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

[摘要] 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 mixingstate, and the supersaturation of the cloud. In-situ data from field studiesprovide a means to assess the relative importance of these parameters.During the 2006 Texas Air Quality – Gulf of Mexico Atmospheric Compositionand Climate Study (TexAQS-GoMACCS), the NOAA RV Ronald H. Brown encountered a wide varietyof aerosol types ranging from marine near the Florida panhandle to urban andindustrial in the Houston-Galveston area. These varied sources provided anopportunity to investigate the role of aerosol sources and chemistry in thepotential activation of particles to form cloud droplets. Measurements weremade of CCN concentrations, aerosol chemical composition in the size rangerelevant for particle activation in warm clouds, and aerosol sizedistributions. Variability in aerosol composition was parameterized by themass fraction of Hydrocarbon-like Organic Aerosol (HOA) for particlediameters less than 200 nm (vacuum aerodynamic). The HOA mass fraction inthis size range was lowest for marine aerosol and highest for aerosolsampled close to anthropogenic sources. Combining all data from theexperiment reveals that composition (defined by HOA mass fraction) explains40% of the variance in the critical diameter for particle activation atthe instrumental supersaturation (S) of 0.44%. Correlations between HOAmass fraction and aerosol mean diameter show that these two parameters areessentially independent of one another for this data set. We conclude that,based on the variability of the HOA mass fraction observed duringTexAQS-GoMACCS, variability in particle composition played a significantrole in determining the fraction of particles that could activate to formcloud droplets. Using a simple model based on Köhler theory and theassumption that HOA is insoluble, we estimate the degree to which calculatedCCN concentrations are under- or overestimated if the variability in the HOAmass fraction that was observed during TexAQS-GoMACCS is neglected. Thepercent under- or overestimation in the CCN concentration is related to thesource of the aerosol. Relative to the mean HOA mass fraction of 0.4±0.2(average ±1σ standard deviation) for the entireexperiment, CCN concentrations are underestimated by up to 50% (at0.22% S) for aerosol sampled far from anthropogenic source regions as ithad a lower HOA mass fraction and overestimated by up to 50% fororganic-rich aerosol sampled near the source as it had a higher HOA massfraction.

[学科分类] 大气科学