Identification of sources and formation processes of atmospheric sulfate by sulfur isotope and scanning electron microscope measurements

[1] Atmospheric sulfate aerosols have a cooling effect on the Earth’s surface and can change cloud microphysics and precipitation. China has heavy loading of sulfate, but their sources and formation processes remain uncertain. In this study we characterize possible sources and formation processes of atmospheric sulfate by analyzing sulfur isotope abundances (S, S, 34S, and S) and by detailed X‐ray diffraction and scanning electron microscope (SEM) imaging of aerosol samples acquired at a rural site in northern China from March to August 2005. The comparison of SEM images from coal fly ash and the atmospheric aerosols suggests that direct emission from coal combustion is a substantial source of primary atmospheric sulfate in the form of CaSO4. Airborne gypsum (CaSO4·2H2O) is usually attributed to eolian dust or atmospheric reactions with H2SO4. SEM imaging also reveals mineral particles with soot aggregates adhered to the surface where they could decrease the single scattering albedo of these aerosols. In summer months, heterogeneous oxidation of SO2, derived from coal combustion, appears to be the dominant source of atmospheric sulfate. Our analyses of aerosol sulfate show a seasonal variation in DS (DS describes either a S excess or depletion relative to that predicted from consideration of classical mass‐dependent isotope effects). Similar sulfur isotope variations have been observed in other atmospheric samples and in (homogenous) gas‐phase reactions. On the basis of atmospheric sounding and satellite data as well as a possible relationship between DS and Convective Available Potential Energy (CAPE) during the sampling period, we attribute the sulfur isotope anomalies (DS and D36S) in Xianghe aerosol sulfates to another atmospheric source (upper troposphere or lower stratosphere).