]1.70 × 104, 4.55 × 106
R205NH2* + H2 → NH3 + H3.0 × 10−11Kasting [1982]-, 4.54 × 101
R206NH2 + HCO→ NH3 + CO1.0 × 10−11Pavlov et al. [2001]-, -
R207NH + HCO → NH2 + CO1.0 × 10−11Pavlov et al. [2001]-, -
R2081CH2 + O2 → H2CO + O3.0 × 10−11Ashfold et al. [1981]-, -
R2093CH2 + O2 → H2CO + O0 -, -
R210C2H2 + hv → C2H + H0, 2.53 × 10−6e-, -
  0, 1.28 × 10−5  
R211C2H2 + hv → C2 + H20, 8.45 × 10−7e-, -
  0, 4.26 × 10−6  
R212C2H4 + hv → C2H2 + H2(0.51) × 3.37 × 10−5Zelikoff and Watanabe [1953]-, -
R2133CH2 + CH3 → C2H4 + H7 × 10−11Baulch et al. [1992]-, -
R214C2H5 + CH3 + M → C3H8 + Mk0: 1.01 × 10−22 exp(341/T), T ≤ 200 K; 2.22 × 10−26 exp(2026/T), T > 200 Kk0: Laufer et al. [1983]; Tsang and Hampson [1986]; Gladstone et al. [1996] kequation image: Sillesen et al. [1993]c-, -
  kequation image: 6.64 × 10−11  
R215C3H8 + OH → C3H7 + H2O8.7 × 10−12 exp(−615/T)Sander et al. [2006]h-, -
R216C3H8 + O → C3H7 + OH1.6 × 10−11 exp(−2900/T) + 2.2 × 10−11 exp(−2250/T)Hampson and Garvin [1977]-, -
R217C3H8 + O(1D) → C3H7 + OH1.3 × 10−10Kasting et al. [1983]-, -
R218C3H7 + H → CH3 + C2H56.0 × 10−13Tsang [1988]-, -
R2193CH2 + 3CH2 → C2H2 + H + H1.8 × 10−10 exp(−400/T)Baulch et al. [1992]-, -
R220C2H2 + OH → CO + CH31.91 × 10−12 exp(−233/T)Atkinson [1986]-, -
R221C2H2 + H + M → C2H3 + Mk0: 3.3 × 10−30 exp(−740/T)Baulch et al. [1992]c-, -
  kequation image: 1.4 × 10−11 exp(−1300/T)  
R222C2H3 + H→ C2H2 + H26.86 × 10−11 exp(23/T)Monks et al. [1995]-, -
R223C2H3 + H2 → C2H4 + H1.57 × 10−20T2.56 exp(−2529/T)Knyazev et al. [1996]-, -
R224C2H3 + CH4 → C2H4 + CH32.4 × 10−24T4.02 exp(−2754/T)Tsang and Hampson [1986]-, -
R225C2H3 + C2H6 → C2H4 + C2H51.0 × 10−21T3.3 exp(−5285/T)Tsang and Hampson [1986]-, -
R226C2H4 + OH → H2CO + CH32.14 × 10−12 exp(411/T)Atkinson [1986]-, -
R227C2H4 + O → HCO + CH35.3 × 10−12 exp(−640/T)Lara et al. [1996]-, -
R228C2H4 + H + M → C2H5 + Mk0: 7.7 × 10−30 exp(−380/T)Baulch et al. [1994]c-, -
  kequation image: 6.6 × 10−15T1.28 exp(−650/T)  
R229C2H + O2 → CO + HCO4.0 × 10−12Tsang and Hampson [1986]-, -
R230C2H + H2 → C2H2 + H1.2 × 10−11 exp(−998/T)Opansky and Leone [1996a]-, -
R231C2H + CH4 → C2H2 + CH31.2 × 10−11 exp(−491/T)Opansky and Leone [1996b]-, -
R232C2H + C2H6 → C2H2 + C2H53.5 × 10−11 exp(2.9/T)Opansky and Leone [1996a]-, -
R233C2H + H + M → C2H2 + Mk0: 1.26 × 10−18T−3.1 exp(−721/T)Tsang and Hampson [1986]c-, -
  kequation image: 3.0 × 10−10  
R234C3H8 + hv → C3H6 + H20, 4.18 × 10−6e-, -
  0, 2.11 × 10−5  
R235C3H8 + hv → C2H6 + 1CH20, 7.87 × 10−7e-, -
  0, 3.94 × 10−6  
R236C3H8 + hv → C2H4 + CH40, 3.14 × 10−6e-, -
  0, 1.71 × 10−5  
R237C3H8 + hv → C2H5 + CH30, 1.83 × 10−6e-, -
  0, 9.17 × 10−6  
R238C2H6 + hv → C2H2 + H2 + H20, 7.85 × 10−7e-, -
  0, 3.94 × 10−6  
R239C2H6 + hv → C2H4 + H + H0, 8.59 × 10−7e-, -
  0, 4.3 × 10−6  
R240C2H6 + hv → C2H4 + H20, 5.99 × 10−7e-, -
  0, 3.02 × 10−6  
R241C2H6 + hv → 2CH30, 2.17 × 10−7e-, -
  0, 1.08 × 10−6  
R242C2H4 + hv → C2H2 + H + H(0.49) × 3.37 × 10−5Back and Griffiths [1967]-, -
R243C3H6 + hv → C2H2 + CH3 + H(0.34) × 3.37 × 10−5assumed to equal JC2H4-, -
R244CH4 + hv3CH2 + 2H0, 1.19 × 10−6e-, -
  0, 5.96 × 10−6  
R245CH4 + hv → CH3 + H0, 2.43 × 10−6e-, -
  0, 1.22 × 10−5  
R246CH + hv → C + H8.48 × 10−9, 2.3 × 10−5e-, -
  5.48 × 10−55, 1.19 × 10−4  
R247CH2CO + hv3CH2 + CO7.92 × 10−5, 8.49 × 10−5e-, -
  0, 2.04 × 10−5  
R248CH3CHO + hv → CH3 + HCO(0.50) × 1.29 × 10−4Demerjian et al. [1974]-, -
R249CH3CHO + hv → CH4 + CO(0.50) × 1.29 × 10−4Calvert and Pitts [1966]-, -
R250C2H5CHO + hv → C2H5 + HCO1.29 × 10−4assumed to equal JCH3CHO-, -
R251C3H3 + hv → C3H2 + H8.48 × 10−9, 2.3 × 10−5e-, -
  5.48 × 10−55, 1.19 × 10−4  
R252CH3C2H + hv → C3H3 + H(0.40) × 2.44 × 10−5Nakayama and Watanabe [1964]-, -
R253CH3C2H + hv → C3H2 + H2(0.15) × 2.44 × 10−5Yung et al. [1984]-, -
R254CH3C2H + hv → CH3 + C2H(0.02) × 2.44 × 10−5 -, -
R255CH2CCH2 + hv → C3H3 + H(0.40) × 5.34 × 10−11Rabalais et al. [1971]-, -
R256CH2CCH2 + hv → C3H2 + H2(0.15) × 5.34 × 10−11Yung et al. [1984]-, -
R257CH2CCH2 + hv → C2H2 + 3CH2(0.06) × 5.34 × 10−11 -, -
R258C3H6 + hv → CH2CCH2 + H2(0.57) × 3.37 × 10−5 -, -
R259C3H6 + hv → C2H4 + 3CH2(0.02) × 3.37 × 10−5 -, -
R260C3H6 + hv → C2H + CH4 + H(0.05) × 3.37 × 10−5 -, -
R261C + OH → CO + H4.0 × 10−11Herbst and Klemperer [1973]-, -
R262C + H2 + M → 3CH2 + Mk0: 7.0 × 10−32k0: Husain and Young [1975] kequation image: Harding et al. [1993]c-, -
  kequation image: 2.06 × 10−11 exp(−57/T)  
R263C + O2 → CO + O3.3 × 10−11Donovan and Hussain [1970]-, -
R264CH + H → C + H21.3 × 10−10 exp(−80/T)Harding et al. [1993]-, -
R265CH + O → CO + H6.6 × 10−11Baulch et al. [1992]-, -
R266CH + H23CH2 + H3.1 × 10−10 exp(−1650/T)Becker et al. [1991]-, -
R267CH + H2 + M → CH3 + Mk0: 1.5 × 10−23T−2.6Fulle and Hippler, [1997]c-, -
  kequation image: 8.55 × 10−11T0.15  
R268CH + O2 → CO + OH2.75 × 10−11Baulch et al. [1992]-, -
R269CH + CO2 → HCO + CO5.9 × 10−12 exp(−350/T)Berman et al. [1982]-, -
R270CH + CH4 → C2H4 + H3.96 × 10−8T−1.04 exp(−36.1/T)Canosa et al. [1997]-, -
R271CH + C2H2 → C3H2 + H1.59 × 10−9T−0.233 exp(−16/T)Canosa et al. [1997]-, -
R272CH + C2H4 → CH3C2H + H3.87 × 10−9T−0.546 exp(−29.6/T)Canosa et al. [1997]-, -
R273CH + C2H4 → CH2CCH2 + H3.87 × 10−9T−0.546 exp(−29.6/T)Canosa et al. [1997]-, -
R2743CH2 + O → CH + OH8.0 × 10−12Huebner and Giguere [1980]-, -
R2753CH2 + O → CO + H + H2.0 × 10−10Baulch et al. [1992]-, -
R2763CH2 + H + M → CH3 + Mk0: 3.4 × 10−32 exp(736/T)Moses et al. [2000]c-, -
  kequation image: 7.3 × 10−12  
R2773CH2 + H → CH + H23.54 × 10−11T0.32Fulle and Hippler [1997]-, -
R2783CH2 + CO + M → CH2CO + Mk0: 1.0 × 10−28k0: Yung et al. [1984]; and kequation image: Laufer [1981]c-, -
  kequation image: 1.5 × 10−15  
R2793CH2 + 3CH2 → C2H2 + H22.0 × 10−11 exp(−400/T)Baulch et al. [1992]-, -
R2803CH2 + C2H2 + M → CH3C2H + Mk0: 6.0 × 10−29 exp(1680/T)k0: Moses et al. [2000] kequation image: Baulch et al. [1992]c-, -
  kequation image: 1.0 × 10−11 exp(−3330/T)  
R2813CH2 + C2H3 → CH3 + C2H23.0 × 10−11Tsang and Hampson [1986]-, -
R2823CH2 + C2H5 → CH3 + C2H43.0 × 10−11Tsang and Hampson [1986]-, -
R283CH2CO + H → CH3 + CO3.0 × 10−11 exp(−1700/T)Baulch et al. [1992]-, -
R284CH2CO + O → H2CO + CO1.3 × 10−12 exp(−680/T)Baulch et al. [1992]-, -
R285CH2CCH2 + H + M → CH3 + C2H2 + Mk0: 8.0 × 10−24T−2.0 exp(−1225/T)Yung et al. [1984]c-, -
  kequation image: 9.7 × 10−13 exp(−1550/T)  
R286CH2CCH2 + H + M → C3H5 + Mk0: 8.0 × 10−24T−2.0 exp(−1225/T)Yung et al. [1984]c-, -
  kequation image: 6.6 × 10−12 exp(−1360/T)  
R287CH3 + O2 + M → CH3O2 + Mk0: 4.0 × 10−31(T/300)−3.6Sander et al. [2006]c-, -
  kequation image: 1.2 × 10−12(T/300)1.1  
R288CH3 + CO + M → CH3CO + Mk0: 1.26 × 10−33 exp(−1636/T)k0: Anastasi and Maw [1982] kequation image: Watkins and Word [1974]c-, -
  kequation image: 2.63 × 10−13 exp(−3007/T)  
R289CH3 + H2CO → CH4 + HCO6.8 × 10−12 exp(−4450/T)Baulch et al. [1992]-, -
R290CH3 + OH → CO + H2 + H26.7 × 10−12Fenimore [1968]-, -
R291CH3 + C2H3 → C3H5 + H2.4 × 10−13Romani et al. [1993]-, -
R292CH3O2 + H → CH4 + O21.4 × 10−11Zahnle [1986]-, -
R293CH3O2 + H → H2O + H2CO1.0 × 10−11Zahnle [1986]-, -
R294CH3O2 + O → H2CO + HO21.0 × 10−11Zahnle [1986]-, -
R295CH3CO + H → CH4 + CO1.0 × 10−10Zahnle [1986]-, -
R296CH3CO + O → H2CO + HCO5 × 10−11Zahnle [1986]-, -
R297CH3CO + CH3 → C2H6 + CO1.4 × 10−10Adachi et al. [1981]-, -
R298CH3CO + CH3 → CH4 + CH2CO1.0 × 10−11Hassinen et al. [1990]-, -
R299CH3CHO + H → CH3CO + H26.8 × 10−15T1.16 exp(−1210/T)Baulch et al. [1992]-, -
R300CH3CHO + O → CH3CO + OH9.7 × 10−12 exp(−910/T)Baulch et al. [1992]-, -
R301CH3CHO + OH → CH3CO + H2O1.95 × 10−14T0.73 exp(560/T)Baulch et al. [1992]-, -
R302CH3CHO + CH3 → CH3CO + CH43.3 × 10−30T5.64 exp(−1240/T)Baulch et al. [1992]-, -
R303CH3C2H + H + M → CH3 + C2H2 + Mk0: 2.0 × 10−29k0: Moses et al. [2000] kequation image: Whytock et al. [1976]c-, -
  kequation image: 3.98 × 10−11 exp(−1152/T)  
R304CH3C2H + H + M → C3H5 + Mk0: 8.0 × 10−24T−2.0 exp(−1225/T)k0: Yung et al. [1984] kequation image: Whytock et al. [1976]c-, -
  kequation image: 6.0 × 10−11 exp(−1233/T)  
R305C2 + O → C + CO5.0 × 10−11Prasad and Huntress [1980]-, -
R306C2 + O2 → CO + CO1.5 × 10−11 exp(−550/T)Baughcum and Oldenborg [1984]-, -
R307C2 + H2 → C2H + H1.77 × 10−10 exp(−1469/T)Pitts et al. [1982]-, -
R308C2 + CH4 → C2H + CH35.05 × 10−11 exp(−297/T)Pitts et al. [1982]-, -
R309C2H + O → CO + CH1.7 × 10−11Baulch et al. [1992]-, -
R310C2H + C3H8 → C2H2 + C3H77.8 × 10−11 exp(3/T)Hoobler et al. [1997]-, -
R311C2H2 + O → 3CH2 + CO3.0 × 10−11 exp(−1600/T)Sander et al. [2006]h-, -
R312C2H2 + OH + M → C2H2OH + Mk0: 5.5 × 10−30Sander et al. [2006]c-, -
  kequation image: 8.3 × 10−13(T/300)2  
R313C2H2 + OH + M → CH2CO + H + Mk0: 5.8 × 10−31 exp(−1258/T)Perry and Williamson [1982]c-, -
  kequation image: 1.4 × 10−12 exp(388/T)  
R314C2H2OH + H → H2O + C2H25.0 × 10−11Miller et al. [1982]-, -
R315C2H2OH + H → H2 + CH2CO3.3 × 10−11 exp(−2000/T)Miller et al. [1982]-, -
R316C2H2OH + O → OH + CH2CO3.3 × 10−11 exp(−2000/T)Miller et al. [1982]-, -
R317C2H2OH + OH → H2O + CH2CO1.7 × 10−11 exp(−1000/T)Miller et al. [1982]-, -
R318C2H3 + O → CH2CO + H1.6 × 10−10Tsang and Hampson [1986]-, -
R319C2H3 + OH → C2H2 + H2O5.0 × 10−11Tsang and Hampson [1986]-, -
R320C2H3 + CH3 → C2H2 + CH43.4 × 10−11Fahr et al. [1991]-, -
R321C2H3 + CH3 + M → C3H6 + Mk0: 6.0 × 10−28 exp(1680/T)k0: Moses et al. [2000] kequation image: Fahr et al. [1991]c-, -
  kequation image: 1.2 × 10−10  
R322C2H3 + C2H3 → C2H4 + C2H22.4 × 10−11Fahr et al. [1991]-, -
R323C2H3 + C2H5 → C2H4 + C2H48.0 × 10−12Tsang and Hampson [1986]-, -
R324C2H3 + C2H5 → CH3 + C3H5k0: 1.9 × 10−27Tsang and Hampson [1986] and Romani et al. [1993]c-, -
  kequation image: 2.5 × 10−11  
R325C2H4 + OH + M → C2H4OH + Mk0: 1.0 × 10−28(T/300)−4.5Sander et al. [2006]c-, -
  kequation image: 8.8 × 10−12(T/300)−0.85  
R326C2H4OH + H → H2O + C2H45.0 × 10−11Zahnle [1986]-, -
R327C2H4OH + H → H2 + CH3CHO3.3 × 10−11 exp(−2000/T)Zahnle [1986]-, -
R328C2H4OH + O → OH + CH3CHO1.7 × 10−10Herron [1988]-, -
R329C2H4OH + OH → H2O + CH3CHO1.7 × 10−11 exp(−1000/T)Zahnle [1986]-, -
R330C2H5 + OH → CH3CHO + H21.1 × 10−10Kasting et al. [1983] and Sander et al. [2006]l-, -
R331C2H5 + O → CH3CHO + H9.1 × 10−11Baulch et al. [1992]-, -
R332C2H5 + CH3 → C2H4 + CH43.25 × 10−11T−0.5Tsang and Hampson [1986] and Laufer et al. [1983]-, -
R333C2H5 + C2H3 → C2H6 + C2H28.0 × 10−13Tsang and Hampson [1986]-, -
R334C2H5 + C2H5 → C2H6 + C2H42.3 × 10−12Tsang and Hampson [1986]-, -
R335C2H5 + H + M → C2H6 + Mk0: 5.5 × 10−22T−2.0 exp(−1040/T)k0: Moses et al. [2000] kequation image: Sillesen et al. [1993]c-, -
  kequation image: 1.66 × 10−10  
R336C2H5 + H → C2H4 + H23.0 × 10−12Tsang and Hampson [1986]-, -
R337C3H2 + H + M → C3H3 + Mk0: 2.52 × 10−28Moses et al. [2000]c-, -
  kequation image: 5.0 × 10−11  
R338C3H3 + H + M → CH3C2H + Mk0: 5.5 × 10−27k0: Moses et al. [2000] kequation image: Homann and Wellmann [1983]c-, -
  kequation image: 1.15 × 10−10 exp(−276/T)  
R339C3H3 + H + M → CH2CCH2 + Mk0: 5.5 × 10−27k0: Moses et al. [2000] kequation image: Atkinson and Hudgens [1999]c-, -
  kequation image: 2.5 × 10−10  
R340C3H5 + H → CH3C2H + H21.4 × 10−11Hanning-Lee and Pilling [1992]-, -
R341C3H5 + H + M → C3H6 + Mk0: 2.0 × 10−28k0: Moses et al. [2000] kequation image: Hanning-Lee and Pilling [1992]c-, -
  kequation image: 2.8 × 10−10  
R342C3H5 + H → CH4 + C2H21.5 × 10−11Yung et al. [1984]-, -
R343C3H5 + CH3 → CH3C2H + CH45.0 × 10−12T−0.32 exp(132/T)Tsang [1991]-, -
R344C3H5 + CH3 → CH2CCH2 + CH41.69 × 10−10T−0.32 exp(66/T)Tsang [1991]-, -
R345C3H6 + OH → CH3CHO + CH34.1 × 10−12 exp(540/T)Hampson and Garvin [1977]-, -
R346C3H6 + O → CH3 + CH3 + CO4.1 × 10−12 exp(−38/T)Hampson and Garvin [1977]-, -
R347C3H6 + H + M → C3H7 + Mk0: 1.3 × 10−28 exp(−380/T)k0: Moses et al. [2000] kequation image: Tsang [1991]c-, -
  kequation image: 2.2 × 10−11 exp(−785/T)  
R348C3H7 + CH3 → C3H6 + CH41.9 × 10−11T−0.32Tsang [1988]-, -
R349C3H7 + OH → C2H5CHO + H21.1 × 10−10Kasting et al. [1983] and Sander et al. [2006]l-, -
R350C3H7 + O → C2H5CHO + H1.1 × 10−10Kasting et al. [1983] and Sander et al. [2006]l-, -
R351H + CH2CCH2 → CH3C2H + H1.0 × 10−11 exp(−1000/T)Yung et al. [1984]-, -
R352O + H2CO → OH + HCO3.4 × 10−11 exp(−1600/T)Sander et al. [2006]h-, -
R3533CH2 + C2H2 + M → CH2CCH2 + M1.0 × 10−11 exp(−3330/T)Baulch et al. [1992]-, -
R354C2H + C2H2 → C4H2(AER) + H1.5 × 10−10Stephens et al. [1987]-, -
R3551CH2 + H23CH2 + H21.26 × 10−11Langford et al. [1983] and Braun et al. [1970]-, -
R356C3H5 + H → CH2CCH2 + H21.4 × 10−11Moses et al. [2000]-, -
R357HCO + H2CO → CH3O + CO1.0 × 10−17Veyret and Lescaux [1981]g-, -
R358CH3O + CO → CH3 + CO22.6 × 10−11 exp(−5940/T)Tsang and Hampson [1986]-, -
R359C2H + CH2CCH2 → C5H4(AER) + H1.5 × 10−10Pavlov et al. [2001] and Stephens et al. [1987]-, -

[5] The resulting set of coupled ordinary differential equations is integrated to steady state using the reverse Euler method. Once steady state is reached, sources and sinks at all altitudes for all chemical species are in balance and all major atmospheric species have equilibrated. We consider the model to have reached this point when input and output fluxes are balanced at all altitudes and no chemical species at any altitude changes more than 1/100th of a percent between time steps. To explore dissipation of sulfur volatiles after an influx from a volcanic event, we take several snapshots of the atmosphere after a specified number of model years (e.g., 1, 10, 100, 1000, etc.)

3. Present-Day Mars

[6] We have performed an important set of simulations to validate our photochemistry model with previous studies of Martian photochemistry. A small number of adaptations to the original terrestrial model are necessary to simulate the current Martian atmosphere. We set the CO2 level at 6 mb, and assign a fixed N2 mixing ratio of 0.027 at the lower boundary of the model. Each layer in the model is 1 km in height. Rainout rates (determined using the method of Giorgi and Chameides [1986]) are adjusted to zero for current Martian conditions. We adopt a temperature profile and an eddy profile matching those of Nair et al. [1994]. Within the lower 20 km of the atmosphere, water abundance is specified at a mixing ratio of 1.5 × 10−4, as in Wong et al. [2003, 2004, 2005], and is allowed to evolve photochemically at higher altitudes. A small amount of volcanic SO2 production (106 cm−2 s−1, approximately 1/1000th of the current terrestrial SO2 flux [Holland, 2002]) is also incorporated into the simulations.

[7] Without imposing any additional constraints or boundary conditions than those specified by Nair et al. [1994], we arrive at results that are highly consistent with those of Nair et al. [1994]. The vertical profiles of major atmospheric constituents and radicals calculated by the model are shown in Figures 2, 3a and 3b. H2 mixing ratios (approximately 35 ppm) are also consistent with the H2 level of 15 ppm detected by Krasnopolsky and Feldman [2001]. Slight variations may be due to choice of solar zenith angle (50° was used for our simulations). We note that our SO2 levels are consistent with the Krasnopolsky [2005] finding of an upper limit of 1 ppb of SO2 for the current Martian atmosphere. For reference, the vertical profile of SO2 is included in Figure 3b.

Figure 2.

Profiles of O, H2, CO, and O2 concentrations in the current Martian atmosphere. Solid lines are our model output; for comparison, dashed lines from Nair et al. [1994] are shown [see also Yung and DeMore, 1999].

Figure 3.

Profiles of (a) OH and H and (b) HO2 and H2O2 concentrations for the current Martian atmosphere. Solid lines are our model output; for comparison, dashed lines from Nair et al. [1994] are shown [see also Yung and DeMore, 1999]. For reference, in Figure 3b a profile of SO2 is also shown.

[8] For our current Mars simulations, we find similar sulfur chemistry to that reported by Krasnopolsky [2005]. Photolysis of SO2 into SO is almost entirely compensated for by recycling back to SO2 involving reactions with SO and O, O2, O3, OH and HO2. The major loss pathways for SO2 are reactions with O and OH to form SO3 and HSO3, respectively, against which the lifetime of SO2 is only 5.79 Earth years.

[9] The present model incorporates more sophisticated sulfur photochemistry than previous models, including the formation of S8 aerosols and a more nuanced scheme of H2SO4 condensation that allows for sulfur atoms incorporated into H2SO4 to be recycled back to SO2. Thus, instead of considering a rate-determined lifetime against certain SO2 destruction paths, we opt to consider e-folding times for SO2 residence in the atmosphere.

4. Ancient Mars

[10] For our simulations of the ancient Martian atmosphere, we extend our model atmosphere to 200 km (by increasing each of the 100 layers to 2 km in thickness) to account for the higher-pressure regime. The atmospheric pressures for CO2 and N2 are set to 500 mb and 100 mb, respectively. In the absence of conclusive information about the abundance of N2 on ancient Mars, a value of 100 mb was chosen as representative of the N2 abundance on early Earth, which may have had a similar size and composition to the atmosphere of early Mars. Additional runs with only 6 mb of N2 (near the estimates of ancient Mars N2 of Fox [1993] and Jakosky et al. [1994]) show the sulfur lifetimes to be largely insensitive to the nitrogen abundance of the ancient atmosphere (varying <5% from baseline simulations). Water vapor is controlled by temperature within the troposphere and at no point exceeds saturation conditions. For CO, we employ a fixed deposition velocity of 10−9 cm s−1, in accordance the abiotic rate determined by Kasting and Catling [2003], and we scale the solar UV flux up by a factor of five to account for higher Lyman-α UV fluxes in the Late Noachian [Ribas et al., 2005]. Our vertical temperature profile, ranging from 258 K at the surface to 168 K in the upper atmosphere, is taken from radiative transfer steady state simulation results for a 500 mb CO2 ancient Martian atmosphere [Johnson et al., 2007; Johnson et al., 2008].

[11] For our simulations, precipitation (again calculated by the terrestrial parameterization of Giorgi and Chameides [1986]) is reduced by a factor of 500 to mimic the hyperarid core of the Atacama Desert in Chile, as recent studies suggest that the geomorphology of Late Noachian basins on Mars are dominated by equal or greater aridity [Stepinski and Stepinski, 2005]. To take into account higher atmospheric pressure, we employ a terrestrial eddy diffusion profile scaled by the square root of density for the Martian atmosphere.

[12] To study photochemical behavior, we begin from a steady state atmosphere containing very little SO2 (f(SO2) < 10−10). In our model, no additional CO2 is injected in association with the eruption because of its negligible mass compared to the background atmosphere. At the beginning of each simulation, we assign a starting SO2 mixing ratio between 10−8 and 10−6. The initial SO2 mixing ratios are only assigned to the lower 20 km of the atmosphere, simulating the vertical reach of a typical Plinian eruption [see Glaze and Baloga, 2002]. We then take snapshots of the atmosphere at several different time steps as the atmosphere returns slowly to steady state. As oxidant rates change minimally from time step to time step, numerical noise is small, thereby enabling our use of the reverse Euler method for time-marching solutions. Figure 4 demonstrates the decline of SO2 mixing ratios with time, as recorded in the lowermost layer of the atmosphere.

Figure 4.

SO2 longevity: f(SO2) versus time in our photochemical model for three initial SO2 mixing ratios. The e-folding times for each simulation, marked by a circle, increase as initial SO2 mixing ratio increases.

5. Sensitivity Studies

[13] We have completed a number of sensitivity studies on our prior simulations with an initial SO2 mixing ratio of 10−6. First, we consider a higher-temperature regime, consistent with a 25 K greenhouse warming effect from a pulse of SO2 in the atmosphere. This vertical temperature profile, ranging from 283 K at the surface to 180 K in the upper atmosphere, is taken from radiative transfer steady state simulation results for a 500 mb CO2 ancient Martian atmosphere within three years of a 1.2 × 1013 kg SO2 pulse (giving rise to a SO2 mixing ratio of 6.14 × 10−6) [Johnson et al., 2007; Johnson et al., 2008].

[14] Second, we explore the effects of a different hydrologic scheme. One set of simulations with a higher rate of precipitation is designed to mimic the rainout regime under conditions an order of magnitude wetter by reducing the original Giorgi and Chameides parameterization by a factor of 50 (rather than 500, and consistent with a few centimeters of precipitation per year). Another set considers even more arid conditions, reducing the Giorgi and Chameides parameterization by a factor of 1000. Finally, a set of simulations studies the role of atmospheric mixing by scaling the eddy diffusion coefficient (K) profile up by a factor of five. We find that changes in temperature and atmospheric mixing have small effects on the overall lifetime of SO2, while changes in precipitation have appreciable effects. The results of these sensitivity studies are shown in Figure 5 and Table 2.

Figure 5.

Sensitivity factors for SO2 longevity: f(SO2) versus time for an initial SO2 mixing ratio of 10−6. The e-folding times for the simulations are marked by a circle.

Table 2. SO2 Longevitya
Initial f(SO2)e-Folding Timeb (in Earth Years)
  • a

    The e-folding times are given in Earth years for f(SO2) in the early Martian atmosphere, including sensitivity factors.

  • b

    A Martian year, the time for Mars to revolve around the Sun, is a factor of 1.88 longer than an Earth year.

Base Code Calculations
10−8333
10−7381
10−6793
Sensitivity Studies
10−6 (higher temperature)751
10−6 (higher precipitation)81
10−6 (lower precipitation)1550
10−6 (higher K)783

6. Discussion

[15] Our study shows that SO2 is likely to persist much longer than has previously been assumed under weakly reducing ancient Martian conditions. Sulfur can leave the atmosphere in three primary ways: as S8 aerosols, as SO4 aerosols, and by SO2 rainout (see Figure 1). The efficiency of these processes ultimately determines the lifetime of SO2 in the atmosphere. The deposition of reduced sulfur is impeded by inefficient photolysis due to enhanced Rayleigh scattering and a ∼30% rise in planetary albedo [Kasting, 1991] in these much thicker CO2 atmospheres. This effective loss of sunlight inhibits the conversion of SO2 to elemental S and on to S8 aerosols (refer to Figure 6). This removal pathway accounts for less than 1% of SO2 loss. As shown in Figure 7, the SO2 photolysis rate decreases linearly with decreasing altitude in the current Martian atmosphere. However, in the substantially thicker atmosphere of the ancient Martian regime, the SO2 photolysis rate drops off dramatically in the troposphere. Furthermore, SO2 photolysis alone cannot be considered as an irreversible loss of sulfur from the atmosphere, as recycling reactions may convert sulfur atoms back to SO2.

Figure 6.

Schematic of the photochemical pathways between SO2 and elemental sulfur. SO2 is converted to elemental S, an important intermediate for S8 aerosols, via photolysis. When photolysis is limited, the formation of elemental S is inhibited.

Figure 7.

SO2 photolysis rates decrease dramatically near the surface under ancient Martian conditions. The current Martian regime is shown in steady state with f(SO2) ∼ 10−12. The ancient Martian regime (initial f(SO2) = 10−6) is shown at the e-folding time of 793 years (f(SO2) = 3.67 × 10−7).

[16] Simultaneously, near-surface SO2 becomes a significant sink for oxidants and, although mixing ratios of H2O reach 10−2 near the bottom of the atmosphere in many simulations, a lack of oxidants precludes effective conversion of SO2 to SO4 aerosols (Figure 8). Figure 9 shows that oxidants, particularly odd hydrogen, are highly depleted below the tropopause at moderate SO2 mixing ratios. In our simulations, there are simply too few oxidants available per second to counter the increasing levels of SO2. Thus, the abundance of aerosols, which tend to cool the atmosphere by reflecting incoming sunlight back to space, remains limited in the model atmosphere. This removal pathway accounts for approximately 10% of SO2 loss.

Figure 8.

Schematic of the photochemical pathways between SO2 and sulfate aerosols. Conversions along these pathways involve radical species (primarily OH). When radicals are in short supply, the intermediates HSO3 and SO3 rarely form.

Figure 9.

Profiles of radicals in ancient Martian regime (initial f(SO2) = 10−6) at the e-folding time of 793 years (f(SO2) = 3.67 × 10−7). For reference, a profile of SO2 is also shown. Under these conditions, oxidants become scavenged near the surface; over the course of the simulation, oxidant concentrations in the lower atmosphere drop 1 to 2 orders of magnitude.

[17] The major remaining loss process for sulfur is SO2 rainout, and when rainout is small, SO2 remains in the atmosphere. Over time, however, SO2 rainout is the dominant mechanism for the destruction of sulfur in the ancient Martian atmosphere, as most (approximately 90%) of the initial influx of sulfur is lost as dissolved SO2. Furthermore, our study conservatively assumes that surface waters are undersaturated with SO2. If, however, shallow lakes on early Mars were saturated with SO2, rainout removal of SO2 would no longer serve as a terminal sink. Sulfur would be outgassed back into the atmosphere, and higher levels of SO2 would persist longer in the atmosphere.

[18] The reduced efficiency on ancient Mars of the processes that destroy atmospheric SO2 results in a longer lifetime for SO2 than assumed for the present day. Our findings suggest that SO2 lifetime is highly dependent on oxidant availability, which cannot be properly modeled if the fates of nonsulfur atmospheric species are neglected. Our investigation indicates that the e-folding time for initial influxes of SO2 is on the order of hundreds of years for moderate atmospheric loadings (10−8f(SO2) ≤ 10−6). This lifetime has important implications for understanding the ancient Martian atmosphere given recent studies assessing the potential for sulfur-induced greenhouse warming. Johnson et al. [2007, 2008] report significant greenhouse warming at higher SO2 mixing ratios, in the range of 10−6 to 10−4. We expect that e-folding times for SO2 will only increase as additional sulfur is loaded into the atmosphere, suggested by the trends in Table 2 and Figure 4. Our results therefore suggest that transient greenhouse warming, arising from the degassing of SO2 volatiles, likely persisted for at least hundreds, if not thousands, of years, allowing for the stability of liquid water in the form of near-surface groundwater, lakes and streams capable of mediating debris flow and generating water-lain sedimentary deposits.

[19] Not only would sulfur in the form of SO2 have had strong atmospheric effects, the deposition of SO2 (eventually oxidized to sulfate at the surface-atmosphere interface) would have generated potent acidity on the ancient Martian terrain. Indeed, extremely low pH levels are recorded in the mineral jarosite, identified in Martian outcrop that dates to the Late Noachian [Squyres et al., 2004]. Sulfuric acid can drastically depress the freezing point of water; for instance, a eutectic aqueous solution of 39% H2SO4 does not freeze until 200 K [Clark, 1999]. It is thus likely that sulfur played a dual role in the early history of the Mars: primarily as a potent greenhouse gas, and secondarily as an agent allowing sulfate-charged waters to remain in liquid form even after surface temperatures dropped below 273 K.

7. Conclusion

[20] The element sulfur may have played an important role in the evolution of early Mars. In this study, we investigate the photochemical longevity of SO2 in the ancient Martian atmosphere. Unlike Bullock and Moore [2007] which obtained their lifetime estimates for the ancient atmosphere from the simple photochemistry of Settle [1979], our model incorporates recycling reactions that can reform SO2, and includes multiple sulfur species and associated reactions, as well as reduced sulfur chemistry and S8 aerosols. We validate our model by replicating the present-day atmospheric results of Nair et al. [1994], which no other model of sulfur photochemistry on Mars has successfully done. We then load a model ancient atmosphere with moderate SO2 mixing ratios (10−8f(SO2) ≤ 10−6) to simulate volcanic degassing, and find e-folding times on the order of hundreds of years. Our results provide new insights into previous analyses [Postawko and Kuhn, 1986; Halevy et al., 2007; Bullock and Moore, 2007; Johnson et al., 2008] that suggest atmospheric SO2 helped generated periods of transient warm, wet conditions, conditions that may have allowed for the periodic development of surface fluvial features.

Acknowledgments

[21] Support for S. Johnson was provided by a National Science Foundation Graduate Fellowship and a NASA Headquarters Planetary Geology and Geophysics Program to M. Zuber. A portion of the research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.