Modified HNO3 seasonality in volcanic layers of a polar ice core: Snow-pack effect or photochemical perturbation?

Using the chemical composition of snow and ice of a central Greenland ice core, we have investigated changes in atmospheric HNO₃ chemistry following the large volcanic eruptions of Laki (1783), Tambora (1815) and Katmai (1912). The concentration of several cations and anions, including SO ₄ ^2– and NO ₃ ^– , were measured using ion chromatography. We found that following those eruptions, the ratio of the concentration of NO ₃ ^– deposited during winter to that deposited during summer was significantly higher than during nonvolcanic periods. Although we cannot rule out that this pattern originates from snow pack effects, we propose that increased concentrations of volcanic H₂SO₄ particles in the stratosphere may have favored condensation and removal of HNO₃ from the stratosphere during Arctic winter. In addition, this pattern might have been enhanced by slower formation of HNO₃ during summer, caused by direct consumption of OH through oxidation of volcanic SO₂.     

The solubility of SO2 and the dissociation of H2SO3 in NaCl solutions

The pK ₁ ^* and pK ₂ ^* of H₂SO₃ have been determined in NaCl solutions as a function of ionic strength (0.1 to 6 m) and temperature (5 and 25 °C). The extrapolated values in water were found to be in good agreement with literature data. The experimental results have been used to determine the Pitzer interaction parameters for SO₂, HSO ₃ ^- and SO ₃ ^- in NaCl solutions. The resultant parameters for NaHSO₃ and Na₂SO₃ were found to be in reasonable agreement with the values for NaHSO₄ and Na₂SO₄. It, thus, seems reasonable to assume that the interactions of Mg²⁺ and Ca²⁺ with HSO ₃ ^- and SO ₃ ^- can be estimated from the values with HSO ₄ ^- and SO ₄ ^- until experimental values are available. Measurements of pK ₁ ^* and pK ₂ ^* in artificial seawater were found to be in good agreement with the calculated values using the derived Pitzer parameters. It is, thus, possible to make reasonable estimates of the activity coefficients of HSO ₃ ^- and SO ₃ ^- ions and pK ₁ ^* and pK ₂ ^* for the ionization of H₂SO₃ in marine aerosols.     

Characteristics of ionic components in precipitation in Kitakyushu City,Japan

Precipitation samples were collected by filtrating bulk sampler in Kitakyushu City, Japan, from January 1988 to December 1990. Volume weighted annual mean of pH was 4.93, but the pH distribution indicated that most probable value lay in the range pH 6.0–6.4. Volume weighted annual mean concentrations of major ionic components were as follows; SO ₄ ^2– : 84.2, NO ₃ ^– : 28.1, Cl^–: 86.3, NH ₄ ⁺ : 45.5, Ca²⁺: 63.3, Mg²⁺: 27.0, K⁺: 3.4, Na⁺: 69.0 µ eq l^–1. The highest concentrations of these ionic components were observed in winter and the lowest occurred in the rainy season. The ratio of ex-SO ₄ ^2– /NO ₃ ^– exhibited the lowest ratio in summer, and the highest ratio in winter. Good correlations were obtained between Cl^– and Na⁺, ex-SO ₄ ²⁺ and ex-Ca²⁺, NO ₃ ^– and ex-Ca²⁺, and NH ₄ ⁺ and ex-SO ₄ ^2– , respectively. However, no correlation between Cl^– and Na⁺ with Ca²⁺ was observed. The relationship of H⁺ with (ex-SO ₄ ^2– + NO ₃ ^– ) - (ex-Ca²⁺ + NH ₄ ⁺ ) indicated positive correlation.     

Aqueous Phase Reaction of HNO4: The Impact on Tropospheric Chemistry

We use a global atmospheric chemistry transport model to study the possible influence of aqueous phase reactions of peroxynitric acid (HNO₄) on the concentrations and budgets of NO_x, SO_x, O₃ and H₂O₂. Laboratory studies have shown that the aqueous reaction of HNO_4aq withHSO^– _3aq, and the uni-molecular decomposition of the NO₄ ^– anion to form NO₂ ^– (nitrite) occur on a time scale of about a second. Despite a substantial contribution of the reaction of HSO^– _3aq with HNO_4aq to the overall in-cloud conversion of SO₂ to SO₄ ^2–, a simultaneous decrease of other oxidants (most notably H₂O₂) more than compensated the increase in SO₄ ^2– production. The strongest influence of heterogeneous HNO₄ chemistry was found in the boundary layer, where calculated monthly average ozone concentrations were reduced between 2% to 10% andchanges of H₂O₂ between –20% to +10%compared to a simulation which ignores this reaction. Furthermore, SO₂ was increased by 10% to 20% and SO₄ ^2–depleted by up to 10%. Since the resolution of our global model does not enable a detailed comparison with measurements in polluted regions, it is not possible to verify whether considering heterogeneous HNO₄ reactions results in a substantial improvement of atmospheric chemistry transport models. However, the conversion of HNO₄ in the aqueous phase seems to be efficient enough to warrant further laboratory investigations and more detailed model studies on this topic.     

In-cloud scavenging of gases and aerosols at a mountain site in Central Switzerland

An in-cloud scavenging case study of the major ions (NH₄ ⁺, SO₄ ^2- and NO₃ ^-) determining the cloudwater composition at a mountain site (1620 m.a.s.l.) is presented. A comparison between in-cloud measurements of the cloudwater composition, liquid water content, gas concentrations and aerosol concentrations and pre-cloud gas and aerosol concentrations yields the following results. Cloudwater concentrations resulted from scavenging of about half of the available NH₃, aerosol NH₄ ⁺, aerosol NO₃ ^-, and aerosol SO₄ ^2-. Approximately a third of the SO₂ was scavenged by the cloudwater and oxidized to SO₄ ^2-. Cloud acidity during the first two hours of cloud interception (pH 3.24) was determined mostly by the scavenged gases (NH₃, SO₂, and HNO₃); aerosol contributions to the acidity were found to be small. Observations of gas and aerosol concentrations at three elevations prior to several winter precipitation events indicated that NH₃ concentrations are typically half (12–80 %) of the total (gas and aerosol) N (-III) concentrations. HNO₃ typically is present at much lower concentrations (1–55 %) than aerosol NO₃ ^-. Concentrations of SO₂ are a substantial component of total sulfur, with concentrations averaging 60 % (14–76 %) of the total S (IV and VI).     

Growth of monodisperse,submicron aerosol particles exposed to SO2, H2O2, and NH3

The growth of monodisperse particles (0.07 to 0.5 µm) exposed to SO₂ (0–860 ppb), H₂O₂ (0–150 ppb) and sometimes NH₃ (0–550 ppb) in purified air at 22 °C at relative humidities ranging from 25 to 75% were measured using the Tandem Differential Mobility Analyzer technique. The experiments were performed in a flow reactor with aqueous (NH₄)₂SO₄ and Na₂SO₄ droplets. For (NH₄)₂SO₄ droplets the fractional diameter growth was independent of size above 0.3 µm but decreased with decreasing size below that. When NH₃ was added the fractional growth increased with decreasing size. Measurements were compared with predictions of a model that accounts for solubility of the reactive gases, the liquid phase oxidation of SO₂ by H₂O₂, and ionic equilibria. Agreement between measured and predicted droplet growth is reasonable when the ionic strength effects are included. Theory and experiments suggest that NH₃ evaporation is responsible for the decrease in relative growth rates for small aqueous ammonium sulfate particles. The observed droplet growth rates are too slow to explain observed growth rates of secondary atmospheric sulfate particles.     

Simulations of seasonal variations of sulfur compounds in the remote marine atmosphere

A photochemical box model is used to simulate seasonal variations in concentrations of sulfur compounds at latitude 40° S. It is assumed that the hydroxyl radical (OH) addition reaction to sulfur in the dimethyl sulfide (DMS) molecule is the predominant pathway for methanesulfonic acid (MSA) production, and that the rate constant increases as the air temperature decreases. Concentration of the nitrate radical (NO₃) is a function of the DMS flux, because the reaction of DMS with NO₃ is the most important loss mechanism of NO₃. While the diurnally averaged concentration of OH in winter is a factor of about 8 smaller than in summer, due to the weak photolysis process, the diurnally averaged concentration of NO₃ in winter is a factor of about 4–5 larger than in summer, due to the decrease of DMS flux. Therefore, at middle and high latitudes in winter, atmospheric DMS is mainly oxidized by the reaction with NO₃. The calculated ratio of the MSA to SO₂ production rates is smaller in winter than in summer, and the MSA to non-sea-salt sulfate (nssSO₄ ^2-) molar ratio varies seasonally. This result agrees with data on the seasonal variation of the MSA/nssSO₄ ^2- molar ratio obtained at middle and high latitudes. The calculations indicate that during winter the reaction of DMS with NO₃ is likely to be a more important sink of NO_x (NO+NO₂) than the reaction of NO₂ with OH, and to serve as a significant pathway of the HNO₃ production. If dimethyl sulfoxide (DMSO) is produced through the OH addition reaction and is heterogeneously oxidized in aqueous solutions, half of the nssSO₄ ^2- produced in summer may be through the oxidation process of DMSO. It is necessary to further investigate the oxidation products by the reaction of DMS with OH, and the possibility of the reaction of DMS with NO₃ during winter.     

A global three-dimensional model of the tropospheric sulfur cycle

The tropospheric part of the atmospheric sulfur cycle has been simulated in a global three-dimensional model. The model treats the emission, transport, chemistry, and removal processes for three sulfur components; DMS (dimethyl sulfide), SO₂ and SO₄ ^2– (sulfate). These processes are resolved using an Eulerian transport model, the MOGUNTIA model, with a horizontal resolution of 10° longitude by 10° latitude and with 10 layers in the vertical between the surface and 100 hPa. Advection takes place by climatological monthly mean winds. Transport processes occurring on smaller space and time scales are parameterized as eddy diffusion except for transport in deep convective clouds which is treated separately. The simulations are broadly consistent with observations of concentrations in air and precipitation in and over polluted regions in Europe and North America. Oxidation of DMS by OH radicals together with a global emission of 16 Tg DMS-S yr^–1 from the oceans result in DMS concentrations consistent with observations in the marine boundary layer. The average turn-over times were estimated to be 3, 1.2–1.8, and 3.2–6.1 days for DMS, SO₂, and SO₄ ^2– respectively.     

Processing of oxidised nitrogen compounds by passage through winter-time orographic cloud

Four case studies are described, from a three-site field experiment in October/November 1991 using the Great Dun Fell flow-through reactor hill cap cloud in rural Northern England. Measurements of total odd-nitrogen nitrogen oxides (NO _y ) made on either side of the hill, before and after the air flowed through the cloud, showed that 10 to 50% of the NO _y , called NO _z , was neither NO nor NO₂. This NO _z failed to exhibit a diurnal variation and was often higher after passage through cloud than before. No evidence of conversion of NO _z to NO₃ ^- in cloud was found. A simple box model of gas-phase chemistry in air before it reached the cloud, including scavenging of NO₃ and N₂O₅ by aerosol of surface area proportional to the NO₂ mixing ratio, shows that NO₃ and N₂O₅ may build up in the boundary layer by night only if stable stratification insulates the air from emissions of NO. This may explain the lack of evidence for N₂O₅ forming NO₃ ^- in cloud under well-mixed conditions in 1991, in contrast with observations under stably stratified conditions during previous experiments when evidence of N₂O₅ was found. Inside the cloud, some variations in the calculated total atmospheric loading of HNO₂ and the cloud liquid water content were related to each other. Also, indications of conversion of NO _x to NO _z were found. To explain these observations, scavenging of NO _x and HNO₂ by cloud droplets and/or aqueous-phase oxidation of NO₂ ^- by nitrate radicals are considered. When cloud acidity was being produced by aqueous-phase oxidation of NO _x or SO₂, NO₃ ^- which had entered the cloud as aerosol particles was liberated as HNO₃ vapour. When no aqueous-phase production of acidity was occurring, the reverse, conversion of scavenged HNO₃ to particulate NO₃ ^-, was observed.     

Behaviour of H2O2, NH3, and black carbon in mixed-phase clouds during CIME

We investigated the partitioning of trace substances during the phase transition from supercooled to mixed-phase cloud induced by artificial seeding. Simultaneous determination of the concentrations of H₂O₂, NH₃ and black carbon (BC) in both condensed and interstitial phases with high time resolution showed that the three species undergo different behaviour in the presence of a mixture of ice crystals and supercooled droplets. Both H₂O₂ and NH₃ are efficiently scavenged by growing ice crystals, whereas BC stayed predominantly in the interstitial phase. In addition, the scavenging of H₂O₂ is driven by co-condensation with water vapour onto ice crystals while NH₃ uptake into the ice phase is more efficient than co-condensation alone. The high solubility of NH₄⁺ in the ice could explain this result. Finally, it appears that the H₂O₂–SO₂ reaction is very slow in the ice phase with respect to the liquid phase. Our results are directly applicable for clouds undergoing limited riming.     

Airborne measurements of dimethylsulfide,sulfur dioxide,and aerosol ions over the Southern Ocean South of Australia

Vertical distributions of dimethylsulfide (DMS), sulfur dioxide (SO₂), aerosol methane-sulfonate (MSA), non-sea-salt sulfate (nss-SO₄ ^2-), and other aerosol ions were measured in maritime air west of Tasmania (Australia) during December 1986. A few cloudwater and rainwater samples were also collected and analyzed for major anions and cations. DMS concentrations in the mixed layer (ML) were typically between 15–60 ppt (parts per trillion, 10^–12; 24 ppt=1 nmol m^–3 (20°C, 1013 hPa)) and decreased in the free troposphere (FT) to about <1–2.4 ppt at 3 km. One profile study showed elevated DMS concentrations at cloud level consistent with turbulent transport (cloud pumping) of air below convective cloud cells. In another case, a diel variation of DMS was observed in the ML. Our data suggest that meteorological rather than photochemical processes were responsible for this behavior. Based on model calculations we estimate a DMS lifetime in the ML of 0.9 days and a DMS sea-to-air flux of 2–3 mol m^–2 d^–1. These estimates pertain to early austral summer conditions and southern mid-ocean latitudes. Typical MSA concentrations were 11 ppt in the ML and 4.7–6.8 ppt in the FT. Sulfur-dioxide values were almost constant in the ML and the lower FT within a range of 4–22 ppt between individual flight days. A strong increase of the SO₂ concentration in the middle FT (5.3 km) was observed. We estimate the residence time of SO₂ in the ML to be about 1 day. Aqueous-phase oxidation in clouds is probably the major removal process for SO₂. The corresponding removal rate is estimated to be a factor of 3 larger than the rate of homogeneous oxidation of SO₂ by OH. Model calculations suggest that roughly two-thirds of DMS in the ML are converted to SO₂ and one-third to MSA. On the other hand, MSA/nss-SO₄ ^2- mole ratios were significantly higher compared to values previously reported for other ocean areas suggesting a relatively higher production of MSA from DMS oxidation over the Southern Ocean. Nss-SO₄ ^2- profiles were mostly parallel to those of MSA, except when air was advected partially from continental areas (Africa, Australia). In contrast to SO₂, nss-SO₄ ^2- values decreased significantly in the middle FT. NH₄ ⁺/nss-SO₄ ^2- mole ratios indicate that most non-sea-salt sulfate particles in the ML were neutralized by ammonium.     

Mass transfer at the air/water interface: Mass accommodation coefficients of SO2, HNO3, NO2 and NH3

We present here experimental determinations of mass accommodation coefficients using a low pressure tube reactor in which monodispersed droplets, generated by a vibrating orifice, are brought into contact with known amounts of trace gases. The uptake of the gases and the accommodation coefficient are determined by chemical analysis of the aqueous phase.We report in this article measurements of _exp=(6.0±0.8)×10^–2 at 298 K and with a total pressure of 38 Torr for SO₂, (5.0±1.0)×10^–2 at 297 K and total pressure of 52 Torr for HNO₃, (1.5±0.6)×10^–3 at 298 K and total pressure of 50 Torr for NO₂, (2.4±1.0)×10^–2 at 290 K and total pressure of 70 Torr for NH₃.These values are corrected for mass transport limitations in the gas phase leading to =(1.3±0.1)×10^–1 (298 K) for SO₂, (1.1±0.1)×10^–1 (298 K) for HNO₃, (9.7±0.9)×10^–2 (290 K) for NH₃, (1.5±0.8)×10^–3 (298 K) for NO₂ but this last value should not be considered as the true value of for NO₂ because of possible chemical interferences.Results are discussed in terms of experimental conditions which determine the presence of limitations on the mass transport rates of gaseous species into an aqueous phase, which permits the correction of the experimental values.     

Seasonal variations of atmospheric sulfur dioxide and dimethylsulfide concentrations at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric sulfur dioxide (SO₂) concentrations were performed from March 1989 to January 1991 at Amsterdam Island (37°50 S–77°30 E), a remote site located in the southern Indian Ocean. Long-range transport of continental air masses was studied using Radon (²²²Rn) as continental tracer. Average monthly SO₂ concentrations range from less than 0.2 to 3.9 nmol m^-3 (annual average = 0.7 nmol m^-3) and present a seasonal cycle with a minimum in winter and a maximum in summer, similar to that described for atmospheric DMS concentrations measured during the same period. Clear diel correlation between atmospheric DMS and SO₂ concentrations is also observed during summer. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces the seasonal variations in the measured atmospheric SO₂ concentrations within ±30%. Comparing between computed and measured SO₂ concentrations allowed us to estimate a yield of SO₂ from DMS oxidation of about 70%.     

Relations between Light Hydrocarbon, Carbon Monoxide, and Carbon Dioxide Concentrations in the Plume from the Combustion of Plant Material in a Furnace

To systematically explain relations between light hydrocarbons, CO, and CO₂ concentrations/emissions of biomassburning, we measured concentrations/emissions of carbon gases – CO,CO₂, light hydrocarbons (CH₄, C₂H₆,C₂H₄, C₂H₂, C₃H₈, C₃H₆,n-C₄H₁₀, i-C₄H₁₀, n-C₅H₁₂,i-C₅H₁₂), and THC (total hydrocarbon) – in the burning of dead plant material, mainly Imperata grass, byclosed-chamber experiments and by time-series analyses of gas concentrations in combustion plumes in relatively efficient and inefficient combustion situations. Concentrations of hydrocarbons measured were well correlated to [CO] although [C₂H₂] was exceptionally well correlated to[CO₂]. The phase diagrams (relation between [CO]/ [CO₂] and [hydrocarbon]/ [CO₂]) obtained by the time-seriesexperiments well illustrated the variation in the overall emission rates of the closed-chamber experiments. The higher rates of decrease in hydrocarbon concentration with increasing carbon number in the efficient case compared with the inefficient case probably reflected the rate of oxidation and the amount of radicals. The overall concentrations (or emissions) of C₂H₄ and C₃H₆ were higher thanthose of C₂H₆ and C₃H₈, suggesting a linkage to mechanisms in whichthe predominant path of hydrocarbon oxidation is through the degradation of alkyl radicals, which can be immediately converted into or formed from alkenes. For C₃ and C₄ species, normal-chain species hadhigher emissions than iso-chain species under lower combustion efficiency. This may be attributable to the presence of tertiary C–H bonds in iso-species,which show more reactivity in the abstraction of H than secondary C–H bonds unless the carbon number is large.     

Features of the atmospheric cycle of aerosol trace elements and sulphur dioxide revealed by baseline observations in Canada

Selected applications of baseline aerosol, SO₂ and deposition chemistry observations in Canada are reviewed to illustrate how new insight can be gained into features of the atmospheric pathway of trace substances such as sources, transformation and removal. A strong annual variation in Arctic aerosol concentration is a manifestation of particle residence times that are much longer in winter than in summer. Differences in the variation of SO₄ ⁼ and V concentrations in the Arctic winter are due to SO₂ oxidation. The mean rate of oxidation between November and April ranges from 0.04 to 0.25%/h and is a minimum in December, January and February. Br measured on filters in the Arctic peaks in concentration later (March and April) than anthropogenic particulate matter suggesting photochemical production. Acidity in Arctic aerosol and in glacial ice are correlated. The relationship yields a best estimate of acidity in the absence of anthropogenic influences of 5.8 mole/l. Coincident air and precipitation measurements of sulphur oxides indicate that on average in eastern Canada 60% of SO₄ ⁼ in rain originates from SO₂ oxidation in the storm. Trends in Arctic ice core acidity and SO₂ emissions in Europe are similar, that is, little variation in the first half of the century and a marked increase since the mid 1950's. This is consistent with meteorological and chemical evidence linking Arctic air pollution with Eurasian sources.     

A Study of DMS Oxidation in the Tropics: Comparison of Christmas Island Field Observations of DMS, SO2, and DMSO with Model Simulations

This study reports comparisonsbetween model simulations, based on current sulfurmechanisms, with the DMS, SO₂ and DMSOobservational data reported by Bandy et al.(1996) in their 1994 Christmas Island field study. For both DMS and SO₂, the model results werefound to be in excellent agreement with theobservations when the observations were filtered so asto establish a common meteorological environment. Thisfiltered DMS and SO₂ data encompassedapproximately half of the total sampled days. Basedon these composite profiles, it was shown thatoxidation of DMS via OH was the dominant pathway withno more than 5 to 15% proceeding through Cl atoms andless than 3% through NO₃. This analysis wasbased on an estimated DMS sea-to-air flux of 3.4 ×10⁹ molecs cm^-2 s^-1. The dominant sourceof BL SO₂ was oxidation of DMS, the overallconversion efficiency being evaluated at 0.65 ± 0.15. The major loss of SO₂ was deposition to theocean's surface and scavenging by aerosol. Theresulting combined first order k value was estimated at 1.6 × 10^-5 s^-1. In contrast to the DMSand SO₂ simulations, the model under-predictedthe observed DMSO levels by nearly a factor of 50. Although DMSO instrument measurement problems can notbe totally ruled out, the possibility of DMSO sourcesother than gas phase oxidation of DMS must beseriously considered and should be explored in futurestudies.     

FT-IR Study of the Kinetics and Products of the Reactions of Dimethylsulphide, Dimethylsulphoxide and Dimethylsulphone with Br and BrO

Kinetics and products of the gas-phase reactions of dimethylsulphide (DMS), dimethylsulphoxide (DMSO) and dimethylsulphone (DMSO₂) with Br atoms and BrO radicals in air have beeninvestigated using on-line Fourier Transform Infrared Spectroscopy (FT-IR) as analytical technique at 740 ± 5 Torr total pressure and at 296 ± 3 K in a480 L reaction chamber. Using a relative rate method for determining the rate constants; the following values (expressed in cm³molecule^–1 s^–1) were found: k_DMS+Br = (4.9 ±1.0) ×10^–14, k_DMSO + Br < 6 × 10^–14,k_{DMSO 2} + Br 1 × 10^–15,k_DMSO + BrO = (1.0 ± 0.3) × 10^–14 andk_{DMSO 2} + BrO 3 × 10^–15 (allvalues are given with one on the experimental data). DMSO, SO₂, COS, CH₃SBr andCH₃SO₂Br were identified as the main sulphur containing products of the oxidation of DMS by Br atoms. From the reaction between DMSO and Br atoms, DMSO₂and CH₃SO₂Br were the only sulphur containing products thatwere identified. DMSO, DMSO₂ and SO₂ were identified as themain sulphur containing products of the reaction between DMS and BrO.DMSO₂ was found to be the only product of the reaction between DMSO and BrO. For the reactions of DMSO₂ with Br and BrO no products were identified because the reactions were too slow.The implications of these results for atmospheric chemistry are discussed.     

临安冬夏季SO2、NO2和O3体积分数特征及与气象条件的关系

对临安大气本底站2003-2004年冬、夏季二氧化氮(NO2)、二氧化硫(SO2)、臭氧(O3)进行了分析.结果表明:冬季NO2和SO2平均体积分数分别为19.48×10-9和35.74 x10-9,而夏季的平均体积分数分别为4.81×10-9和8.12×10-9,冬季高于夏季;O3在夏季的平均体积分数为33.55×10-9,略高于冬季的25.44×10-9;夜间NO2和SO2体积分数比白天高,并且NO2呈明显的单峰单谷型分布,O3也呈单峰型但峰值出现在白天.NO2、SO2体积分数存在着明显的“假日效应”,假日比非假日低,周五高于假日和非假日;但O3体积分数没有明显的假日效应.降水对SO2有明显的清除作用,但对NO2的清除作用不明显.与风向对比发现,夏季高体积分数的NO2、SO2都受到NW、WNW风的影响,冬季则分别受NE和SW、SSW风的影响;而O3受风向的影响较复杂,与局地光化学反应有关.     

Model studies of the impact of chemical inhomogeneity on SO2 oxidation in warm stratiform clouds

A one-dimensional, time-dependent model of the physics and chemistry of a warm stratiform cloud is used to study the possible impact of chemical inhomogeneity among cloud and raindrops on the oxidation of SO₂ in clouds. The effects of chemical inhomogeneity are examined using two contrasting models: In Model 1 a bulk-solution parameterization is adopted which effectively treats all cloud and raindrops as if they are chemically homogeneous; in Model 2 we allow the cloud and raindrops to have a dichotomous distribution. The dichotomous distribution in Model 2 is simulated by assuming that the two groups of cloud droplets nucleate from two chemically distinct populations of condensation nuclei; one being acidic and the other being alkaline. While the two models yield essentially identical results when the ambient levels of H₂O₂ are greater than the ambient levels of SO₂, the rate of conversion of SO₂ to sulfuric acid and the amount of sulfate removed in the precipitation can be significantly enhanced in Model 2 over that of Model 1 under conditions of oxidant limitation (i.e., H₂O₂ < SO₂). This enhancement is critically dependent upon the fraction of alkaline nuclei assumed to be present in Model 2 and arises from the rapid increase in the aqueous-phase reaction between O₃+S_IV at high pH. Our results suggest that cloud models which adopt a bulk-solution parameterization for cloud droplet chemistry, may underestimate the amount of in-cloud SO₂ oxidation under oxidant-limited conditions.