Black carbon linked aerosol hygroscopic growth: Size and mixing state are crucial

Abstract

The objective of this study is to characterise urban refractory black carbon (rBC) mixing state relative to hygroscopic growth factor (HGF) of size selective aerosols to better constrain the aerosol indirect effect. The Aitken mode range (≤100 nm) is dominated by particles that have relatively low hygroscopicity and comprised freshly emitted hydrophobic rBC, however, our observations suggest that BC mixing states in Aitken range (down to 70 nm) still govern the hygroscopic properties. Conversely, the accumulation mode range (>100 nm) dominated by particles that have relatively high hygroscopicity consisted of oxidized organic compounds and inorganic salts. Single particle soot photometer (SP2) measurement further revealed that particles at lower size are mostly incandescent type dominated by refractory component whereas higher size particles are mostly scattering type dominated by nonrefractory component. The lower hygroscopicity parameter (κ) (0.26 ± 0.08) obtained for Aitken mode particles suggests that they may contain levoglucosan and levoglucosan-OH oxidation products, and are possibly from biomass burning sources whereas accumulation range particles with higher κ value (0.39 ± 0.03) have ammonium sulfate, ammonium bisulfate and malonic acid in their composition and their possible source would be secondary in origin. These findings are important because for the first time, BC mixing state and the impact of size selective rBC on HGF are determined directly over an urban region which have implications to precipitation.