Particle phase-state variability in the North Atlantic free troposphere during summertime is determined by atmospheric transport patterns and sources
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 
Published: July 2022

Abstract. 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 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 lead to an overestimation of the viscosity of free tropospheric particles.
Our findings warrant future investigation aiming at the quantitative assessment of
the influence of internal mixing on the phase states of the individual
particles. This study also provides insights into the chemical composition
and phase state of free tropospheric particles, which can help models to
reduce uncertainties about the effects of ambient aerosol particles on climate.