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

Abstract

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.

Size-Resolved Elemental Composition of Respiratory Particles in Three Healthy Subjects

“…EDX analysis is considered semiquantitative for lighter elements, and contributions of C and O from the substrate are expected. 37 Previous studies have shown that elemental composition derived from EDX spectra is consistent with the composition of laboratory-generated standards and with the composition of ambient aerosols analyzed in bulk. 35,37−39 Based on the elemental percentages, we used k-means clustering 40 to classify each particle as one of five types: sodium-rich salt (Na-rich salt, Na ≥ 1%, and Na > K and Na > P), potassium-rich salt (K-rich salt, K > Na, K > P, and K ≥ 5%), phosphorus-rich salt (P-rich salt, P > Na, P > K, and P ≥ 1%), carbonaceous (CNO, C + N + O ≥ 97% and Na < 1%), or mixed salt for those that did not fit into one of the previously listed categories (Table S1).…”

“…The particles were dominated by C (90% averaged across all samples) and contained a substantial amount of O (5%), Na (2%), and Cl (2%). The percentages for C, N, and O should be considered semiquantitative, due to limitations of the method . The respiratory particle samples contained small amounts of N, Mg, Al, Si, P, S, K, Ca, Mn, Fe, and Zn, each of which accounted for less than 1% of the particles.…”

“…We used another EDX instrument (Oxford Instruments) to conduct elemental mapping to investigate the spatial distribution of different elements. EDX analysis is considered semiquantitative for lighter elements, and contributions of C and O from the substrate are expected . Previous studies have shown that elemental composition derived from EDX spectra is consistent with the composition of laboratory-generated standards and with the composition of ambient aerosols analyzed in bulk. ,− …”

Physicochemical Characterization and Source Apportionment of Arctic Ice Nucleating Particles Observed in Ny-Ålesund in Autumn 2019

“…(e) and (f) PM10 samples. Note that the categorization based on elemental compositions was slightly different for selected impinger (Cheng et al, 2022) and PM10 (Hiranuma et al, 2013) samples. The group C+N (CNO) include particles containing only carbon and nitrogen (and oxygen), mainly representative of (oxygenated) carbonaceous particles.…”

An automated size and time-resolved aerosol collector platform integrated with environmental sensors to study the vertical profile of aerosols

“…One potential explanation is that these particles might be in a liquid state with low viscosity so that they spread out and deform to a at shape when they impact the substrate. 55,91,92 Another explanation is that large particles could bounce from higher to lower stages. 89 carbonaceous particles are different, suggesting different atmospheric processing along the vertical column.…”