Influence of Gas Phase Compounds on CCN Activity Observed during the Eastern Pacific Cloud Aerosol Precipitation Experiment

MARKUS PETTERS, Elavarasi Ravichandran, Sanghee Han, Abigail Williams, Jeramy Dedrick, Christian Pelayo, Nattamon (Jeep) Maneenoi, Lauren Robinson, Rachel Chang, Michael Wheeler, Jeremy Wentzell, John Liggio, Lynn M. Russell, University of California, Riverside

     Abstract Number: 445
     Working Group: Coast to Coast Campaigns on Aerosols, Clouds, Chemistry, and Air Quality

Abstract
One of the less-well understood influences on the CCN activity of particles, particularly under ambient conditions, is the role of the gas phase in modifying activation supersaturation of a particle. Co-condensation of gas-phase species can dissolve into the growing droplet adding to the solute mass. Organic trace gasses can adsorb to the water/air interface and modify the surface tension of the drop. These effects may alter the activation supersaturation relative to that attributable to dry particle composition alone. Here we show observational evidence of the influence of gas-phase compounds on the activation supersaturation obtained during the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) campaign. The mobility size distribution and size-resolved hygroscopicity parameter were measured by interfacing a scanning mobility particle spectrometer with a condensation particle counter and a continuous flow CCN counter sampling between 0.2 and 1% supersaturation. Optionally, particles exiting the differential mobility analyzer column were denuded prior to entering the CCN counter. The denuder removed volatile organic compounds from the gas-phase. Cloud droplet residuals were sampled behind a counterflow virtual impactor inlet. In parallel, the particle size distribution D > ~0.15 μm was measured using a portable optical particle spectrometer. The hygroscopicity parameter κ typically varied between 0.2 and 0.4 before, during and after the cloud event for all supersaturations except s = 0.2%, which often showed much higher κ values. These results are broadly consistent with hygroscopicity parameters measured at the Scripps Pier using a humidified tandem differential mobility analyzer. Inclusion of the denuder perturbed the derived κ by up to 50%. The effect is intermittent but was observed throughout the study period between Feb and August 2024. When present, the effect of denuding is often, but not always to render the particles less CCN active. These data demonstrate a surprisingly strong and not yet fully understood influence of the gas phase on CCN activity.