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

Abstract

[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 ( 32 S, 33 S, 34 S, and 36 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 CaSO 4 . Airborne gypsum (CaSO 4 ·2H 2 O) is usually attributed to eolian dust or atmospheric reactions with H 2 SO 4 . 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 SO 2 , derived from coal combustion, appears to be the dominant source of atmospheric sulfate. Our analyses of aerosol sulfate show a seasonal variation in D 33 S (D 33 S describes either a 33 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 D 33 S and Convective Available Potential Energy (CAPE) during the sampling period, we attribute the sulfur isotope anomalies (D 33 S and D 36 S) in Xianghe aerosol sulfates to another atmospheric source (upper troposphere or lower stratosphere).Citation: Guo, Z., Z. Li, J. Farquhar, A. J. Kaufman, N. Wu, C. Li, R. R. Dickerson, and P. Wang (2010), Identification of sources and formation processes of atmospheric sulfate by sulfur isotope and scanning electron microscope measurements,

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“…8a), which indicates that the sulfate was derived from different sources or formation processes (Guo et al, 2010). It has been reported that sea salt sulfate is characterized by a d 34 S value of þ21‰ (Tostevin et al, 2014).…”

“…Sulfur from coal combustion is an important source for atmospheric sulfur in China (Mukai et al, 2001;Guo et al, 2010). It is reported that the d 34 S values of atmospheric sulfate in different regions of China are associated with local coal combustion (Mukai et al, 2001).…”

“…The remaining solution was acidified with HCl to a pH to <2, heated up to boiling, and the dissolved sulfate was precipitated as barium sulfate by adding 25 ml of 8.5% BaCl 2 solution. The solution was kept at 80 C for 3 h. Thereafter, 0.22 mm acetate millipore filters were used to collect the barium sulfate precipitate, which was rinsed with 150 ml Milli-Q water (Guo et al, 2010). The precipitates were dried in an oven at 45 C for 48 h and later analyzed for their sulfur and oxygen isotopic compositions.…”

“…However, the sources and formation processes of PM 2.5 in the atmosphere are not well constrained and differ from region to region. Hence, detailed investigations of the chemical and physical characteristics of PM 2.5 advance our understanding of their sources and formation mechanisms (Yao et al, 2002;Guo et al, 2010; R.J. .…”

“…Guo et al, 2016). Many investigations have been conducted in Europe (Sinha et al, 2008), China (Mukai et al, 2001;Guo et al, 2010;, Japan (Sakata et al, 2013) and Antarctica (Kunasek et al, 2010) to trace atmospheric sulfate sources using sulfur isotopes. Recently, Z. investigated the sulfur isotopic composition of PM 2.5 in Nanjing and identified the major sulfur sources of PM 2.5 .…”