Dominating influence of nh3 on the oxidation of aqueous SO2: The coupling of NH3 and SO2 in atmospheric water (original) (raw)
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The importance of liquid water concentration in the atmospheric oxidation of SO2
Atmospheric environment, 1987
The concentration of condensed water available for aqueous chemical reactions is viewed as a fundamental parameter of the heterogeneous conversion of gaseous SO2 to particulate sulfate. New results from a series of dispersed-phase experiments in a cloud chamber, in which the magnitude of this parameter was allowed to vary widely, demonstrate that the heterogeneous SO, conversion rate in hazes is generally limited by the smallconcentration ofcondensed water. This limitation precludes the heterogeneous oxidation pathway from being important in the atmosphere during haze episodes except under extreme conditions of high humidities and aerosol loadings. In clouds, on the other hand, the liquid water co~nt~tions are relativeiy large, permitting chemically related factors, such as pti-dependent equilibria and oxidant abundances, to limit the SO, conversion rate. Key word index: Aqueous-phase reactions. cloud chamber, SO1 oxidation, acidity, cloud chemistry, aerosols, liquid water content. NOMENCLATURE Nucleation fraction fraction of SO2 converted net expansion ratio generic soluble gas species first-order rate constant (s-r) rate constant used by Radojevic (1984) (M-112 s-1) rate constant used by Penkett et al. (1979) (M-'.73S-f) second-order rateconstants used by Hoffmann (1986) (M-' s-') overall Henry's Law constant (M ppm-')
29-Sulphur dioxide atmospheric oxidation in the presence of ammonia
Water, Air and Soil Pollution, 1981
Concentrations of SO2 and NH3 were measured daily for 3 yr. Monthly average concentrations obtained front daily data showed two seasonal cycles: in winler high SO2 and low NH3 concentrations; in suntmer low SO2 and high NH3 concentrations.
1] Models of aerosol scavenging and aqueous-phase oxidation of SO 2 by H 2 O 2 and O 3 in a cloud updraft are compared. Bulk models considering only a single droplet size are compared with size-resolved models that explicitly simulate multiple aerosol and drop sizes. All models simulate growth of cloud drops on a lognormal ammonium bisulfate aerosol distribution, and subsequent aqueous-phase chemistry during adiabatic ascent. In agreement with earlier published studies, it is found that relative to bulk models, the sizeresolved cloud chemical models consistently calculate 2-3 times more oxidation via the SO 2 + O 3 pathway, due to calculated variability of cloud water pH among cloud drops. All models calculate high scavenging of the input dry aerosol mass, but the calculated number of cloud drops formed varies from 275-358 drops cm À3 . Differences in the calculated number of cloud drops formed result from the treatment of gaseous species uptake, solution thermodynamics, applied water condensation mass accommodation coefficient, and bin size range definitions over which the input aerosol distribution is numerically approximated. The difference in calculated cloud drop number can under many conditions propagate to appreciable variations in cloud albedo. It is found that the modifications to the aerosol size and mass spectrum are sensitive to the number of cloud drops formed, and differences in the processed aerosol spectra were found to induce up to 13% differences in calculated light extinction properties of the modified particle distributions. These significant discrepancies among cloud aerosol chemistry interaction models, even when used to simulate relatively simple conditions, suggest that parameterizations of these processes used in larger-scale cloud, regional and longer-term climate models can contain high levels of uncertainty. Modification of aerosol mass and size distribution due to aqueous-phase SO 2 oxidation in clouds: Comparisons of several models,
SO2 oxidation in cloud drops containing NaCl or sea salt as condensation nuclei
Atmospheric environment, 1987
Experimental results from cloud-chamber studies provide direct evidence that N&I, artificial sea salt and natural sea salt promote faster rates ofaqueous SOs oxidation than observed in the absence of these salts. However, the chemical basis for this effect has not been clarified. Oxidation rates > 30% h-' are observed in cloud-chamber experiments with 0.4 pmdiamcter salt particles as cloud nuclei and a cloud pH of 6. SO2 oxidation under similar atmospheric conditions might account for the rapid formation of sulfate observed in marine fogs.
Chemosphere, 2004
We have demonstrated the use of Se as a tracer to quantitatively determine in situ SO 2À 4 production from SO 2 oxidation in clouds and fogs. Until now, it has not been possible to study the kinetics of SO 2 oxidation because the aerosol sampling interval for Se determination was limited to 2 h or longer. Here we report results of 5-min aerosol measurements carried out at Lahore, Pakistan, during January 9-11, 2001, using new methodology for Se analysis coupled with hydride generation and ICP-MS detection. These improvements will enable the tracer technique to determine in situ SO 2À 4 production in clouds and fogs on a time scale of several minutes and possibly 1 min. The method may prove useful for kinetic studies of in-cloud SO 2 oxidation and in the study of other phenomena such as atmospheric mixing, cloud drop lifetimes, and aerosol formation that occur on the time scale of a few minutes.
We examine factors controlling the photochemical oxidation of SO2 in tropospheric aerosols using a gas-aqueous photochemical model. Over a range of liquid water contents (3x10 -4g H20 m -3 to 9 g H20 m -3) and pH values (0 to 8), we find that H202(aq) and O3(aq) provide the major sinks for SO2 in the aqueous phase when pH is held constant at below 5 and larger than 6, respectively. OH(aq) may be an important oxidant of SO2 in the aqueous phase when pH is held constant between 5 and 6 and H2 02 is depleted in an air parcel. When pH is allowed to vary during the integration, H202(aq) is the most important oxidant in the aqueous phase. O3(aq) is important primarily when the liquid water content is large (> 1 g m -3) and the solution pH is above 4.0 3 (aq) is also important when the pH is initially high (> 6) for quickly oxidizing SO2 and, thereby, reducing the pH into the pH region where H202(aq) is the most important oxidant. OH(aq) may be important when H20 2 is depleted and the liquid water content is large. When aerosols are present during noncloudy. days in summer, the aqueous-phase oxidation of SO2 is insignificant compared with the gas-phase oxidation of SO2. We find, however, that the SO2 oxidation in wet aerosols may be enhanced in winter or when the temperature is low (273 K) and the relative humidity is high. Uncertainties in the reaction rate coefficients may significantly affect the concentrations of oxidants and other compounds of photochemical origin. Using a relatively stringent criterion, a compressed gas-aqueous phase chemical mechanism for photochemical oxidation of SO2 is proposed for global tropospheric modeling. dioxide in aerosols is much slower than that in clouds [Jacobson, 1997a,b]. However, because aerosols are ubiquitous in the free troposphere, they may contribute to significant aqueous-phase formation of sulfate on a global scale.
Field studies of the SO2/aqueous S(IV) equilibrium in clouds
Atmospheric Environment. Part A. General Topics, 1990
Concentrations of S(IV) were measured in cloudwater at Great Dun Fell and compared with theoretical HSO; assuming equilibrium between aqueous and gaseous phases in cloud. Detectable concentrations of S(IV) in the range of 1 x 10-6 to 17.2 x 10-6 moldm-3 were observed only in samples which contained low H202 concentrations, generally < 1 x 10-6 mol dm-3. Concentrations of S(IV) were below the detection limit of 1 x 10-6 moldm-3 in samples which contained high H202 levels (1 x 10-6-80 x 10-6 moldm-3) confirming that either SO2 or H202 acts as the limiting reagent in the oxidation of SO2 in cloudwater. Equilibrium HSO 3 concentrations were estimated from the measured cloudwater pH, the gas phase SO 2 concentration and the ambient temperature and found to be on average about 5 times lower than the measured S(IV) concentrations. The possible role of formaldehyde in stabilizing S(IV) in cloudwater is discussed. The kinetic data available in the literature suggest that the complexation reaction between S(IV) and HCHO is too slow to account for the observed difference between measured and calculated S(IV) concentrations over the typical lifetime of clouds in our study. S(IV) accounted for up to 10% of the SO 2-measured in stored cloudwater samples.
The contribution of in-cloud oxidation of SO2 to wet scavenging of sulfur in convective clouds
Atmospheric Environment (1967), 1985
The sensitivity of in-cloud oxidation of SO1 in convective clouds to a number of chemical and physical parameters is examined. The parameterization of precipitation growth processes is based on the work of Scott (1978) and Hegg (1983). A chemical model predicts gas and aqueous phase distributions of soluble gases and in-cloud uncatalyzed oxidation of SO1 by O3 and H202. Sulfate aerosol and SO1, C02, NH,, HsOz and 0s gases and their aqueous phase dissociation products are treated. The results indicate that incloud conversion is an important removal mechanism for SO2 and accounts for a significant fraction of the precipitation sulfate. However, except at low SO2 concentrations, the precipitation sulfate concentration is insensitive to the initial SO2 concentration; the sulfate concentration is most sensitive to the initial Hz02 and NH, concentrations. At low SO2 concentrations, the precipitation sulfate concentration is determined primarily by the initial sulfate aerosol concentration. The feedback between sulfate production and pH is important in limiting SO2 oxidation by 0s. If gas phase HsOs of order 1 ppb is the major source ofaqueous phase Hz02 for S(W) oxidation, it is likely that the oxidation reaction is oxidant limited. The sulfate concentration is a decreasing function of the precipitation rate. At low rainfall rates (< 1 mm h-'), ice phase growth decreases the sulfate concentration. However, the results are insensitive to an ice phase origin at moderate and high rainfall rates.
Atmospheric Environment (1967), 1982
A~rr~~t-A~~l~ §i~ of&e simultaneous absorption and chemical reaction of SO, in water droplets falling in a non-uniform temperature field is extended to include the ef%cts of raindrop size distribution, aerosol scavenging and S(W) oxidation. Presented results indicate that non-isothermal effects on SO, absorption by water droplets can be important in gas-absorption calculations and interpretation of ground 1eveX samples. For example, for low initial pH rains the effect of rainfall rate on ground level bulk samples is dramatically different than that predicted by isotherma models. Furthermore, in the belaw-cloud region both SO2 absorption alzd sulfate aerosol scavenging are shown to impact significantly on the solution chemistry, wbercas, the effects ofSfW> oxidation are not important. In addition, simufation rest&s areanalyzed in terms of washout ratios and scavenging coefT&ents; which, in turn, are parameterized by drop size, pM, rainfail intensity and temperature.