11 Feb 2022
11 Feb 2022
Status: this preprint is currently under review for the journal ACP.

Particles' phase state variability in the North Atlantic free troposphere during summertime determined by different atmospheric transport patterns and sources

Zezhen Cheng1, Megan Morgenstern2, Bo Zhang3, Matthew Fraund4, Nurun Nahar Lata1, Rhenton Brimberry1, Matthew A. Marcus4, Lynn Mazzoleni2, Paulo Fialho5, Silvia Henning6, Birgit Wehner6, Claudio Mazzoleni2, and Swarup China1 Zezhen Cheng et al.
  • 1Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, USA
  • 2Atmospheric Sciences Program, Michigan Technological University, Houghton, Michigan, 49921, USA
  • 3National Institute of Aerospace, Hampton, VA 23666, USA
  • 4Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
  • 5Institute of Volcanology and Risk Assessment – IVAR, Rua da Mãe de Deus, 9500-321 Ponta Delgada, Portugal
  • 6Leibniz Institute for Tropospheric Research, Permoserstraße 15, 04318 Leipzig, Germany

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.

Zezhen Cheng et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-84', Anonymous Referee #1, 03 Mar 2022
  • RC2: 'Comment on acp-2022-84', Anonymous Referee #2, 13 Apr 2022
  • EC1: 'Comment on acp-2022-84', Sergey A. Nizkorodov, 18 Apr 2022
  • RC3: 'Comment on acp-2022-84', Anonymous Referee #3, 21 Apr 2022

Zezhen Cheng et al.

Zezhen Cheng et al.


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Short summary
We observed a high abundance of liquid and internally mixed particles in samples collected in the North Atlantic free troposphere during summer. We also found several solid and semisolid particles for different emission sources and transport patterns. 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.