Kinetics of CCN activation and droplet growth observed in recent field campaigns

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Description

Atmospheric aerosols can indirectly influence global climate budget by changing the microphysical structure, lifetime, and coverage of clouds. While it is generally agreed that aerosol indirect effects act to cool the Earth-atmosphere system by increasing cloud reflectivity and coverage, the magnitudes of the indirect effects are poorly understood. The formation of cloud droplets from aerosol particles is kinetically controlled by the availability of water vapor, equilibrium water vapor pressure above the growing droplet surface, and both the gas phase and aerosol phase mass transfer resistances. It has been hypothesized that the formation of surface organic films or the delay in dissolution of solute could significantly delay the growth of cloud droplets. Such delay could lead to a higher maximum supersaturation within a rising cloud parcel, and therefore a higher droplet number concentration and smaller droplet size at constant liquid water content. When only a subset of the droplets experiences significant growth delay, the overall droplet size spectrum will be broadened, which facilitates the formation of precipitation.

During three recent field campaigns (CalNex-LA, CARES, and the Aerosol Intensive Observation Period at Brookhaven National Laboratory), the CCN activity and droplet growth of size-selected particles ranging from 25 to 320 nm were characterized by a cloud condensation nuclei (CCN) counter under supersaturations from 0.1% to 0.8%. The three campaigns allow us to examine the droplet growth for many representative organic aerosol types, including biogenic secondary organic aerosol (SOA), anthropogenic SOA, and organic aerosols from biomass burning. The droplet growth of size-selected ambient particles inside the CCN counter was found to be influenced by a number of parameters, including particle critical supersaturation, heterogeneity in particle composition, and particle concentration. For example, reduced droplet growth due to water vapor depletion was observed when particle concentration was higher than 200 cm^-3. The influences of the different parameters on droplet growth were modeled and compared with measurements. The potential impact of surface organic film on droplet growth was isolated by comparing the droplet growth of size-selected ambient particles to that of ammonium sulfate particles with the same critical supersaturation, and the results are discussed.