(341b) Laboratory Measurements of Cloud Condensation Nuclei Activation, Volatility and Growth Kinetics

Understanding cloud condensation nuclei (CCN) activation is a vital step towards decreasing the uncertainty in the aerosol indirect effect on climate. In this presentation we will present work characterizing the CCN properties of anthropogenic and biogenic secondary organic aerosols (SOA) produced in the Carnegie Mellon University smog chamber. The smog chamber is a 10 cubic meter temperature-controlled reactor outfitted with ultraviolet light bulbs and instrumentation to monitor gas-phase and particle-phase species. Biogenic precursors included isoprene, the most abundant biogenic non-methane volatile organic carbon, beta-caryophyllene, and a mixure of monoterpenes. Toluene was selected as a model anthropogenic system. SOA was formed from homogeneous nucleation initiated by oxidation. Oxidants included ozone in the dark and hydroxyl radicals from either HONO or HOOH subjected to ultraviolet radiation. CCN concentrations, activation diameter and droplet growth kinetic information were monitored as a function of supersaturation for several hours using a cylindrical continuous-flow streamwise thermal gradient CCN counter developed by Droplet Measurement Technologies (DMT). Some experiments also included activation diameter measurements from a static diffusion CCN counter from DH Associates. We will summarize the findings for CCN activity, aging, and droplet growth kinetics. The hygroscopicity parameter, kappa will allow us to compare across systems while Kohler theory analysis will allow us to highlight some pertinent details of the system CCN activity. All of the CCN systems studied are moderately CCN-active organic compounds.

Filter samples were collected during a number of experiments in order to perform offline CCN analysis by extracting the water soluble organic carbon (WSOC) collected and re-atomizing it into the DMT CCN counter. Köhler Theory Analysis (KTA) is then used to infer the molar mass of the SOA sampled online and offline from atomized filter samples. Assuming complete solubility, the estimated average molar mass of online monoterpene SOA was determined to be 180 plus or minus 55 grams per mole. This value is consistent with existing monoterpene SOA speciation studies. KTA suggests that the aged aerosol (both from alpha-pinene and the mixed monoterpene oxidation) is primarily water-soluble (around 65 percent). CCN activity measurements of the monoterpene SOA mixed with ammonium sulfate suggest that the organic can depress surface tension up to 10 milliNewtons per meter (with respect to pure water). The droplet growth kinetics of monoterpene SOA samples are similar to ammonium sulfate. The CCN activation diameter of alpha-pinene and mixed monoterpene SOA can be modelled to within 10-15 percent of experiments by a simple implementation of Koehler theory, assuming complete dissolution of the particles, no dissociation into ions, a molecular weight of 180 grams per mole, a density of 1.5 grams per cubic centimeter, and the surface tension of water. Isoprene SOA and toluene SOA display similar activation diameters as observed in monoterpene SOA. Finding a small range of activation diameters or a single kappa value for a broad array of precursors would be quite useful for global models and studies of the indirect effect of aerosols on climate.

The model sesquiterpene system, beta-caryophyllene, shows that the CCN contain small amounts of hygroscopic material. Although beta-caryophyllene produces higher yields than monoterpene systems the aerosols are less effective as CCN in the atmosphere. The two CCN counters used in this study have fundamentally different methods for supersaturation generation. The static CCN counter relies upon the nonlinear dependence of water vapor pressure upon temperature to generate supersaturation whereas the DMT CCN counter exploits the higher diffusivity of water than that of heat. The result of these unique methods is that the DMT CCN counter heats the particles along the length of the column causing a small amount of the particle mass to evaporate whereas the static CCN counter cools the particles. A major finding in this work is that the hygroscopic component of beta-caryophyllene SOA is volatile. This leads to discrepancies between CCN instrumentation not observed in monoterpene systems. The finding is potentially quite important for atmospheric measurements impacted by diurnal temperature cycles and thus, potentially, CCN activity diurnal cycles. Evidence of a kinetic barrier to droplet activation was revealed from a comparison of droplet growth kinetics of the CCN and the fraction of WSOC. Filter samples show that the WSOC fraction of the SOA is composed of low molecular weight (less than 200 grams per mole) compounds that are slightly surface active. This is similar to the findings for monoterpene SOA systems indicating that exact speciation knowledge might not be required for predictive understanding of CCN.