Hygroscopic growth of submicron and supermicron aerosols in the marine boundary layer

We investigate hygroscopic growth of marine aerosols from three research cruises: Texas Air Quality Study-Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS) 2006, International Chemistry Experiment in the Arctic Lower Troposphere (ICEALOT) 2008, and California Research at the Nexus of Air Quality and Climate Change (CalNex) 2010. Particle hygroscopic growth was characterized by measuring the effect of water uptake under subsaturated conditions on the aerosol light extinction at 532 nm. Mie theory calculations were utilized to convert the observed optical growth factors (f(ext)(RH)) into physical growth factors (GF) at 85% RH. GF is found to be a more robust measure of aerosol hygroscopic growth than f(ext)(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 GF(OM)(85%) specifically was found to range from 1.0 to 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 (GF(dust)(85%) = similar to 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.