Influence of particle size and chemistry on the cloud nucleating properties of aerosols

The ability of an aerosol particle to act as a cloud condensation nuclei (CCN) is a function of the size of the particle, its composition and mixing state, and the supersaturation of the cloud. In-situ data from field studies provide a means to assess the relative importance of these parameters. During the 2006 Texas Air Quality – Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS), the NOAA RV <i>Ronald H. Brown</i> encountered a wide variety of aerosol types ranging from marine near the Florida panhandle to urban and industrial in the Houston-Galveston area. These varied sources provided an opportunity to investigate the role of aerosol sources and chemistry in the potential activation of particles to form cloud droplets. Measurements were made of CCN concentrations, aerosol chemical composition in the size range relevant for particle activation in warm clouds, and aerosol size distributions. Variability in aerosol composition was parameterized by the mass fraction of Hydrocarbon-like Organic Aerosol (HOA) for particle diameters less than 200 nm (vacuum aerodynamic). The HOA mass fraction in this size range was lowest for marine aerosol and highest for aerosol sampled close to anthropogenic sources. Combining all data from the experiment reveals that composition (defined by HOA mass fraction) explains 40% of the variance in the critical diameter for particle activation at the instrumental supersaturation (<i>S</i>) of 0.44%. Correlations between HOA mass fraction and aerosol mean diameter show that these two parameters are essentially independent of one another for this data set. We conclude that, based on the variability of the HOA mass fraction observed during TexAQS-GoMACCS, variability in particle composition played a significant role in determining the fraction of particles that could activate to form cloud droplets. Using a simple model based on Köhler theory and the assumption that HOA is insoluble, we estimate the degree to which calculated CCN concentrations are under- or overestimated if the variability in the HOA mass fraction that was observed during TexAQS-GoMACCS is neglected. The percent under- or overestimation in the CCN concentration is related to the source of the aerosol. Relative to the mean HOA mass fraction of 0.4±0.2 (average ±1σ standard deviation) for the entire experiment, CCN concentrations are underestimated by up to 50% (at 0.22% <i>S</i>) for aerosol sampled far from anthropogenic source regions as it had a lower HOA mass fraction and overestimated by up to 50% for organic-rich aerosol sampled near the source as it had a higher HOA mass fraction