Indications of nonlinearities in processes of wet deposition

Numerical simulations have been carried out with a model consisting of clear-air chemistry, in- cloud chemical reactions, and dynamic processes of cloud development in order to examine the time history of cloudwater pH and sulfate production in a cumulus cloud and the relationship between pollutant precursors and corresponding acidic chemical species in wet deposition. Preliminary results indicate that the molar ratio SO₄²⁻/NC₃⁻ in cloud water increases as the ratio SO₂/NO₂ increases, that the relationship between the increase of precursor SO₂/NO₂ and the increase of SO₄²⁻/NO₃⁻ in cloud water is nonlinear, and that the degree of this nonlinearity becomes more significant for cases when the cloud condensation nuclei content in air is assumed to be invariant.     

Observations and modelling of aerosol growth in marine stratocumulus—case study

Airborne measurements of the growth of the marine accumulation mode after multiple cycles through stratocumulus cloud are presented. The nss-sulphate cloud residual mode was log-normal in spectral shape and it’s mode radius was observed to progressively increase in size from 0.78 to 0.94 μm over 155 min of air parcel evolution through the cloudy marine boundary layer. The primary reason for this observed growth was thought to result from aqueous phase oxidation of SO₂ to aerosol sulphate in activated cloud drops. An aqueous phase aerosol–cloud-chemistry model was used to simulate this case study of aerosol growth and was able to closely reproduce the observed growth. The model simulations illustrate that aqueous phase oxidation of SO₂ in cloud droplets was able to provide enough additional sulphate mass to increase the size of activated aerosol. During a typical cloud cycle simulation, ≈4.6 nmoles kg^-1_air (0.44 μg m^-3) of sulphate mass was produced with ≈70% of sulphate production occurring in cloud droplets activated upon sea-salt nuclei and ≈30% occurring upon nss-sulphate nuclei, even though sea-salt nuclei contributed less than 15% to the activated droplet population. The high fraction of nss-sulphate mass internally mixed with sea-salt aerosol suggests that aqueous phase oxidation of SO₂ in cloud droplets activated upon sea-salt nuclei is the dominant nss-sulphate formation mechanism and that sea-salt aerosol provides the primary chemical sink for SO₂ in the cloudy marine boundary layer.     

Removal of sulphur dioxide in a two-dimensional rain system based on a scale analysis of the conservation equations

Since rain systems show a wide variation in structure in both time and space, it is virtually impossible to model in detail the behaviour of sulphur passing through a rain system. Instead, an attempt has been made to determine the scale of the major processes going on in a frontal rain system based on the conservation equations for cloudwater, rainwater, SO₂ in air, sulphate in cloud and sulphate in rainwater. When this procedure is followed it is found that cloud and rainwater amounts are determined largely as a dynamic balance between cloudwater condensation and accretion of cloud drops by rain. The removal of SO₂ in rain is mainly determined by the oxidation of SO₂ in cloud, enhanced by oxidation in rain. In the case when oxidation of SO₂ by O₃ is the primary oxidation pathway a simple formula is derived for the fractional removal efficiency, which shows which parameters are of greatest importance and has potential use in the current generation of long-range transport models. This formula shows that the removal efficiency is a strongly non-linear function of sulphur dioxide concentration. At regional average SO₂ concentrations removal is efficient, but decreases rapidly at higher SO₂ concentrations.     

Electron microscopy of acidic aerosols collected over the northeastern United States

As part of the acid precipitation experiment (APEX) conducted in the northeastern U.S. by the National Center for Atmospheric Research and cooperating universities, aerosols were collected from an aircraft in different seasons, locations and meteorological conditions. Particles were impacted on electron microscope grids for morphological analysis and thin-film chemical tests for sulfate and nitrate. Under most conditions the accumulation mode aerosols (c. 0.1–1.0 μm diameter) collected in the boundary layer were composed of sulfate particles of uniform composition (i.e. an internally mixed aerosol), indicating that individual particle composition could be inferred from bulk measurements. Externally mixed aerosols (i.e. assemblages of different kinds of particles) were found to exist near certain sources (e.g. power plants), in urban plumes, and near fair weather cumulus clouds. Direct evidence of rapid oxidation of SO₂ to H₂SO₄ in cloud droplets was obtained in samples collected near clouds in northern New York and central Illinois, and this represents a potentially major pathway for SO₂ oxidation in the lower troposphere.     

An Eulerian model for scavenging of pollutants by raindrops

An Eulerian model for simulating the coupled processes of gas-phase depletion and aqueousphase accumulation of the pollutant species during a rain event has been formulated. The model is capable of taking into account any realistic vertical profile of pollutant species concentrations and time-dependent initial aqueous-phase concentrations at the cloud base. The model considers the processes of single species absorption and dissociation in the aqueous phase. The coupled partial differential equations constituting the model are discretized into a set of ordinary differential equations by using the Galerkin method with chapeau functions as the basis functions. These equations are solved to obtain the pollutant concentrations of the gas phase and raindrops as well as the pH of raindrops as a function of time and distance below cloud-base.Simulations are performed for scavenging of gaseous HNO₃, H₂O₂, SO₂, formaldehyde and NH₃. For the case of highly soluble HNO₃ and H₂O₂, raindrops are far from equilibrium with the gas phase and their capacity for absorption of these gases is undiminished even as they reach ground level. The gas-phase concentrations for these species decrease exponentially with time and the washout is determined primarily by the rain intensity and mass-transfer coefficient of the gaseous species to the raindrops. The pollutant species concentrations in raindrops are an almost linear function of the distance below the cloud base. For the simulation conditions considered in this study, the half-life periods of these gases for removal from the atmosphere range from 15 to 40 min.For SO₂ and formaldehyde, the aqueous-phase concentrations approach equilibrium as the drops fall to ground level and the gas-phase concentrations show large gradients in the vertical. Half-life periods for SO₂ range from 1.3 to 13 h depending on the initial raindrop pH and rain intensity. For formaldehyde, the half-life ranges from 19 to 63 min.Solubility of NH₃ is a strong function of the raindrop pH. As NH₃ is absorbed, the raindrop pH increases and NH₃ solubility decreases. For pre-acidified drops (pH = 4.6), ammonia solubility is very high and the drops are far from equilibrium with the gas phase throughout the falling period. The half-life for ammonia ranges from 11 min to over 3 h in our simulations.     

Transport and transformation of sulfur compounds over East Asia during the TRACE-P and ACE-Asia campaigns

On the basis of the recently estimated emission inventory for East Asia with a resolution of 1×1°, the transport and chemical transformation of sulfur compounds over East Asia during the period of 22 February through 4 May 2001 was investigated by using the Models-3 Community Multi-scale Air Quality (CMAQ) modeling system with meteorological fields calculated by the regional atmospheric modeling system (RAMS). For evaluating the model performance simulated concentrations of sulfur dioxide (SO₂) and aerosol sulfate (SO₄²⁻) were compared with the observations on the ground level at four remote sites in Japan and on board aircraft and vessel during the transport and chemical evolution over the Pacific and Asian Pacific regional aerosol characterization experiment field campaigns, and it was found that the model reproduces many of the important features in the observations, including horizontal and vertical gradients. The SO₂ and SO₄²⁻ concentrations show pronounced variations in time and space, with SO₂ and SO₄²⁻ behaving differently due to the interplay of chemical conversion, removal and transport processes. Analysis of model results shows that emission was the dominant term in regulating the SO₂ spatial distribution, while conversion of SO₂ to SO₄²⁻ in the gas phase and the aqueous phase and wet removal were the primary factors that controlled SO₄²⁻ amounts. The gas phase and the aqueous phase have the same importance in oxidizing SO₂, and about 42% sulfur compounds (25% in SO₂) emitted in the model domain was transported out, while about 57% (35% by wet removal processes) was deposited in the domain during the study period.     

The contribution of in-cloud oxidation of SO2 to wet scavenging of sulfur in convective clouds

The sensitivity of in-cloud oxidation of SO₂ in corrective 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 SO₂ by O₃ and H₂O₂. Sulfate aerosol and SO₂, CO₂, NH₃, H₂O₂ and O₃ gases and their aqueous phase dissociation products are treated.The results indicate that in-cloud 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 H₂O₂ and NH₃ 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 O₃. If gas phase H₂O₂ of order 1 ppb is the major source of aqueous phase H₂O₂ for S(IV) 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⁻¹), ice phase growth decreases the sulfate concentration. However, the results are insensitive to an ice phase origin at moderate and high rainfall rates.     

Fenton氧化法同时脱硫脱硝的实验研究

应用Fenton液相氧化吸收法进行同时脱硫脱硝实验。首先,利用单因素实验,分别考察了H2O2浓度、Fe2+投加量、初始pH值、UV照射和温度对脱硫脱硝的影响。结果表明,SO2和NO去除率随着H2O2浓度和Fe2+投加量的增大而提高;初始pH对SO2和NO的去除有较大影响;UV能促进SO2和NO的净化;温度对脱硫效率影响不大,但对NO的去除有显著作用,适当升温可以提高脱硝效率。随后,考察了SO2对NO去除率的影响。通过单独脱硝和同时脱硫脱硝的对比实验发现,SO2的加入对NO的去除有一定的促进作用,Fenton法可同时获得起始约80%的脱硝效率和98%以上的脱硫效率。     

SO2 oxidation in cloud drops containing NaCl or sea salt as condensation nuclei

Experimental results from cloud-chamber studies provide direct evidence that NaCl, artificial sea salt and natural sea salt promote faster rates of aqueous SO₂ oxidation than observed in the absence of these salts. However, the chemical basis for this effect has not been clarified. Oxidation rates > 30% h⁻¹are observed in cloud-chamber experiments with 0.4 μm-diameter 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.     

湿法烟气脱硫反应过程的实验研究

在石灰石/石膏湿法烟气脱硫中试台上,系统开展了浆液pH值、飞灰浓度、液气比、入口SO2浓度、烟气速度和氧化方式等对脱硫反应过程影响的实验研究。实验表明,脱硫效率随着石膏浆液pH值、液气比的升高而增加,且入口SO2浓度越高,液气比越低,影响效应越明显;脱硫效率随着烟气速度、烟气温度和入口SO2浓度的增加而下降;石膏浆液中飞灰含量对系统脱硫效率具有一定的促进作用:pH值>5.6,飞灰浸出液中Fe3+含量相对较低,Fe3+对脱硫反应过渡态催化氧化影响程度较轻,不同工况脱硫效率差别不大。pH值<5.6,飞灰浸出液中Fe3+含量随pH值降低而增大,增效效果逐渐显著;氧化方式对脱硫反应过程有明显的影响,强制氧化工艺的脱硫效率比自然氧化的高5%左右。     

The oxidation rate and residence time of sulphur dioxide in the arctic atmosphere

The Arctic air mass contains gaseous and particulate compounds that originate mainly from fossil fuel combustion at mid-latitudes and especially from the Eurasian sector. Observations of the temporal variation of SO₂, SO₄²⁻ and V concentrations in the North American and Norwegian Arctic are presented. At Igloolik, Canada, 3–7-day average SO₂ concentrations ranged from 2.3 to 4.3 μg m⁻³ in February to much lower values in spring and fall. The most probable cause of similar strong variations in the ratio of SO₄⁻² to V observed throughout the North American Arctic is a seasonally varying SO₂ oxidation rate. Interpreted in the light of a Lagrangian transport model, observations indicate that the mean SO₂-oxidation rate between Eurasian sources and the North American Arctic is 0.1 % h⁻¹ in early December, 0.04 % h⁻¹ in late February, and 0.1–0.2% h⁻¹ in early April. The residence time of SO₂, controlled not only by chemical conversion to sulphate but also by dry deposition, is 14–20 days in late fall, 16–32 days at mid-winter and 10–19 days in April. The estimated rates of SO₂ oxidation cannot be explained by photochemical oxidation mechanisms at least when reactive hydrocarbons are ignored.     

Numerical studies of acidification processes within and below clouds with a flow-through chemical reactor model

A flow-through chemical reactor model has been exercised to assess the importance of various oxidation reactions and cloud processes on wet removal and redistribution of atmospheric pollutants and to investigate the effect of in-cloud acidification on precipitation chemistry at the surface. Preliminary results indicate that in-cloud acidification accounts for more than 60% of the wet deposited acids derived from acidification of initial SO₂, that 42–57% of water-soluble, non-reactive NH₃ and HNO₃ are removed by wet deposition. The pseudo-first-order conversion rate of SO₂ to SO₄²⁻ ranges from 3 to 25% h ⁻¹ depending on initial and boundary conditions.Sensitivity studies have been carried out to test the importance of time evolution of clouds on partitioning of pollutants in the atmosphere and to investigate the variability of precipitation chemistry due to changes in rate constants. The distributions of NH₃ and HNO₃ are found to be dependent largely on the cloud microphysical parameters, while the distributions of H₂O₂ and SO₂ depend largely on initial conditions of both species. Individual physical and chemical mechanisms can determine the overall rate of sulfate wet deposition at different stages of cloud evolution.     

Formation of particulate sulfur species (sulfate and methanesulfonate) during summer over the Eastern Mediterranean: A modelling approach

To improve our understanding of the mechanisms of particulate sulfur formation (non sea-salt sulfate, nss-SO₄²⁻) and methanesulfonate (MS_x used here to represent the sum of gaseous methanesulfonic acid, MSA, and particulate methanesulfonate, MS⁻) in the eastern Mediterranean and to evaluate the relative contribution of biogenic and anthropogenic sources to the S budget, a chemical box model coupled offline with an aerosol–cloud model has been used.Based on the measurements of gaseous dimethyl sulfide (DMS) and methanesulfonic acid (MSA) and the MSA sticking coefficient determined during the Mediterranean Intensive Oxidant Study (MINOS) experiment, the yield of gaseous MSA from the OH-initiated oxidation of DMS was calculated to be about 0.3%. Consequently, MSA production from gas-phase oxidation of DMS is too small to explain the observed levels of MS⁻. On the other hand, heterogeneous reactions of dimethyl sulfoxide (DMSO) and its gas-phase oxidation product methanesulfinic acid (MSIA) can account for most of the observed MS⁻ levels. The modelling results indicate that about 80% of the production of MS⁻ can be attributed to heterogeneous reactions.Observed submicron nss-SO₄²⁻ levels can be fully explained by homogeneous (photochemical) gas-phase oxidation of sulfur dioxide (SO₂) to sulfuric acid (H₂SO₄), which is subsequently scavenged by (mainly submicron) aerosol particles. The predominant oxidant during daytime is hydroxyl radical (OH) showing very high peak levels in the area during summer mostly under cloudless conditions. Therefore, during summer in the east Mediterranean, heterogeneous sulfate production appears to be negligible. This result is of particular interest for sulfur abatement strategy. On the other hand only about 10% of the supermicron nss-SO₄²⁻ can be explained by condensation of gas-phase H₂SO₄, the rest must be formed via heterogeneous pathways.Marine biogenic sulfur emissions contribute up to 20% to the total oxidized sulfur production (SO₂ and H₂SO₄) in good agreement with earlier estimates for the area.     

Acidity of raindrop by uptake of gases and aerosol pollutants

Below-cloud raindrops acidification simulated with a simple model incorporating gas–liquid equilibriums, gas-phase mass transfer, and catalyzed SO₂ oxidation in aqueous phase with uptake of gases and scavenging of particles. Ionic contents of various species in raindrops of different size and pH are computed using one-dimensional time-variant model. The model results are based on SO₂ and NH₃ absorption and collection of calcium aerosols by raindrops with various collection mechanisms. Aqueous concentrations of (SO₂)_l and (NH₃)_l and their ionic components in raindrops are found to be increased with the fall distance from cloud base and decrease of drop size. The overall magnitude of pH enhances with the increase in drop size and transient position of raindrops in the atmosphere below the cloud base. The elevated ionic calcium in raindrops by impaction of calcium aerosols of higher inertia neutralizes the acidic components. Acidic ion contents in smaller droplets are found to be significant and resulted pH of raindrop increases with the size and neutralizing potential of alkaline species. The pH values of rainwater contents of predominant size raindrops in bulk samples corresponding to various rainfall intensities are higher as against the individual non-evaporating smaller raindrops. Results are important in view of the impact of showers on earth surfaces during rain containing large number of smaller droplets as compared to the acidification studies of bulk rainwater.     

A modeling study of SOx-NOx-Hydrocarbon plumes and their transport to the background troposphere

A model which emulates the behavior of urban-industrial plumes has been developed and used to analyze the chemical reaction processes occurring as a polluted air mass is transported from an urban area. A 73-step reaction mechanism describing hydrocarbon/NO_xSO_x chemistry was used, with photolytic rate constants depending on the latitude, time of day and time of year. The model includes the physical processes of plume dilution, entrainment and dry deposition, and is simulated under a diurnally varying mixing layer or neutral atmospheric stability conditions.Simulation results are compared with reported field measurements for plumes from St. Louis, Milwaukee, and a power plant plume entrained in the Milwaukee urban plume. The agreement with field concentrations and SO₂ transformation rate data is good, the latter ranging from 1 to 12 % h⁻¹. The study was extended to hypothetical plumes for parametric analysis. In every case considered, the classic O₃ peak occurred at about 3:30 p.m., essentially independent of initial concentrations and plume departure time. The analysis also indicated that substantial SO₂ oxidation via homogeneous gas phase chemistry can occur at night-time, the prerequisite being a high HG/NO_x ratio.     

An Eulerian transport/transformation/removal model for SO2 and sulfate—I. Model development

A combined transport/chemistry model which simulates the regional distribution of SO₂ and sulfate within the lower troposphere is described. The mathematical analysis is based on the coupled three-dimensional advection-diffusion equations for SO₂ and sulfate, and incorporates chemical transformations as well as the physical phenomena of dry deposition at the surface. The analysis also considers spatial variations in topography and spatial and temporal variations in both the mixing layer heights and the wind field. Based on the results from a series of numerical experiments, the dynamic model employs a Galerkin method for the numerical solution of the partial differential equations.A SO₂ photochemical oxidation mechanism is incorporated into the transport model. The SO₂ photochemical oxidation rate is based on a set of 27 reactions used to estimate the hydroxyl and peroxyl radical concentrations. The kinetic mechanism has been tested in simulations of smog chamber studies and yields realistic concentrations and conversion rates in model simulations of both urban and natural tropospheres.Other major facets treated in the formulation of the model include the interpretation and use of data available on dry deposition and the development of procedures to calculate meteorological model inputs (e.g., eddy diffusivities, dry deposition velocities, the three components of wind velocity, etc.) from routinely measured meteorological data. Simulations using the analysis are presented in a companion paper.     

Photochemical oxidation and dispersion of gaseous sulfur compounds from natural and anthropogenic sources around a coastal location

The photochemical oxidation and dispersion of reduced sulfur compounds (RSCs: H₂S, CH₃SH, DMS, CS₂, and DMDS) emitted from anthropogenic (A) and natural (N) sources were evaluated based on a numerical modeling approach. The anthropogenic emission concentrations of RSCs were measured from several sampling sites at the Donghae landfill (D-LF) (i.e., source type A) in South Korea during a series of field campaigns (May through December 2004). The emissions of natural RSCs in a coastal study area near the D-LF (i.e., source type N) were estimated from sea surface DMS concentrations and transfer velocity during the same study period. These emission data were then used as input to the CALPUFF dispersion model, revised with 34 chemical reactions for RSCs. A significant fraction of sulfur dioxide (SO₂) was produced photochemically during the summer (about 34% of total SO₂ concentrations) followed by fall (21%), spring (15%), and winter (5%). Photochemical production of SO₂ was dominated by H₂S (about 55% of total contributions) and DMS (24%). The largest impact of RSCs from source type A on SO₂ concentrations occurred around the D-LF during summer. The total SO₂ concentrations produced from source type N around the D-LF during the summer (a mean SO₂ concentration of 7.4 ppbv) were significantly higher than those (≤0.3 ppbv) during the other seasons. This may be because of the high RSC and SO₂ emissions and their photochemistry along with the wind convergence.     

Transport and oxidation of SO2 in a stagnant foggy valley

The fate of SO₂ emitted in the San Joaquin Valley of California under stagnant foggy conditions was determined by the release of an inert tracer and the concurrent monitoring of SO₂ and SO₄²⁻ concentrations. At night, SO₂ was found to be trapped in a dense fog layer below a strong and persistent inversion based a few hundred meters above the valley floor. This lack of ventilation led to the accumulation of SO₂ and SO₄²⁻ over a major SO₂ source region in the valley. The rate of oxidation of SO₂ to SO₄²⁻ in fog was estimated at 3 ± 2%h⁻¹. Production of acidity from the oxidation of SO₂ fully titrated the NH₃(g) present before the fog, and led to a progressive drop of the fogwater pH over the course of the night. In the afternoon, the valley was found to be efficiently ventilated by a buoyant upslope flow through the inversion. The tracer data indicated that about 40 % of the air transported upslope in the afternoon was returned to the valley in the night-time drainage flow. The fates of SO₂ and SO₄²⁻ in the valley during extended highinversion episodes appear to depend considerably on the presence of fog or stratus, and on the extent of daytime insolation.     

Application of chemical oxidation to remediate HCH-contaminated soil under batch and flow through conditions

This is the first study describing the chemical oxidation of hexachlorocyclohexanes (HCHs) in contaminated soil under water saturated and unsaturated flow through conditions. Soil contaminated with β-HCH (45 mg kg^?1) and γ-HCH (lindane, 25 mg kg^?1) was sampled from former lindane waste storage site. Efficiency of following treatments was tested at circumneutral pH: H₂O₂ alone, H₂O₂/Fe^II, Na₂S₂O₈ alone, Na₂S₂O₈/Fe^II, and KMnO₄. Experimental conditions (oxidant dose, liquid/solid ratio, and soil granulometry) were first optimized in batch experiments. Obtained results revealed that increasing dose of H₂O₂ improved the oxidation efficiency while in Na₂S₂O₈ system, maximum HCHs were removed at 300 mM. However, oxidation efficiency was slightly improved by Fe^II-activation. Increasing the solid/liquid ratio decreased HCH removal in soil samples crushed to 500 μm while an opposite trend was observed for 2-mm samples. Dynamic column experiments showed that oxidation efficiency followed the order KMnO₄ > Na₂S₂O₈/Fe^II > Na₂S₂O₈ whatever the flow condition, whereas the removal extent declined at higher flow rate (e.g., ~50% by KMnO₄ at 0.5 mL/min as compared to ~30% at 2 mL/min). Both HCH removal and oxidant decomposition extents were found higher in saturated columns than the unsaturated ones. While no significant change in relative abundance of soil mineral constituents was observed before and after chemical oxidation, more than 60% of extractable organic matter was lost after chemical oxidation, thereby underscoring the non-selective behavior of chemical oxidation in soil. Due to the complexity of soil system, chemical oxidation has rarely been reported under flow through conditions, and therefore our findings will have promising implications in developing remediation techniques under dynamic conditions closer to field applications.     

Influence of NO2 on S(IV) oxidation in aqueous suspensions of aerosol particles from two different origins

The influence of soluble compounds leached from real atmospheric aerosol particles (size range D_ae: 0.17–1.6 μm) and dissolved NO₂ on S(IV) oxidation in aqueous solution is presented. Experiments were conducted with aerosol particles of two different origins (i.e., urban and industrial) and at concentrations of trace gases in the gas mixtures (SO₂/air and SO₂/NO₂/air) typical for a polluted atmosphere. During the introduction of SO₂/air into the aqueous aerosol suspensions under dark conditions at pH 4, the formation of SO₄²⁻ was very slow with a long induction period. However, in the presence of NO₂ the oxidation rate of dissolved SO₂ in suspensions of aerosols from both origins increased substantially (about 10 times). The results suggest that soluble compounds eluted from atmospheric aerosols have not only a catalytic (e.g. Fe, Mn), but also a pronounced inhibiting effect (e.g., oxalate, formate, acetate, glycolate) on S(IV) autoxidation. When NO₂ was also introduced into the aerosol suspensions, the inhibition was not so highly expressed. An explanation for this is that the radical chain mechanism is mainly initiated by the interaction of dissolved NO₂ and HSO₃⁻. Therefore, at conditions typical for a polluted atmosphere dissolved NO₂ can have a significant influence on the secondary formation of SO₄²⁻.