Cloud activation properties of organic aerosols observed at urban sites during CALNEX-LA and CARES

 
Poster PDF

Authors

Jian Wang — Washington University in St. Louis
Qi Zhang — University of California, Davis
Jose-Luis Jimenez — University of Colorado
Fan Mei — Pacific Northwest National Laboratory

Category

Aerosol Properties

Description

Atmospheric aerosols strongly influence the global energy budget. Currently, the indirect effects of aerosols remain the most uncertain components in forcing of climate change over the industrial period. This large uncertainty is in part due to our incomplete understanding of the ability of aerosol particles to form cloud droplets under climatically relevant supersaturations. During two recent field campaigns, size-resolved cloud condensation nuclei (CCN) spectrum and aerosol chemical composition were characterized at urban supersites in Pasadena, California (CALNEX-LA) and Cool, California (CARES) in summer 2010. At both sites, monodispersed aerosol particles were first classified using a differential mobility analyzer at sizes ranging from 25 to 320 nm. The activation efficiency of the classified aerosol, defined as the ratio of its CCN concentration (characterized by a DMT CCN counter) to total CN concentration (measured by a condensation particle counter, TSI 3771), is derived as a function of both particle size and supersaturation, which ranges from 0.08% to 0.39% during CAlNEX-LA and 0.15% to 0.45% during CARES. Aerosol chemical composition was characterized using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). At both sites, increases in aerosol mode diameter, organics mass loading, and aerosol organics volume fraction were often observed from ~10:00 AM to 16:00 PM during weekdays. Some of these increases are attributed to the morning traffic pollution from downtown Los Angeles and downtown Sacramento, respectively. Positive matrix factorization (PMF) analyses of AMS measurements at both sites were carried out, and the organics O:C ratios were examined. Particle overall hygroscopicity (κ, Petters and Kreidenweis, 2007, ACP) was derived from the size-resolved CCN measurements ranging from 0.2 to 0.3 under the range of measured supersaturations. The derived particle κ increases with increasing particle diameter, which is consistent with observed decrease in organics volume fraction as particle size increases from 100 nm to 300 nm. Based on the particle hygroscopicity and aerosol chemical composition, the organics hygroscopicity (κOrg) was derived and correlated with the O:C ratio. The comparison of aerosol source, aerosol chemical composition, and organics hygroscopicity between the two sites will be discussed. Besides particle critical supersaturation, the influence of organics species on droplet growth is also examined.