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Discussion papers | Copyright
https://doi.org/10.5194/acp-2017-1222
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Feb 2018

Research article | 13 Feb 2018

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This discussion paper is a preprint. A revision of the manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP).

Diverse Chemical Mixing States of Aerosol Particles in the Southeastern United States

Amy L. Bondy1, Daniel Bonanno2, Ryan C. Moffet2, Bingbing Wang3,a, Alexander Laskin3,b, and Andrew P. Ault1,4 Amy L. Bondy et al.
  • 1Department of Chemistry, University of Michigan, Ann Arbor, MI, 48109, USA
  • 2Department of Chemistry, University of the Pacific, Stockton, C A, 95211, USA
  • 3Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, 99354, USA
  • 4Department of Environmental Health Sciences, University of Mich igan, Ann Arbor, MI, 48109, USA
  • apresent address: State Key Laboratory of Marine Environmental S cience, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
  • bpresent address: Department of C hemistry, Purdue University, We st Lafayette, IN, 47907, USA

Abstract. Aerosols in the atmosphere are chemically complex with thousands or more chemical species distributed in different proportions across individual particles in an aerosol population. An internal mixing assumption, with species present in the same proportions across all aerosols, is used in many models and calculations of secondary organic aerosol (SOA) formation, cloud activation, and aerosol optical properties. However, many of these effects depend on the distribution of species within individual particles, and important information can be lost when internal mixtures are assumed. Herein, we show that during the Southern Oxidant and Aerosol Study (SOAS) in Centreville, Alabama, at a rural, forested location, that aerosols frequently are not purely internally mixed, even in the accumulation mode (0.2–1.0µm). A range of aerosol sources and mixing states were obtained using computer controlled scanning electron microscopy with energy dispersive X-ray spectroscopy (CCSEM-EDX) and scanning transmission X-ray microscopy-near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS). Particles that were dominated by SOA and inorganic salts were the majority of particles by number fraction from 0.2–5 microns with an average of 78% SOA in the accumulation mode. However, during certain periods contributions by sea spray aerosol (SSA) and mineral dust were significant to accumulation (22% SSA and 26% dust) and coarse mode number concentrations (38% SSA and 63% dust). The fraction of particles containing key elements (Na, Mg, K, Ca, and Fe) were determined as a function of size for specific classes of particles. Within internally mixed SOA/sulfate particles <5% contained Na, Mg, K, Ca, or Fe, though these non-volatile cations were present in particles from the other sources (e.g. SSA and dust). Mass estimates of the aerosol elemental components were used to determine the extent of internal versus external mixing by calculating the mixing state index (χ). The aerosol population was more externally mixed than internally mixed during all time periods analysed. Accumulation mode aerosol ranged from mostly internally mixed during SOA periods to mostly externally mixed during dust periods. Supermicron aerosol were most externally mixed during SOA time periods, when more SOA particles added a distinct supermicron class, and more internally mixed when dominated by a single particle type (e.g. SSA or dust). These results emphasize that neither external nor internal mixtures fully represent the mixing state of atmospheric aerosols, even in a rural, forested environment, which has important implications for air quality and climate modelling.

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To determine important sources of aerosols during the Southern Oxidant and Aerosol Study (SOAS), as well as their mixing with secondary species, individual particles were analyzed with electron and X-ray microscopy to determine size and chemical composition. Secondary organic aerosol, sea spray aerosol, and mineral dust each dominated during different periods. Particles were less similar chemically to each other than is commonly assumed, which is important for air quality and climate models.
To determine important sources of aerosols during the Southern Oxidant and Aerosol Study (SOAS),...
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