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

Recently air pollution is seriously threatening the health of millions of people in China. The multiple sulfur isotopic composition of sulfate in PM_2.5 samples collected in Beijing is used to better constrain potential sources and formation processes of sulfate aerosol. The Δ³³S values of sulfate in PM_2.5 show a pronounced seasonality with positive values in spring, summer and autumn and negative values in winter. Positive Δ³³S anomalies are interpreted to result from SO₂ photolysis with self-shielding, and may reflect air mass transport between the troposphere and the stratosphere. The negative Δ³³S signature (-0.300‰ < Δ³³S < 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 Δ³³S anomalies (-0.664‰ < Δ³³S ← 0.300‰) in winter and positive Δ³³S anomalies (0.300‰ < Δ³³S < 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.