Multiple Sulfur Isotope Constraints on Sources and Formation Processes of Sulfate in Beijing PM_2.5 Aerosol

Abstract

Recently air pollution is seriously threatening the health of millions of people in China. The multiple sulfur isotopic composition of sulfate in PM samples collected in Beijing is used to better constrain potential sources and formation processes of sulfate aerosol. The ΔS values of sulfate in PM show a pronounced seasonality with positive values in spring, summer and autumn and negative values in winter. Positive ΔS anomalies are interpreted to result from SO photolysis with self-shielding, and may reflect air mass transport between the troposphere and the stratosphere. The negative ΔS signature (-0.300‰ < ΔS < 0‰) in winter is possibly related to incomplete combustion of coal in residential stoves during the heating season, implying that sulfur dioxide released from residential stoves in more rural areas is an important contributor to atmospheric sulfate. However, negative ΔS anomalies (-0.664‰ < ΔS ← 0.300‰) in winter and positive ΔS anomalies (0.300‰ < ΔS < 0.480‰) in spring, summer, and autumn suggest sulfur isotopic equilibrium on an annual time frame, which may provide an implication for the absence of mass-independent fractionation of sulfur isotopes (S-MIF) in younger sediments. Results obtained here reveal that reducing the usage of coal and improving the heating system in rural areas will be important for efficiently decreasing the emissions of sulfur in China and beyond.

Seasonality in the Δ&lt;sup&gt;33&lt;/sup&gt;S measured in urban aerosols highlights an additional oxidation pathway for atmospheric SO&lt;sub&gt;2&lt;/sub&gt;

“…These also cannot be explained by the experimentally determined 34 α and 33 α-values currently available in the literature (implicating O2+TMI, H2O2 and OH; Harris et al (2013a) and NO2 Au Yang et al (2018)) or their potential combination that predict Δ 33 S-values centered around 0‰ ; i.e. at odds with available data for urban aerosols (Guo et al, 2010;Han et al, 2017;Romero and Thiemens, 2003;Shaheen et al, 2014;Lin et al, 2018b).…”

“…Previous studies showed that sulfates in urban aerosols display δ 34 S-values from 0 to 20‰ and Δ 33 S-values from -0.6 to 0.5‰ (Guo et al, 2010;Han et al, 2017;Romero and Thiemens, 2003;Shaheen et al, 2014;Lin et al, 2018b). Two main lines of reasoning are usually evoked to explain such S-isotope compositions.…”

Possible heterogeneous chemistry of hydroxymethanesulfonate (HMS) in northern China winter haze

“…Organosulfates, formed through reactions of gaseous organics (e.g., epoxides and aldehydes) and particulate inorganic sulfate, also seem to represent minor contributions to OS based on the following evidence. The most common organosulfates identified previously in ambient aerosols, such as glycolic acid sulfate (C 2 H 3 SO − 6 ), 2methylglyceric acid sulfate (C 4 H 7 SO − 7 ), isoprene epoxydiols sulfate (C 5 H 11 SO − 7 ), and benzyl (or methyl phenyl) sulfate (C 7 H 7 SO − 4 ) (Froyd et al, 2010;Hatch et al, 2011a;Ma et al, 2014), were not detected by SPAMS measurements taken in Beijing winter. The production of organosulfates is enhanced by increased aerosol acidity (Surratt et al, 2010;Hatch et al, 2011b;Riva et al, 2016), whereas the relatively high pH values of about 4 to about 5 for winter haze aerosols in northern China may represent a limiting factor .…”

“…The negative ion peaks at m/z −155, −187, −199, and −215 were also analyzed in order to detect individual organosulfate species. This technique has been employed to measure individual organosulfates in different regions around the world (Hatch et al, 2011a;Wang et al, 2017).…”

Possible heterogeneous hydroxymethanesulfonate (HMS) chemistry in northern China winter haze and implications for rapid sulfate formation

“…For anion detection in IC, pH of the carrier fluid (known as eluent, a solution of KOH, NaOH, NaHCO3, etc.) is usually greater than 9 (Wang et al, 2005;Cao et al, 2012;Liu et al, 2016;Han et al, 2017). HMS is unstable under such an alkaline environment and dissociates rapidly into SO 3 2− and HCHO (Seinfeld and Pandis, 2016) CH 2 (OH)SO 3 − + OH − → HCHO…”