posted on 2007-05-03, 00:00authored byJ. E. Shilling, S. M. King, M. Mochida, D. R. Worsnop, S. T. Martin
Oxidative processing (i.e., “aging”) of organic aerosol particles in the troposphere affects their cloud
condensation nuclei (CCN) activity, yet the chemical mechanisms remain poorly understood. In this study,
oleic acid aerosol particles were reacted with ozone while particle chemical composition and CCN activity
were simultaneously monitored. The CCN activated fraction at 0.66 ± 0.06% supersaturation was zero for
200 nm mobility diameter particles exposed to 565 to 8320 ppmv O3 for less than 30 s. For greater exposure
times, however, the particles became CCN active. The corresponding chemical change shown in the particle
mass spectra was the oxidation of aldehyde groups to form carboxylic acid groups. Specifically, 9-oxononanoic
acid was oxidized to azelaic acid, although the azelaic acid remained a minor component, comprising 3−5%
of the mass in the CCN-inactive particles compared to 4−6% in the CCN-active particles. Similarly, the
aldehyde groups of α-acyloxyalkylhydroperoxide (AAHP) products were also oxidized to carboxylic acid
groups. On a mass basis, this conversion was at least as important as the increased azelaic acid yield. Analysis
of our results with Köhler theory suggests that an increase in the water-soluble material brought about by the
aldehyde-to-carboxylic acid conversion is an insufficient explanation for the increased CCN activity. An
increased concentration of surface-active species, which decreases the surface tension of the aqueous droplet
during activation, is an interpretation consistent with the chemical composition observations and Köhler theory.
These results suggest that small changes in particle chemical composition caused by oxidation could increase
the CCN activity of tropospheric aerosol particles during their atmospheric residence time.