Photochemical Oxidation Of SO2 Research Articles

Fast Sulfate Formation Initiated by the Spin-Forbidden Excitation of SO2 at the Air-Water Interface.

The multiphase oxidation of SO2 to sulfate in aerosol particles is a key process in atmospheric chemistry. However, there is a large gap between the observed and simulated sulfate concentrations during severe haze events. To fill in the gaps in understanding SO2 oxidation chemistry, a combination of experiments and theoretical calculations provided evidence for the direct, spin-forbidden excitation of SO2 to its triplet states using UVA photons at an air-water interface, followed by reactions with water and O2 that facilitate the rapid formation of sulfate. The estimated reaction energy for the whole process, 3SO2 + H2O + 1/2O2 → HSO4- + H+ (298 K, 1 M), was ΔGr = -107.8 kcal·mol-1. Moreover, calculations revealed that this was a multistep reaction involving submerged, small energy barriers (∼10 kcal·mol-1). These results indicate that photochemical oxidation of SO2 at the air-water interface with solar actinic light may be an important unaccounted source of sulfate aerosols under polluted haze conditions.

Variability and Source Characterization of Regional PM of Two Urban Areas Dominated by Biomass Burning and Anthropogenic Emission

Identifying the sources and formation pathways of particulate matter (PM) and its components is important to determine the impact of atmospheric aerosols on human health and the warming of the global atmosphere. Here, we characterized the variability of the eleven-year concentration and the origin of PM2.5 in two urban regions (Metro Manila and Boracay) in the Philippines, a country in South East Asia (SEA) with high local anthropogenic emissions and frequently impacted by transboundary pollution. The surface concentration of PM2.5 was analyzed using simulated and reanalyzed satellite data (MERRAero), complemented, and validated with filter measurements for PM2.5 and black carbon (BC). The PM2.5 masses in Metro Manila (12.3 ± 2.4 µg m–3) and Boracay (12.9 ± 3.6 µg m–3) were comparable yet with varying concentrations of aerosol components Anthropogenic related components of PM (organic carbon, sulfate, and BC) were more enhanced in the metropolis region (4.9 µg m–3) than in the prime tourist spot (2.72 µg m–3). Sulfate formation impacted the aerosol mass in Metro Manila, with the photochemical oxidation of SO2 as the predominant production pathway of sulfates. Analysis of regional organic matter (OM) and BC in Boracay revealed extensive transboundary transport of biomass burning (BB) plumes from equatorial SEA countries. Also, the ratio of OM and SO4 was utilized as a quick proxy for source characterization. Metro Manila and Boracay reported 0.77 ± 0.20 and 1.06 ± 0.55, highlighting the evident impact of traffic emissions in the PM mass in the metropolis region. For Boracay, the mixed contributions of BB and vehicular activities influenced the formation of PM. Overall, the reanalysis of satellite data captured the long-term variability and origins of surface PM in two vital urban regions in the Philippines. The utilization of MERRAero will be an important procedure in making regulatory decisions on regional pollution control.

Atmospheric oxidation of isoprene and 1,3-butadiene: influence of aerosol acidity and relative humidity on secondary organic aerosol

Abstract. The effects of acidic seed aerosols on the formation of secondary organic aerosol (SOA) have been examined in a number of previous studies, several of which have observed strong linear correlations between the aerosol acidity (measured as nmol H+ m−3 air sample volume) and the percent change in secondary organic carbon (SOC). The measurements have used several precursor compounds representative of different classes of biogenic hydrocarbons including isoprene, monoterpenes, and sesquiterpenes. To date, isoprene has displayed the most pronounced increase in SOC, although few measurements have been conducted with anthropogenic hydrocarbons. In the present study, we examine several aspects of the effect of aerosol acidity on the secondary organic carbon formation from the photooxidation of 1,3-butadiene, and extend the previous analysis of isoprene. The photooxidation products measured in the absence and presence of acidic sulfate aerosols were generated either through photochemical oxidation of SO2 or by nebulizing mixtures of ammonium sulfate and sulfuric acid into a 14.5 m3 smog chamber system. The results showed that, like isoprene and β-caryophyllene, 1,3-butadiene SOC yields linearly correlate with increasing acidic sulfate aerosol. The observed acid sensitivity of 0.11% SOC increase per nmol m−3 increase in H+ was approximately a factor of 3 less than that measured for isoprene. The results also showed that the aerosol yield decreased with increasing humidity for both isoprene and 1,3-butadiene, although to different degrees. Increasing the absolute humidity from 2 to 12 g m−3 reduced the 1,3-butadiene yield by 45% and the isoprene yield by 85%.

Elemental and ionic components of atmospheric aerosols and associated gaseous pollutants in and near Dar es Salaam, Tanzania

Elemental and water-soluble ionic compounds (WSICs) of atmospheric aerosols (total suspended particulate – TSP) and some gaseous pollutants (SO2, NO2 and O3) from a coastal, semi-urban and rural site in and near Dar es Salaam, Tanzania were investigated during dry and wet seasons of January 2005–November 2007. Na+, Ca2+, SO42−, NO3− and Cl− made up the dominant fraction of WSICs during the dry season with average concentrations ranging from non-detectable (n.d.)–5.4, 0.26–2.6, 0.74–14.7, 0.4–1.5 and 1.1–3.4 μg m−3, respectively, while in the wet season, from n.d. up to 1.7, 1.2, 4.4, 2.1 and 3.0 μg m−3, respectively. The total air concentrations of the detected elements (Al, Si, S, Cl, K, Ca, Fe and Zn) showed seasonal and site-specific variation in the range of 7.5–26.6 with an average of 14.5 μg m−3. Most of the air concentrations of pollutants were observed to decrease with increasing distance from the coastal site, which is under urban and industrial pollutant emissions. Sulphur and nitrogen oxidation ratios during the dry season ranged from 0.08 to 0.91 and 0.013 to 0.049, respectively, while they were between 0.09–0.65 and 0.002–0.095, respectively, in the wet season. These values indicate the photochemical oxidation of SO2 and a high extent of NO3−formation in the atmosphere. Neutralization ratios revealed the presence of acidic SO42− and NO3− aerosols. Principal component analysis identified sea spray, local combustion, vehicular traffic, biomass burning and re-suspended road dust as dominant sources of aerosols at the studied coastal and semi-urban sites. However, at the rural site, besides sea spray, crustal sources, soil dust re-suspension and long-range transport are the possible origins of suspended particulates.

Particle growth in urban and industrial plumes in Texas

Particle size distributions and gas‐phase particle precursors and tracer species were measured aboard an aircraft in the plumes downwind from industrial and urban sources in the vicinity of Houston, TX during the daytime in late August and early September 2000. Plumes originating from the Parish gas‐fired and coal‐fired power plant, petrochemical industries along the Houston ship channel, the petrochemical facilities near the Gulf coast, and the urban center of Houston were studied. Most of the particle mass flux advected downwind of Houston came from the industries and electrical utilities at the periphery of the city rather than from sources in the urban core. In SO2‐rich plumes that did not contain elevated concentrations of volatile organic compounds (VOCs), particle volume increased with increasing plume oxidation (age) at a rate consistent with condensation and neutralization of the gas‐phase oxidation products of SO2. In plumes that were rich in both SO2 and VOCs, observed particle growth greatly exceeded that expected from SO2 oxidation, indicating the formation of organic particulate mass. In plumes that were enhanced in VOCs but not in SO2, and in the plume of the Houston urban center, no particle volume growth with increasing plume oxidation was detected. Since substantial particle volume growth was associated only with SO2‐rich plumes, these results suggest that photochemical oxidation of SO2 is the key process regulating particle mass growth in all the studied plumes in this region. However, uptake of organic matter probably contributes substantially to particle mass in petrochemical plumes rich in both SO2 and VOCs. Quantitative studies of particle formation and growth in photochemical systems containing nitrogen oxides (NOx = NO + NO2), VOCs, and SO2 are recommended to extend those previously made in NOx–VOC systems.

The influence of light intensity, SO2, NOx and C3H6 on sulfate aerosol formation

The photochemical oxidation of SO2 in the presence of NO and C3H6 was studied in a 18.2 liter pyrex reactor. When light intensity, irradiation time and SO2 concentration were constant, SO42- concentration, derived from the total volume of aerosol produced, peaked when [C3H6]/[NO] was approximately 6.0. Another increase im SO42- formation was reached at very high ratios (>50). The experimental observations are consistent with the two SO2 oxidation mechanisms. At low [C3H6]/[NO] ratios, the processes proceed via the HO−SO2 reaction, while at high ratios the O3−C3H6 adduct is assumed to oxidize SO2 to produce SO42- aerosols.

Photochemical aerosol formation from SO2: A theoretical analysis of smog chamber data

A simplified theoretical model is proposed for evaluating the dynamic behavior of photochemically generated aerosols in the SO2—air system. The key simplifying assumption is that evaporation rates are negligible compared with condensation rates for molecular clusters and particles of all sizes. The theory accounts for the formation of condensable molecules (monomer) by chemical reaction, and coagulation of these monomer with clusters of condensed molecules to form larger particles. Cluster coagulation is also included. For a constant rate of monomer production, the equations can be cast in a dimensionless form which is independent of the rate of monomer formation by chemical reaction. These dimensionless equations have been solved numerically, both including and neglecting the effect of London—van der Waals forces on the coagulation rate. Agreement between theory and the data ofW. E. Clark (“Measurements of Aerosols Produced by the Photochemical Oxidation of SO2 in Air,” Ph.D. thesis, University of Minnesota, Minneapolis, 1972) and G. Madelaine, M. Perrin, and A. Renoux (J. Aerosol Sci. 12, 202 (1979)) is good, particularly when London—van der Waals forces are considered. Because the solutions are presented in dimensionless form, direct application of these results for arbitrary rates of aerosol formation and for aerosols with arbitrary physical properties is possible.

The size distribution and interaction of radioactive and natural aerosols in the stratosphere

Artificial radioactivity which persists in the stratosphere on a time scale of years is shown to be associated with particles below 0.02 μ radius above 27 km and with particles very nearly 0.1 μ radius between 21 km and the tropopause. Assuming the artificial radioactivity to be associated with natural aerosols at each level, the radioactive particle size data provide insight on the size distribution of micrometeorites and other particulates in the upper stratosphere and their interaction with sulfate particles in the lower stratosphere. The decrease in specific radioactivity with increased sulfate particle radius near 20 km suggests that the photochemical oxidation of SO2 may not be the important mechanism for large particle formation at this level. Chemical factors and residence time considerations support the view that Aitken nuclei in the upper troposphere are sulfate particles of sufficient size and population to account for the production of large sulfate particles near and above the tropopause by Aitken nuclei coagulation. Approximate estimation of the total sulfate mixing ratio with altitude above 5 km indicates a broad maximum in the upper troposphere. Photochemical and radiochemical evidence for the rapid oxidation of SO2 in the stratosphere reinforces these views. It is tentatively concluded that Aitken particles in the troposphere account for most of the sulfate in the atmosphere and that there is no stratospheric sulfate layer but only a stratospheric “large particle” layer. DOI: 10.1111/j.2153-3490.1966.tb00261.x

The size distribution and interaction of radioactive and natural aerosols in the stratosphere

Artificial radioactivity which persists in the stratosphere on a time scale of years is shown to be associated with particles below 0.02 μ radius above 27 km and with particles very nearly 0.1 μ radius between 21 km and the tropopause. Assuming the artificial radioactivity to be associated with natural aerosols at each level, the radioactive particle size data provide insight on the size distribution of micrometeorites and other particulates in the upper stratosphere and their interaction with sulfate particles in the lower stratosphere. The decrease in specific radioactivity with increased sulfate particle radius near 20 km suggests that the photochemical oxidation of SO2 may not be the important mechanism for large particle formation at this level. Chemical factors and residence time considerations support the view that Aitken nuclei in the upper troposphere are sulfate particles of sufficient size and population to account for the production of large sulfate particles near and above the tropopause by Aitken nuclei coagulation. Approximate estimation of the total sulfate mixing ratio with altitude above 5 km indicates a broad maximum in the upper troposphere. Photochemical and radiochemical evidence for the rapid oxidation of SO2 in the stratosphere reinforces these views. It is tentatively concluded that Aitken particles in the troposphere account for most of the sulfate in the atmosphere and that there is no stratospheric sulfate layer but only a stratospheric “large particle” layer.DOI: 10.1111/j.2153-3490.1966.tb00261.x