Cloud Condensation Nuclei Activity of Internally Mixed Particles at a Remote Marine Free Troposphere Site in the North Atlantic Ocean

ZEZHEN CHENG, Megan Morgenstern, Silvia Henning, Bo Zhang, Greg Roberts, Matthew Fraund, Matthew A. Marcus, Nurun Nahar Lata, Paulo Fialho, Lynn Mazzoleni, Birgit Wehner, Claudio Mazzoleni, Swarup China, Pacific Northwest National Laboratory

     Abstract Number: 286
     Working Group: Aerosols, Clouds and Climate

Abstract
In this study, we report results from research conducted at the Observatory of Mount Pico (OMP), 2225 m above mean sea level on Pico Island in the Azores archipelago in June and July of 2017. We investigated the chemical composition, mixing state, and cloud condensation nuclei (CCN) activities of long-range transported free tropospheric (FT) particles. FLEXible PARTicle Lagrangian particle dispersion model (FLEXPART) simulations reveal that most air masses that arrived at the OMP during the sampling period originated in North America and were highly aged (average CO age >10 days). We probed size-resolved chemical composition, mixing state, and hygroscopicity parameter (κ) of individual particles using computer-controlled scanning electron microscopy with an energy-dispersive X-ray spectrometer (CCSEM-EDX). Particles were mostly carbonaceous (~29.7-68.9% by number), but there was a significant fraction of highly aged sea salt (~8.8 to 31.6 %) and sea salt with sulfate (~5.2 to 31.5 %) particles since air masses were transported above the North Atlantic Ocean. Based on the estimated elemental composition of individual particle mass, we calculated the mixing state index, χ. During our study, most FT particles were internally mixed (χ of samples are between 78% and 93%), owing to the long atmospheric aging time. We used data from a miniature Cloud Condensation Nucleus Counter (miniCCNC) to derive the hygroscopicity parameter, κCCNC. Combining κCCNC and FLEXPART, we found that air masses recirculated above the North Atlantic Ocean with lower mean altitude had higher κCCNC due to the higher contribution of sea salt particles. We also used CCSEM-EDX and phase state measurements to predict κ (κCCSEM-EDX) values. The κCCSEM-EDX values overlap with the lower range of κCCNC measured below 0.15% supersaturation. Therefore, CCSEM-EDX measurements can be useful in predicting the lower bound of κ, which can be used in climate models to predict CCN activities.