Particle phase-state variability in the North Atlantic free troposphere during summertime is determined by atmospheric transport patterns and sources

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Cheng, Zezhen, Morgenstern, Megan, Zhang, Bo, Fraund, Matthew, Lata, Nurun Nahar, Brimberry, Rhenton, Marcus, Matthew A., Mazzoleni, Lynn, Fialho, Paulo, Henning, Silvia, Wehner, Birgit, Mazzoleni, Claudio, China, Swarup
• Format: Text
• Language: unknown
• Published: Digital Commons @ Michigan Tech 2022
• Subjects: Mount Pico; North Atlantic
• Online Access: https://digitalcommons.mtu.edu/michigantech-p/16205; https://doi.org/10.5194/acp-22-9033-2022

Free tropospheric aerosol particles have important but poorly constrained climate effects due to transformations of their physicochemical properties during long-range transport. In this study, we investigate the chemical composition and provide an overview of the phase states of individual particles that have undergone long-range transport over the North Atlantic Ocean in June and July 2014, 2015, and 2017 to the Observatory of Mount Pico (OMP) in the Azores. The OMP is an ideal site for studying long-range-transported free tropospheric particles because local emissions have a negligible influence and contributions from the boundary layer are rare. We used the FLEXible PARTicle Lagrangian particle dispersion model (FLEXPART) to determine the origins and transport trajectories of sampled air masses and found that most of them originated from North America and recirculated over the North Atlantic Ocean. The FLEXPART analysis showed that the sampled air masses were highly aged (average plume age >10 d). Size-resolved chemical compositions of individual particles were probed using computer-controlled scanning electron microscopy with an energy-dispersive X-ray spectrometer (CCSEM-EDX) and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS). 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 %). We used a tilted stage interfaced within an environmental scanning electron microscope (ESEM) to determine the phase states of individual submicron particles. We found that most particles (∼ 47 % to 99 %) were in the liquid state at the time of collection due to inorganic inclusions. Moreover, we also observed substantial fractions of solid and semisolid particles (∼ 0 % to 30 % and ∼ 1 % to 42 %, respectively) during different transport patterns and events, reflecting the particles' phase-state variability for different atmospheric ...