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 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 in June and July 2014, 2015, and 2017 to the Observatory of Mount Pico (OMP), in the Azores. OMP is an ideal site for studying long-range transported free tropospheric particles with negligible influence from local emissions and rare contributions from the boundary layer. We used the FLEXible PARTicle Lagrangian particle dispersion model (FLEXPART) to determine the origin and transport trajectories of sampled air masses and found that most originated from North America and recirculated over the North Atlantic Ocean. The FLEXPART analysis show that the sampled air masses were highly aged (average plume age > 10 days). 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 show 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 state 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 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. Our findings warrant future investigation on the quantitative assessment of the influence of internal mixing on the phase state of the individual particles. This study also provides insights into the chemical composition and phase state of free tropospheric particles, which can benefit models to reduce uncertainties in ambient aerosol particles' effects on climate.
Highlights
45 Atmospheric aerosol particles play a vital role in regional and global climates
We classified each particle into 6 typical classes based on the volume fraction of organic carbonaceous (OC), elemental carbon (EC), and inorganic components (In) components: 1) OC (OC is greater than 96 % and both EC and In are less than 2 %); 2) EC (EC is greater than 96 % and both OC and In are less than 2 %); 3) In (In is greater than 96 % and both OC and EC are less than 2 %); 4) OCEC; 5) OCIn; and 6) OCInEC (OC, EC, and inorganics 210 are all greater than 2 %)
We analyzed the vertical distribution of air masses residence time (Fig. S5 for S3-1 to S4-4 and Fig. S6 (b, d, f, h, j, i) for S1 to S6) and the temperature and relative humidity (RH) during transport for a better understanding of ambient conditions that are critical for aerosol property transition
Summary
Free tropospheric aerosol particles have important but poorly constrained climate effects due to transformations of their physicochemical properties during long-range transport. 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. This study provides insights into the chemical composition and phase state of free tropospheric particles, which can benefit models to reduce uncertainties in ambient aerosol particles’ effects on climate. We observed a high abundance of liquid and internally mixed particles in samples collected in the North Atlantic free troposphere during summer. Our results suggest that considering the mixing state, emission source, and transport patterns of particles is necessary to estimate their phase state in the free troposphere, which is critical for predicting their effects on climate
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