PNNL

Abstract

This study reports 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 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 age >10 days). We probed size-resolved chemical composition, mixing state, and hygroscopicity parameter (?) 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 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 individual particle mass from elemental composition, we calculated the mixing state index, ?. During our study, most FT particles were internally mixed (? 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, ?CCNC. Combining ?CCNC and FLEXPART, we found that air masses recirculated above the North Atlantic Ocean with lower mean altitude had higher ?CCNC due to the higher contribution of sea salt particles. We used CCSEM-EDX and phase state measurements to predict single-particle ? (?CCSEM-EDX) values, which overlap with the lower range of ?CCNC measured below 0.15% supersaturation. Therefore, CCSEM-EDX measurements can be useful in predicting the lower bound of ?, which can be used in climate models to predict CCN activities, especially in remote locations where online CCN measurements are unavailable.