AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Hygroscopic Growth of Super-micron Particles in the Coastal Marine Atmosphere
XIAOLU ZHANG, Christopher Cappa, Paola Massoli, Patricia Quinn, Timothy Bates, University of California, Davis
Abstract Number: 207 Working Group: Aerosols, Clouds, and Climate
Abstract Aerosol hygroscopicity is a key factor in determining the direct aerosol radiative forcing of Earth’s climate, as much of the aerosol light scattering is attributable to water uptake by aerosols at high RH. Here we investigate the hygroscopic growth of aerosols in the marine atmosphere using optical measurements made during the TexAQS-GoMACCCS in 2006. Bulk sub-saturated growth factors (GF) for both sub- and super-micron aerosols were derived from light extinction coefficients at 532 nm under dry and elevated RHs measured with a cavity ring-down aerosol extinction spectrometer. The calculated growth factors at 85% RH for sub- and super-micron aerosols were averaged at 1.58(±0.14) and 1.71(±0.18), respectively. Variability in super-micron GFs (GFsup) during the campaign appeared to be driven by changes in the chemical composition of super-micron aerosols. Sea-salt and dust are the dominant components, together accounting for 78% on average of the dry super-micron aerosol mass. GFsup was well correlated with the volume fraction of sea-salt aerosols for most of the study, with the exception of when substantial contribution from organics was observed. Two types of dust with distinct hygroscopicities were identified based on their sources and composition. The dust aerosols that originated from North Africa were somewhat hygroscopic, with an average GF of ~1.34, whereas dust from local sources was less hygroscopic, with an average GF ~1.0. A hygroscopic closure calculation based on a volume-weighted mixing rule was performed using literature documented GFs for the non-dust inorganic components and GFs for the two dust types determined in this study. Minimization of the difference between observed and predicted GFs indicated a campaign-averaged GF of 1.59 at 85% RH for the super-micron organics, suggesting super-micron organics may be largely different from sub-micron organics in terms of hygroscopicity. This study provides new insights on hygroscopicity of super-micron aerosols in the marine atmosphere.