In-cloud sulfate addition to single particles resolved with sulfur isotope analysis during HCCT-2010

Abstract

Abstract. In-cloud production of sulfate modifies aerosol size distribution, with important implications for the magnitude of indirect and direct aerosol cooling and the impact of SO 2 emissions on the environment. We investigate which sulfate sources dominate the in-cloud addition of sulfate to different particle classes as an air parcel passes through an orographic cloud. Sulfate aerosol, SO 2 and H 2 SO 4 were collected upwind, in-cloud and downwind of an orographic cloud for three cloud measurement events during the Hill Cap Cloud Thuringia campaign in autumn 2010 (HCCT-2010). Combined SEM and NanoSIMS analysis of single particles allowed the δ 34 S of particulate sulfate to be resolved for particle size and type.The most important in-cloud SO 2 oxidation pathway at HCCT-2010 was aqueous oxidation catalysed by transition metal ions (TMI catalysis), which was shown with single particle isotope analyses to occur primarily in cloud droplets nucleated on coarse mineral dust. In contrast, direct uptake of H 2 SO 4 (g) and ultrafine particulate were the most important sources modifying fine mineral dust, increasing its hygroscopicity and facilitating activation. Sulfate addition to "mixed" particles (secondary organic and inorganic aerosol) and coated soot was dominated by in-cloud aqueous SO 2 oxidation by H 2 O 2 and direct uptake of H 2 SO 4 (g) and ultrafine particle sulfate, depending on particle size mode and time of day. These results provide new insight into in-cloud sulfate production mechanisms, and show the importance of single particle measurements and models to accurately assess the environmental effects of cloud processing.

Influence of cloud processing on CCN activation behaviour in the Thuringian Forest, Germany during HCCT-2010

“…The isotopic analyses showed that the sulfate content of particles increased following cloud processing during HCCT-2010 by > 10-40 % depending on particle type (cf. Table 5 in Harris et al, 2014). Consistent with our results of increased hygroscopicity, both offline (impactor) and online (aerosol mass spectrometer) measurements of the chemical aerosol composition during HCCT often indicate an increased mass fraction of sulfate in aerosol particles after their passage through a cloud (van Pinxteren, Poulain, D'Anna, personal communications, 2013, data yet to be published in forthcoming companion papers of this special issue).…”

“…This estimate is supported by measurement results from other groups during HCCT-2010 who focused on the chemical and isotopic signatures of the particle populations; for example, sulfur isotope analysis of the particulate material was used to investigate the in-cloud production of sulfate. Combined gas-phase and single-particle measurements allowed the dominating sulfate production sources to be identified (Harris et al, 2014). Direct sulfate uptake, through dissolution of H 2 SO 4 gas and scavenging of ultrafine particulate, as well as in-cloud aqueous SO 2 oxidation by H 2 O 2 , Fig.…”

“…Using previous measurements of sulfur isotope fractionation factors characteristic of different oxidation pathways, e.g. oxidation by OH, H 2 O 2 or transition metal ion catalysis (Harris et al, 2012(Harris et al, , 2013a, the isotopic analyses made during HCCT-2010 allow dominant sulfate production pathways to be determined and resolved for different particle types, as described in Harris et al (2014).…”

In situ chemical composition measurement of individual cloud residue particles at a mountain site, southern China

“…Although the in-cloud addition of sulfate can be produced from aqueous Fe-catalyzed or oxidation by H 2 O 2 /O 3 reactions (Harris et al, 2014), sulfate abundance was found in the Fe cloud residues relative to nonactivated particles, but no enhancement relative to ambient particles was found. Previous studies also showed that the mass or NF of sulfate-containing particles in the cloud residues changed between ambient and nonactivated particles (Drewnick et al, 2007;Twohy and Anderson, 2008;Schneider et al, 2017).…”

“…The nitrate-containing particles accounted for only 46 % of the aged EC cloud residues, which is significantly less than the sulfate-containing particles. Previous field studies have found that aged EC (soot) fog/cloud residues are mainly internally mixed with sulfate (Pratt et al, 2010a;Harris et al, 2014;Bi et al, 2016). Aged EC particles mixed with sulfate are good CCN (Bi et al, 2016;Roth et al, 2016).…”