Particles’ Phase State Variability in the North Atlantic Free Troposphere during Summertime Determined by Different Atmospheric Transport Patterns and Sources

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

     Abstract Number: 439
     Working Group: Aerosol Chemistry

Abstract
Free tropospheric aerosol particles have important but poorly constrained climate effects due to transformations of their Physico-chemical properties during long-range transport. We investigated the chemical composition and provided an overview of the phase state of individual particles that have been long-range transported over the North Atlantic Ocean and were collected on June and July 2014, 2015, and 2017 at the Observatory of Mount Pico (OMP), in the Azores. The FLEXible PARTicle Lagrangian particle dispersion model (FLEXPART) showed that most air masses arrived during the sampling period at OMP originated from North America and recirculated over the North Atlantic Ocean, and they were highly aged (average plume age >10 days). Computer-controlled scanning electron microscopy with an energy dispersive X-ray spectrometer (CCSEM-EDX) results showed that the most abundant particle types were carbonaceous (~29.9 to 82.0 %), sea salt (~0.3 to 31.6 %), and sea salt with sulfate (~2.4 to 31.5 %). Tilted SEM images indicated that most particles (~47 to 99 %) were in the liquid state at the time of collection due to inorganic inclusions. Moreover, we also observed a substantial fraction of solid and semisolid particles (~0 to 30 % and ~1 to 42 %, respectively) during different transport patterns/events, reflecting the particles’ phase state variability for different atmospheric transport events and sources. Combining phase state measurements with FLEXPART CO tracer analysis, we found that wildfire-influenced plumes can result in particles with a wide range of viscosities after long-range transport in the free troposphere. We also used temperature and RH values extracted from the Global Forecast System (GFS) along the FLEXPART simulated path to predict the phase state of the particles during transport and found that neglecting internal mixing with inorganics would overestimate the viscosity of free tropospheric particles.