"Hygroscopic Growth of Submicron and Supermicron Aerosols in the Marine Boundary Layer"

We investigate hygroscopic growth of marine aerosols from three research cruises: TexAQS-GoMACCS 2006, ICEALOT 2008 and CalNex 2010. Particle hygroscopic growth was characterized by measuring the effect of water uptake under sub-saturated conditions on the aerosol light extinction at 532 nm. Mie theory calculations were utilized to convert the observed optical growth factors (fext(RH)) into physical growth factors (GF) at 85% RH. GF is found to be a more robust measure of aerosol hygroscopic growth than fext(RH), which can be biased by changes in aerosol dry size. Consistent with previous observations, the overall GF(85%) for submicron aerosol depended on the fraction of organics. The submicron GFOM(85%) specifically was found to range from 1.0-1.3 for all three campaigns. A robust positive linear dependence of the overall supermicron GF(85%) on the mass fraction of sea salt was observed. During TexAQS, two types of dust particles with distinct hygroscopic properties were identified in the supermicron mode; one that originated from the Sahara desert was moderately hygroscopic (GFdust(85%) = ~1.4) and the other from continental sources was nearly hydrophobic. The GF(85%) of supermicron organics was estimated through hygroscopicity closure calculations. Supermicron organics that originated from marine sources were found to be substantially more hygroscopic than those from continental sources, with the latter having a GF(85%) similar to that of the submicron organics. This study demonstrates the potential of using aerosol optical measurements to retrieve hygroscopic growth factor and underlines the importance and need for future investigations on the hygroscopic properties of marine supermicron aerosols.