Calculating CCN Activity of Ambient ATOFMS Measurements During a Period of Changing Meteorological Conditions

Aerosols have both direct and indirect effects on climate. The direct effect is characterized by a particle’s ability to scatter and absorb radiation. These particles can also nucleate cloud droplets. Once activated, these cloud condensation nuclei (CCN) can go on to form clouds, which also scatter and absorb radiation (the indirect aerosol effect). Changing aerosol size and chemistry will affect CCN activity and cloud microphysical properties. Here we show that CCN activity can change dramatically in a short amount of time due to shifting winds and changing meteorological conditions. During a study in Riverside, CA in September of 2007, single particle dual polarity Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) measurements were made that showed three time periods where the aerosol chemistry was dominated by one particle type. A large increase in salt concentration was observed when the winds were coming from the west. Then, a sudden shift to winds from the east resulted in a large concentration of elemental carbon mixed with organic carbon (ECOC), followed closely by a rise in biomass burning particles. A Kohler theory model was used to estimate the number of particles likely to activate and a ratio of activated to total particles was calculated for particles assuming the dominant particle type made up all of the particles (CCNpure) and including all of the particle types (CCNmixed). These calculations suggest that the salt and ECOC periods' CCN activity agreed fairly well with CCNmixed/CCNpure of 1.00 and 0.998, respectively. The biomass period however had less agreement with CCNmixed/CCNpure of 0.988. Our results show that local CCN activity is highly variable on a short time scale and that the dominant particle type can be used to predict CCN activity if it is 50 percent or more of the total particles.