Hydrogen peroxide serves as pivotal fountainhead for aerosol aqueous sulfate formation from a global perspective

Traditional atmospheric chemistry posits that sulfur dioxide (SO2) can be oxidized to sulfate (SO42-) through aqueous-phase reactions in clouds and gas-phase oxidation. Despite adequate knowledge of traditional mechanisms, several studies have highlighted the potential for SO2 oxidation within aerosol water. Given the widespread presence of tropospheric aerosols, SO42- production through aqueous-phase oxidation in aerosol water could have a pervasive global impact. Here, we quantify the potential contributions of aerosol aqueous pathways to global sulfate formation based on the GEOS-Chem simulations and subsequent theoretical calculations. Hydrogen peroxide (H2O2) oxidation significantly influences continental regions both horizontally and vertically. Over the past two decades, shifts in the formation pathways within typical cities reveal an intriguing trend: despite reductions in SO2 emissions, the increased atmospheric oxidation capacities, like rising H2O2 levels, prevent a steady decline in SO42- concentrations. Abating oxidants would facilitate the benefit of SO2 reduction and the positive feedback in sulfate mitigation.

Acidity across the interface from the ocean surface to sea spray aerosol

Sea spray aerosol, produced through breaking waves, is one of the largest sources of environmental particles. Once in the atmosphere, sea spray aerosol influences cloud formation, serves as microenvironments for multiphase atmospheric chemical reactions, and impacts human health. All of these impacts are affected by aerosol acidity. Here we show that freshly emitted sea spray aerosol particles become highly acidic within minutes as they are transferred across the ocean−air interface. These results have important implications for atmospheric chemistry and climate, including aerosol/gas partitioning, heterogeneous reactions, and chemical speciation at the surface and within sea spray aerosol.

Aerosols impact climate, human health, and the chemistry of the atmosphere, and aerosol pH plays a major role in the physicochemical properties of the aerosol. However, there remains uncertainty as to whether aerosols are acidic, neutral, or basic. In this research, we show that the pH of freshly emitted (nascent) sea spray aerosols is significantly lower than that of sea water (approximately four pH units, with pH being a log scale value) and that smaller aerosol particles below 1 μm in diameter have pH values that are even lower. These measurements of nascent sea spray aerosol pH, performed in a unique ocean−atmosphere facility, provide convincing data to show that acidification occurs “across the interface” within minutes, when aerosols formed from ocean surface waters become airborne. We also show there is a correlation between aerosol acidity and dissolved carbon dioxide but no correlation with marine biology within the seawater. We discuss the mechanisms and contributing factors to this acidity and its implications on atmospheric chemistry.

Air pollution characteristics and their relationship with emissions and meteorology in the Yangtze River Delta region during 2014-2016

With rapid economic growth and urbanization, the Yangtze River Delta (YRD) region in China has experienced serious air pollution challenges. In this study, we analyzed the air pollution characteristics and their relationship with emissions and meteorology in the YRD region during 2014-2016. In recent years, the concentrations of all air pollutants, except O₃, decreased. Spatially, the PM_2.5, PM₁₀, SO₂, and CO concentrations were higher in the northern YRD region, and NO₂ and O₃ were higher in the central YRD region. Based on the number of non-attainment days (i.e., days with air quality index greater than 100), PM_2.5 was the largest contributor to air pollution in the YRD region, followed by O₃, PM₁₀, and NO₂. However, particulate matter pollution has declined gradually, while O₃ pollution worsened. Meteorological conditions mainly influenced day-to-day variations in pollutant concentrations. PM_2.5 concentration was inversely related to wind speed, while O₃ concentration was positively correlated with temperature and negatively correlated with relative humidity. The air quality improvement in recent years was mainly attributed to emission reductions. During 2014-2016, PM_2.5, PM₁₀, SO₂, NO_x, CO, NH₃, and volatile organic compound (VOC) emissions in the YRD region were reduced by 26.3%, 29.2%, 32.4%, 8.1%, 15.9%, 4.5%, and 0.3%, respectively. Regional transport also contributed to the air pollution. During regional haze periods, pollutants from North China and East China aggravated the pollution in the YRD region. Our findings suggest that emission reduction and regional joint prevention and control helped to improve the air quality in the YRD region.

Characterization of PM2.5 and identification of transported secondary and biomass burning contribution in Seoul, Korea

The chemical and seasonal characteristics of fine particulates in Seoul, Korea, were investigated based on 24-h integrated PM_2.5 measurements made over four 1-month periods in each season between October 2012 and September 2013. The four-season average concentration of PM_2.5 was 37 μg m^-3, and the major chemical components were secondary inorganic aerosol (SIA) species of sulfate, nitrate, and ammonium (49%), followed by organic matter (34%). The mass concentration and most of the chemical components of PM_2.5 showed clear seasonal variation, with a winter-high and summer-low pattern. The winter-to-summer sulfate ratio and the winter organic carbon (OC)-to-elemental carbon (EC) ratio were unusually high compared with those in previous studies. Strong correlations of both the sulfate level and the sulfur oxidation ratio with relative humidity, and between water-soluble OC (WSOC) and SIA in winter, suggest the importance of aqueous phase chemistry for secondary aerosols. A strong correlation between non-sea salt sulfate and Na⁺ levels, a high Cl^-/Na⁺ ratio, and an unusual positive correlation between the nitrogen oxidation ratio and temperature during the winter indicate the influence of transported secondary emission sources from upwind urban areas and from China across the Yellow Sea. Despite the absence of local forest fires and the regulation of wood burning, a high levoglucosan concentration and its correlations with OC and WSOC indicate that Seoul was affected by biomass burning sources in the winter. The unusually high water-insoluble OC (WIOC)-to-EC ratio in winter implies additional transported combustion sources of WIOC. The strong correlation between WIOC and levoglucosan suggests the likely influence of transported biomass burning sources on the high WIOC/EC ratio during the winter.

Synergistic formation of sulfate and ammonium resulting from reaction between SO2 and NH3 on typical mineral dust

A synergistic effect between SO₂ and NH₃ on typical mineral dust.

Presence of the most abundant ionic species and their contribution to PM2.5 mass, in the city of Guadalajara, Jalisco (Mexico)

Ambient PM2.5 samples were taken at 24 h intervals at two sites (Centro and Miravalle) in the city of Guadalajara from January to June 2008. The Centro site is located in the downtown, while the Miravalle site is located in an industrial zone south of the downtown. For both sites the higher concentrations of PM2.5 were between January and May. High correlation coefficients between sulfate, nitrate and ammonium of 0.95, 0.92 and 0.91, respectively, showed low variations in the concentrations of these species in the city. It was estimated that sulfate, nitrate and ammonium represented almost 47% of the PM2.5 mass in June at the Centro site, but in general the contributions in the other months were less than 21%, while at Miravalle this percentage was between 7.7% and 27.6%.

A simple model which predicts some non-linear features between atmospheric sulphur and sulphur emissions

This paper develops a simple model and suggests a plausible chemico-physical mechanism for a non-linear response between atmospheric sulphur and sulphur emissions. It contains simplified representations of transport, deposition and conversion processes and uses a proxy in-cloud oxidant-limited reaction along a pathway connecting an emission source with a receptor site. Individual pathway responses to emissions show linear behaviour above a threshold. However, by averaging the values of SO2 at the receptor site from different pathways a continuous non-linear relationship is obtained. As emissions reduce, distant emission sources become less significant contributors of sulphur dioxide at a receptor site but their emissions are still counted in an emission inventory, leading to an apparent non-linearity. Sulphate is always found to contribute a signal to the receptor site total. This model goes someway to explaining a proposed 'crossover' between observed proportions of wet and dry deposited sulphur in the UK as emissions have been reduced.