Sulfur Chemistry in the Middle Atmosphere of Venus
Zhang et al. (2010)
Abstract
Section 1. Introduction
1. Big picture review…
2. Measurement summary (gases), especially SO and SO2 from Belyaev et al. (2010)
3. Summary of Zhang et al. (2010). ). It proposed a mechanism above 80 km but
didn’t reproduce the SO2 measurement below 80 km. Related to the night
temperature and aerosol measurement, summary
4. Aerosol profile
Above the middle cloud top (~57 km), the aerosols are found to have a bi-modal
distribution. We combine the upper haze profiles from Wilquet et al. (2010) above
72 km and upper cloud particle profiles from Knollenberg and Hunten (1980) from
58-65 km. Interpolation is used for the intermediate altitudes (65-72km) due to the
lack of data at present. Fig. 1 shows the bi-modal aerosol profiles (left panel).
From the aerosol abundances, we can roughly estimate the sulfur content. The mode
1 aerosols are roughly 0.2 micron in radius for all altitudes constantly. For the mode
2 aerosols, we use 0.7 micron above 72 km (Wilquet et al. 2010) for the upper haze
and 1.1 micron below for the cloud layer particle (Knollenberg and Hunten, 1980).
The right panel of Fig. 1 shows the sulfur content mixng ratio converted from the
H2SO4 aerosol (solid line) abundances by assuming the H2SO4 aerosol density is
2g/cc and weight percent to be 85% and 75% below and above 72 km, respectively.
The sulfur content in the H2SO4 droplets is close to 1 ppm at all altitudes, which is
enough for the enhancement of sulfur oxides above 80 km. In addition, if the
polysulfur (Sx) is the unknown UV absorber, previous study (Carlson, et al. 2010)
estimated that the elemental sulfur is about 1% of H2SO4 content. We assume the
radius of elemental sulfur to be the half of the H2SO4 droplet radius and the density
is also roughly 2g/cc. We found that the sulfur content in elemental sulfurs is in the
ppb level at all altitude (Fig. 1, right panel, dashed line), which is also enough to
produce the sulfur species if the Sx aerosol is a steady source (see discussion later).
Since both the H2SO4 and Sx could provide the source of sulfur oxides above 90 km,
we will discuss the two candidates respectively in this study.
According to the turning point of SO2 profile around 80 km, we separate the Venus
atmosphere above the middle cloud top (~58 km) into two regions: the lower
region (below 80 km) and the upper region (above 80 km). In this paper, we will
first introduce the photochemical model in section 2, and discuss the chemistry in
the lower region and compare the model results with the observations in section 3.
The upper region chemistry is in section 4, where we provide the detailed
discussion on the two possible sulfur sources above 80 km, H2SO4 and Sx,
respectively. In what roles they play in the sulfur chemistry, their implications and
how to distinguish the two sources by the future observations. Then we come to the
summary of the chemical mechanisms and conclusion remarks.
Section 2: Model Description
Our photochemical model is based on the 1-D Caltech/JPL kinetics code for Venus
(Allen, Yung, Mills) with updated reaction rate coefficients. The model solves the
coupled continuity equations with chemical kinetics and diffusion processes, as
function of time and altitude from 58 to 112 km. We use 32 altitude grids with the
increment of 0.4 km from 58 to 60 km and 2 km from 60 to 112 km. The diurnally
averaged radiation field from 100-800 nm is calculated from a modified radiative
trasfer scheme including the gas absorption, Rayleigh scattering and aerosol
scattering with wavelength-dependent optical properties based on Mie calculations
(see Appendix I). The unknown UV absorber is approximated as suggested by Crisp
et al. (1985). The calculations are set at mid-latitude (45N) and we use the low solar
activity flux from ???.
In this study we select 43 species in the reference model (model A), including O,
O(1D), O2, O2(1D), O3, H, H2,OH, HO2, H2O, H2O2, N2, Cl, Cl2, ClO, HCl, HOCl, ClCO,
COCl2, ClC(O)OO,CO, CO2, S, S2, S3, S4, S5, S7, S8, SO, (SO)2, SO2, SO3, S2O, HSO3,
H2SO4, OCS, OSCl, ClSO2, ClS, ClS2, Cl2S, and Cl2S2. The last four chlorosulfanes
(SmCln) are included because they open an important pathway to form S2 and
polysulfur Sx(x=2→8) in the upper cloud region (Mills 2007), although their chemistry
is not yet known very well. The chlorosulfanes chemistry is not important for the
sulfur cycles above ~80 km because the SmCln abundances are less. Nitrogen
species especially NO2 have been found to be important in converting SO to SO2 and
O to O2 (Krasnopolsky 2006). In order to simulate the full Nitrogen chemistry, 8
Nitrogen species N, NO, NO2, NO3, N2O, HNO, HNO2, and HNO3 need to be included.
The Nitrogen chemistry is not important above 80 km either. In the region from 5880 km, we will compare three models: model A (the reference model), model B
(same as model A but without Chlorosulfanes chemistry) and model C (same as
model A but with Nitrogen chemistry). And in section 4 for the sulfur chemistry
above 80 km, we will use model A although the results would not change for other 2
models.
In Zhang et al. (2010), the chemistry is simplified because (SO)2, S2O and HSO3 are
only considered as the sinks of the sulfur species. Instead, a full chemical reaction
set with 35 photodissociation reactions and 260 neutral reactions are used in model
A and listed in Table A1 and A2, respectively. We take the ClCO thermal equilibrium
constant from the 1-sigma model in Mills (2007) so that we can constrain the total
O2 column abundances below 2e18 cm-2. In addition, for the first time we introduce
the heterogeneous nucleation processes of elemental sulfurs (S, S2 and polysulfur)
because these sulfur species are readily to stick onto the sulfuric acid droplet and
may provide the required albedo of the unknown UV absorber (Carlson et al. 2010).
But we neglect all the heterogeneous reactions within the condensed elemental
sulfurs on the droplet surface. The calculation of the heterogeneous condensation
rates is described in Appendix II. The accommodation coefficient  is varying from
0.01 to 1 for the sensitivity study. The dayside temperature profile and original eddy
diffusion coefficient profile from Yung and DeMore (1982) are shown in Fig. 2 (solid
line). But the SO2 mixing ratio at the 58 km and the eddy diffusivity profile can be
adjusted to match the SO2 observations around 70 km (Belyev et al., 2008). The
upper and lower boundary conditions for the important species are listed in Table 1.
We set the HCl as 4 ppm at 58 km, which is about factor of 2 larger than the Venus
Express observations (Bertaux et al. (2007)) but other observations also reported 4
ppm HCl (Dalton et al. (2000), Krasnopolsky (2010)). Since ClC(O)OO is key species
to convert CO and O2 to CO2, 0.4 ppm HCl is needed in our model to constrain the
total column abundances of O2.
Section 3: Chemistry between 58-80 km
Reference Model
First we build a reference model (model A) where we fixed the vertical profiles of
N2, H2O, and H2SO4, which is appropriate if the phase change processes
(evaporation, condensation and nucleation) are much faster than the gas-phase
chemical reactions. The N2 profile has a constant mixing ratio of 3.4%. The H2O
profile (see Fig. 4) is prescribed based on the Venus Express observations (Bertaux,
et al., 2007). The H2SO4 vapor pressure is calculated based on H2SO4 weight
percent as 85% below 70 km, 75% from 70 to 90 km and 100% (i.e., pure sulfuric
acid) above 90 km (see section 4 for details). Model A uses the daytime H2SO4
abundances (see Fig. 14). The accommodation coefficient of the sulfur nucleation is
set as 1 (the upper limit). Around the turning point of the observed SO2 mixing ratio
profile, we reduce the eddy diffusivity by a factor of 4 in 74-80 km to reproduce the
data. In fact the eddy diffusivity also needs to be decreased by a factor of 4 in 80-86
km in order to match the SO2 profile above 80 km when we add the sulfur sources
in the upper region later. The new eddy diffusion coefficient profile is plotted in
dashed line in Fig. 1. Model A requires 5 ppm SO2 the lower boundary to match the
SO2 observations from Belyaev et al. (2010).
Figures 3-8 show the volume mixing ratios of oxygen species, hydrogen species
(including HOx), chlorine species, sulfur oxides, chlorine-sulfur species, and
elemental sulfurs, respectively. The photolysis of the parent species CO2, OCS, SO2,
H2O and HCl, which are transport by diffusion from 58 km, provides the sources of
the other species. Although the sulfur cycle is closely coupled with the oxygen and
chlorine cycles in the upper cloud layer, the sulfur species have little effect on the
abundances of the oxygen (including HOx) and chlorine species above the cloud top,
but not vice versa. In other words, the model without sulfur cycle would produce
roughly the same amount of oxygen and chlorine species as the model with sulfur
species does in the upper region where the sulfur species are less abundant. The
free oxygen and chlorine radicals, such as O, OH, Cl, ClO, etc., are the key catalysts in
recycling sulfur species in their inner cycle, but the sulfur species do not act as the
catalysts in the Venus chemistry. Therefore, to some extent, the sulfur chemistry in
the mesosphere can be separated from the other cycles above the cloud top. Fig. 9
illustrates the important pathways of the sulfur cycle. The chlorosulfane chemistry
not shown explicitly here should locate in the upper left part with elemental sulfurs
(see Mills (2007) for detailed discussion). The polysulfur chemistry within the
allotropes is not illustrated in Fig. 9. (see Yung et al. (2008) for details). A fast inner
cycle exists among the sulfur species, including the phtodissociation and oxidization
processes. If ignoring the aerosol evaporation above 80 km at this moment, H2SO4
and Sx act as the ultimate sinks rather than the sources of the sulfur species. The
total production rate of H2SO4 from 58-112 km is 1.2e12 cm-3s-1 with peaks 2.8e6
cm-3s-1 at around 62-64 km, while the total loss rate of gaseous elemental sulfurs to
aerosol through heterogeneous nucleation processes is 6.4e12 cm-3s-1, equivalent
to sulfur atom loss rate ~1.6e13 cm-3s-1. Therefore, the major sink of sulfur species
is to form the polysulfur aerosol in model A. The polysulfur sinks decreases with
altitude mainly because both the aerosol and elemental sulfurs are more abundant
at lower altitude.
Below ~65 km, SO2 is roughly in equilibrium with ClSO2. SO2 reacts with chlorine
radical: Cl + SO2 + CO2 → ClSO2 + CO2, and ClSO2 reacts with O, S, S2, SO, ClSO2, etc.
(Reactions 292-299 in table A1) to return back SO2 and produce chlorine species
including chlorosulfanes. Above ~65 km, the photolysis of SO2 becomes the
dominant sink with some oxidization to SO3. The three body reaction O + SO
produces more SO2, and ClO and ClC(O)OO reacting with SO also play important
roles in SO2 production. Fig. 10 shows the main production and loss pathways of
SO2 in model A.
The OCS mixing ratio in the cloud layer is puzzling. OCS is believed to be transported
from the lower atmosphere. At 33 km the maximum mixing ratio is ~6 ppm at
equator observed by the Venus Express and ground-based telescope IRTF
observation (see Yung et al. (2009)). But 6 ppm OCS at the lower boundary (58 km)
is only able to produce ~1 ppb and 0.02 ppb OCS at 65 km and 70 km, respectively.
Although 1 ppb OCS lies in the observation range 0.3-9 ppb reported by
Krasnopolsky (2010), it is difficult for model A to achieve the 10 ppb level OCS at 65
km. Krasnopolsky (2008) reported even larger values, ~14 ppb around 65 km and
~2 ppb around 70 km and the Venus Express results suggest that the upper limit of
OCS should be 1.6 ± 2 ppb in 70-90 km. Besides, the scale height of OCS in the model
A is only ~1 km at 65 km, which is only the lower limit of the observations (1-4 km
from Krasnopolsky (2010)). It seems the eddy transport in model A between 58-70
km may not be efficient to transport the OCS upward. The eddy mixing in the cloud
layer could have large variations, resulting in the large variation of the detected OCS
values. A large implication of the unexpected large amount of OCS is to change the
polysulfur production pathway. In model A, the primary source of atomic sulfur
below ~62 km is from the photolysis of OCS instead of SO and ClS. And the reaction
rate of S+OCS is as large as ClS2 photolysis below 60 km. So the large amount of OCS
may greatly enhance the production of Sx around the lower boundary and even shift
the production peak down to 58-60 km region.
Supersaturation of elemental sulfurs
Even under the fastest heterogeneous nucleation processes (unity accommodation
coefficient), the model A results show the S2, S3, and S4 are highly supersatured in
the Venus atmosphere (see Fig. 8) based on the monoclinic sulfur vapor pressures
over the solid phase from Lyons (2008). The column abundances of gaseous S2, S3,
and S4 above 58 km are 8.1e13 cm-3s-1, 4.6e11 cm-3s-1, and 3.7e12 cm-3s-1,
respectively. In fact S5 is also modestly supersaturated with saturation ratio ~10 at
58 km but decreases quickly below the saturated abundance above 60 km. The
saturation ratio of S4 is about 1e7 at the lower boundary and becomes unsaturated
above 76 km. S3 is oversaturated by a factor of 1e3-1e7 from 58 km-100km. S2 is
extremely supersaturated at all altitudes. The saturation ratio is 1e7 at the bottom
and 1e5 at the top, with the peak in 1e15 at 90 km, where the aerosol
heterogeneous nucleation is negligible compared with the production processes
from atomic sulfur through the three body reaction 2S+M, and the major loss
processes are the oxidization to SO and photolysis to atomic sulfur. As illustrated in
Fig. 9, the main production processes of Sx can be summarized as S+Sx-1->Sx and
S2+Sx-2->Sx, but the ClS+S2 and S2O+S2O are also important for S3 production in
the bottom and top atmosphere, respectively. The loss mechanisms of Sx include the
heterogeneous nucleation, conversion to other allotropes, and oxidization through
Sx+O->Sx-1+SO, which we think may provide the sources of the upper atmosphere
sulfur oxides and will be discussed in the later sections. Fig. A3 shows the diurnalaveraged photolysis timescales of S2, S3 and S4 in comparison with the nucleation
timescale and eddy transport timescale. The S2 loss process is dominated by the
condensation from 58 km to about 72 km till the photolysis is as fast as the
condensation loss, but the conversion from S2 to S4 is also important around 60 km.
The photolysis timescales of S3 and S4 are in the order of 1s, much smaller than the
nucleation timescale (~10 s at 60 km and ~100 s at 70 km). Therefore, for S3 and
S4, photolysis by the visible light is the major loss pathway and the heterogeneous
nucleation processes are negligible. Since the S3 and S4 aerosols are one of the
leading candidates of the unknown UV absorbers although they are unstable
(Carlson, et al., 2010), the condensed S3 and S4 are probably produced from the
heterogeneous Sx chemistry over the H2SO4 droplet surfaces (Lyons, 2008). Due to
the high uncertainty of the Sx chemistry, the reality may be more complicated than
what we discussed here. A proper treatment of the microphysical processes coupled
with atmosphere dynamical processes within the cloud layer is needed to help
elucidate the Sx chemistry.
Sensitivity to the accommodation coefficient
Due to the uncertainty of accommodation coefficient, we slow the heterogeneous
nucleation processes by reducing . As  decreases, the formation of Sx aerosols is
slower so there are more sulfur species in the gas phase. Therefore, the model
requires less SO2 at the lower boundary and smaller eddy diffusivity between 74-80
km to reproduce the SO2 observations. In table 1 we summarized the 3 cases with
1 (model A), 0.1 and 0.01, respectively. All the three cases show that most of
sulfur species are eventually going into the polysuflur aerosols. As the polysulfur
sinks are reduced, the H2SO4 production rate is also decreased due to lower SO2
abundances around 62-64 km where the production peak is. The total column
abundances of O2 are nearly no difference when  changes. The production rates of
condensed Sx aerosol are almost the same because: (1), The other sinks of elemental
sulfur include the photolysis of S3 and S4, and oxidization which is controlled by the
abundances O2 below 65 km. But there are only small differences of the O2
concentrations among these models (all are 1-sigma ClCO stability model (Mills et
al., 2007)); (2), The net production of S2 is from OCS in the 58-60 km region and the
chlorosulfane reactions (Mills et al., 2007) above 60 km. However, the
chlorosulfanes don’t vary too much when changing the heterogeneous nucleation
rate. The OCS profile does not change much either because it is mainly determined
by the eddy transport and photodissociation processes. Therefore, more future
modeling work and observations of the OCS in the upper cloud layer are needed to
help identify the Sx aerosol nucleation process.
Comparison between different chemistry schemes between 58-80 km
Since the chlorosulfane chemistry still has some uncertainties (Mills et al., 2007), we
also test the model (model B) without chlorosulfanes (ClS, ClS2, Cl2S, and Cl2S2)
and their chemistry for comparison. And another model (model C) with Nitrogen
species and Nitrogen chemistry (from Yung and DeMore (1982) and Mills (1998)) is
also discussed here. Fig. 12 is the mixing ratio profiles of Nitrogen species from
model C, which is in good agreement of observations (Krasnopolsky (2005)) .
However, we are not trying to discuss the Nitrogen chemistry and chlorosulfane
chemistry in details here (see Yung and DeMore (1982) and Mills et al. (2007)).
Table 2 summarizes the results of model A, B and C. model without chlorosulfane
chemistry (model B) would have slower loss of sulfur oxides in the cloud layer than
model A, so it requires less sulfur flux from the lower boundary (58 km) to match
the SO2. The side effect is that, the sulfuric acid production would decrease from
1.2e12 cm-3s-1 in the model A to 5.9e11 cm-2s-1 in the model B. And the model
with Nitrogen species (model C) would convert more SO to SO2 around 70-80 km
(~15% of the SO2 production rate at those altitudes), the SO2 lower boundary is
smaller than model A and hence the H2SO4 production rate is less (~9e11 cm-2s-1).
For reference, the Krasnopolsky and Pollack [1994] requires the H2SO4 production
rate of 2.2e12 cm-3s-1. And some previous models range from 9e11 to 1e13 cm-2s1 (Yung and DeMore 1982 and Krasnopolsky and Pollack 1994). The SO2
production and loss mechanisms are roughly the same for the 3 models, except that
below 65 km, ClSO2 in model B does not reacts with elemental sulfur to produce
SO2 and chlorosulfanes. And the SO2 production rate in model C is larger than that
of model A and B due to the NO2 oxidization in 70-80 km region. The total SO2
production rates for the three models are shown in table 2.
Section 4: Chemistry above 80 km
Large enhancement of SO2 and SO above 80 km is not expected from the models in
section 2, especially SO2, because it is thought to be one of the parent species that is
transported from the lower atmosphere. This phenomenon should not be mixed
with another similar phenomenon just revealed by Venus Express as well. That is,
there exist local minima between 85-90 km for all of the observed profiles of CO,
H2O, HDO, HCl and HF. These local minima are hard to be explained by the
chemistry but probably caused by the atmospheric dynamics. The SO2 minimum is
different because: (1). The SO2 minimum occurs around 80 km, about 5 km lower
than that of other species. (2). From the observations, SO2 mixing ratio decreases
by 2 order of magnitude from 58 km to 80 km, while the other minima are only
factor of 2-4 less than the maximum values in the lower region. (3), The CO, H2O,
HDO, HCl and HF profiles were assumed to be roughly constant below 90 km from
all the previous models because they don’t have very large loss mechanism. But SO2
profile was predicted to be decreasing with altitude due to the photodissociation
and oxidization processes. Therefore, the SO2 and SO enhancement should be
treated differently.
Transport doesn’t account
Volcano cannot reach high, Volcano (~69 km)
Sudden Injection (SO2 lifetime), SO2 lifetime plot (Fig. 12)
4.1 H2SO4 aerosol as the sources (model D)
need a table summarizing the assumptions in all models/ add to table 3
H2SO4 vapor pressure, weight percent and vertical profile (Fig. 13, 14)
H2SO4 photolysis cross section (with large UV, or w/o large UV but required
scaling) (Fig. 15)
Results: Sulfur species compared with model A (Fig. 16)
Chemistry discussion, SO, SO2 and SO3 production and loss (Fig. 17, 18, 19)
4.2 Sx aerosol as the sources (model E)
Sx aerosol has to be transported from below since the production is not enough.
Thus it could be considered as the constant source. That’s why the ppb level Sx
could produce the 10-100 ppb level SO and SO2.
Sx night time profile (Fig. 20)
model E description, Sx above 82 km (where the SO2 inversion occurs). Sx below 82
km is fixed as the reference model (model A) output.
Model E results: Sulfur species compared with model A (Fig. 21)
Sx+O reactions discussions, SO, SO2 and SO3 production and loss rates (Fig. 22, 23,
24)
4.3 summary of the upper region chemistry
H2SO4 -> SO3 v.s. Sx -> SO
Ratio: [S]:[SO]:[SO2]:[SO3]:[OCS]
Sensitivity study
SO2 mixing v.s. H2SO4*photo/ SO2 mixing v.s. Sx*rxnrates (Fig. 25)
SO3 is the key species to distinguish the two mechanisms
Section 5: Summary and conclusion remarks
5.1 summary of the chemistry
Difference between the lower and upper region chemistry (upper chemistry is much
simpler but aerosol chemistry is the big uncertainty)
5.2 Discussion on rxns
SO + O + M = SO2 + M coef, we need 3e-30 at 100km(168 K) from observations
Temperature dependence infer from the observation
Puzzle: why the SO2/SO ratio increases with Temperature at 100 km ? (since the
recombination rate should decrease with T, so high T we should have more SO
rather than SO2)
Maybe other rxns….
Final conclusion!