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Volume 18, issue 21 | Copyright
Atmos. Chem. Phys., 18, 16099-16119, 2018
https://doi.org/10.5194/acp-18-16099-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Nov 2018

Research article | 08 Nov 2018

Is there an aerosol signature of chemical cloud processing?

Barbara Ervens1,2,a, Armin Sorooshian3,4, Abdulmonam M. Aldhaif3, Taylor Shingler5,6, Ewan Crosbie5,6, Luke Ziemba6, Pedro Campuzano-Jost2,7, Jose L. Jimenez2,7, and Armin Wisthaler8,9 Barbara Ervens et al.
  • 1NOAA/ESRL/Chemical Sciences Division, Boulder, CO, USA
  • 2CIRES, University of Colorado, Boulder, CO, USA
  • 3Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, USA
  • 4Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA
  • 5Science Systems and Applications, Inc., Hampton, VA, USA
  • 6NASA Langley Research Center, Hampton, VA, USA
  • 7Department of Chemistry, University of Colorado, Boulder, CO, USA
  • 8Department of Chemistry, University of Oslo, Oslo, Norway
  • 9Institute for Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria
  • anow at: Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, Clermont-Ferrand, France

Abstract. The formation of sulfate and secondary organic aerosol mass in the aqueous phase (aqSOA) of cloud and fog droplets can significantly contribute to ambient aerosol mass. While tracer compounds give evidence that aqueous-phase processing occurred, they do not reveal the extent to which particle properties have been modified in terms of mass, chemical composition, hygroscopicity, and oxidation state. We analyze data from several field experiments and model studies for six air mass types (urban, biogenic, marine, wild fire biomass burning, agricultural biomass burning, and background air) using aerosol size and composition measurements for particles 13–850nm in diameter. We focus on the trends of changes in mass, hygroscopicity parameter κ, and oxygen-to-carbon (OC) ratio due to chemical cloud processing. We find that the modification of these parameters upon cloud processing is most evident in urban, marine, and biogenic air masses, i.e., air masses that are more polluted than very clean air (background air) but cleaner than heavily polluted plumes as encountered during biomass burning. Based on these trends, we suggest that the mass ratio (Rtot) of the potential aerosol sulfate and aqSOA mass to the initial aerosol mass can be used to predict whether chemical cloud processing will be detectable. Scenarios in which this ratio exceeds Rtot ∼ 0.5 are the most likely ones in which clouds can significantly change aerosol parameters. It should be noted that the absolute value of Rtot depends on the considered size range of particles. Rtot is dominated by the addition of sulfate (Rsulf) in all scenarios due to the more efficient conversion of SO2 to sulfate compared to aqSOA formation from organic gases. As the formation processes of aqSOA are still poorly understood, the estimate of RaqSOA is likely associated with large uncertainties. Comparison to Rtot values as calculated for ambient data at different locations validates the applicability of the concept to predict a chemical cloud-processing signature in selected air masses.

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The paper presents a new framework that can be used to identify emission scenarios in which aerosol populations are most likely modified by chemical processes in clouds. We show that in neither very polluted nor in very clean air masses is this the case. Only if the ratio of possible aerosol mass precursors (sulfur dioxide, some organics) and preexisting aerosol mass is sufficiently high will aerosol particles show substantially modified physicochemical properties upon cloud processing.
The paper presents a new framework that can be used to identify emission scenarios in which...
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