Evolution of refractory black carbon mixing state in an urban environment.

The size distribution and hygroscopicity of the refractory black carbon (rBC) aerosol are critical properties for understanding its impact on human health, the extent to which the particles disperse through the atmosphere, and the effect that the particles have on clouds and the climate system. The hygroscopicity of black carbon is primarily controlled by the degree to which the black carbon cores are coated with other compounds. Here we show new measurements of rBC obtained from a single particle soot photometer at an urban location in Houston, TX, USA during the Department of Energy TRacking Aerosol Convection Interactions ExpeRiment (TRACER) campaign. We classified rBC particles into uncoated, thinly coated and thickly coated, where the latter is defined as the optical coating thickness exceeding 10 nm. To understand how aerosol aging and the prevailing winds interact to affect rBC coatings, a slice of the size distribution with rBC core diameter 143–166 nm is considered. For this slice, the average coating thickness of thickly coated particles was 29 ± 8 nm. The rBC number concentration, the fraction of thinly and thickly coated particles, and the average coating thickness strongly vary with the wind direction. We found that onshore flows are associated with lower number concentrations, more thinly coated particles, and thinner coatings when compared to offshore flows. Diurnal profiles show that the fraction of coated particles increases during daytime, likely due to photochemical aging. The estimated fraction of rBC particles that may activate into cloud droplets at 0.1% and 1% water supersaturation was generally ∼0.6% and ∼9%, respectively. This suggests that substantial further aging is needed to precondition the majority of rBC particles for incorporation into cloud droplets. • Onshore flows are associated with lower concentration and thinner coatings. • Photochemical aging increases the fraction of coated particles during daytime. • Fraction of black carbon particles that may activate into cloud droplets is <9%. [ABSTRACT FROM AUTHOR]