Laboratory studies of SOA CCN activity as a function of oxygen-to-carbon ratio for a range of organic precursors

Description

Currently the greatest challenge for climate-predictive models is the reliable characterization of climate forcing effects of organic aerosols that constitute 20% to 90% of the ambient particle mass. The difficulty of characterizing such aerosols is due to their chemical and physical complexity and to the incomplete understanding of multi-step processes such as oxidative aging. Specifically, oxidative aging affects the cloud-forming potential (CCN activity) of the aerosol. Inadequate representation of CCN activity and resulting effects on cloud albedo and cloud lifetime are a particularly large source of uncertainty in climate models. We present results of laboratory experiments with secondary organic aerosol (SOA) produced by oxidation of both gas-phase and particle-phase precursors. Ten gas-phase and two particle-phase precursors representing atmospherically relevant biogenic and anthropogenic sources were studied. SOA particles were generated via controlled exposure of precursors to the radicals OH and/or O3 in a flow reactor. The precursors were exposed to oxidants equivalent to 0.3–15 days of atmospheric aging. The goal was to generate SOA with similar chemical characteristics to those measured in the field. Chemical composition and CCN activity of the SOA particles were measured as a function of oxidant exposure. The oxygen-to-carbon (O/C) ratio as measured by an aerosol mass spectrometer (AMS) ranged from 0.01–1.4, which encompasses the O/C ratio measured in the field. The CCN activity of the SOA was characterized in the form of the hygroscopicity parameter, κ. In the figure, κ is plotted as a function of O/C ratio. As is evident, the CCN activity is positively correlated with SOA chemical composition (i.e. O/C). A best-fit straight line is shown in the figure. This result suggests that given a measured O/C ratio, CCN activity (κ) may be predicted to within ±20% for organic aerosol with O/C = 0.4–1.4. Measurements relating SOA hydrogen-to-carbon (H/C) to the O/C ratio as a function of OH exposure will also be presented.