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

International audienceThe 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 Ronald~H.~Brown 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, chemistry, and size in the activation of particles to form cloud droplets. Measurements were made of CCN concentrations, aerosol chemical composition in the size range relevant for particle activation, 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 0.44% supersaturation (S). 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, composition played a significant role in determining the fraction of particles that could activate to form cloud droplets. In addition, we estimate the error that results in calculated CCN concentrations if the HOA mass fraction is neglected (i.e., a fully soluble composition of (NH4)2SO4 is assumed) for the range of mass fractions and mean diameters observed during the experiment. This error is then related to the source of the aerosol. At 0.22 and 0.44% S, the error is considerable (>50%) for anthropogenic aerosol sampled near the source region as this aerosol had, on average, a high HOA mass fraction in the sub-200 nm diameter size range (vacuum aerodynamic). The error is lower for aerosol distant from anthropogenic source regions as it had a lower HOA mass fraction. Hence, the percent error in calculated CCN concentration is larger for organic-rich aerosol sampled near the source and smaller for aerosol sampled away from sources of anthropogenic particulate organic matter (POM)