ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-11165-2012 Supermicron modes of ammonium ions related to fog in rural atmosphere Yao X. H. 1 Zhang L. 2 Key Lab of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada 26 11 2012 12 22 11165 11178 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/11165/2012/acp-12-11165-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/11165/2012/acp-12-11165-2012.pdf

Fog-processed aerosols were identified and analyzed in detail from a large-sized database in which size-segregated atmospheric particles and gases were simultaneously measured at eight Canadian rural sites. In ten samples collected during or following fog events, at least one supermicron mode of particulate NH<sub>4</sub><sup>+</sup> was observed. The supermicron modes were likely associated with fog events since they were absent on non-fog days. The supermicron mode of NH<sub>4</sub><sup>+</sup> in the 5–10 μm size range probably reflected the direct contribution from fog droplets. Based on detailed analysis of the chemical compositions and the extent of neutralization, the supermicron mode of NH<sub>4</sub><sup>+</sup> in the 1–4 μm size range was believed to be caused by fog-processing of ammonium salt aerosols. These aerosol particles consisted of incompletely neutralized sulfuric acid aerosols in NH<sub>3</sub>-poor conditions or a mixture of ammonium nitrate and ammonium sulfate aerosols in NH<sub>3</sub>-rich conditions. Interstitial aerosols and fog droplets presented during fog events likely yielded a minor direct contribution to the measured NH<sub>4</sub><sup>+</sup>. The mass median aerodynamic diameter (MMAD) of the 1–4 μm mode of NH<sub>4</sub><sup>+</sup> strongly depended on ambient temperature (<i>T</i>) and can be grouped into two regimes. In one regime, the MMAD was between 1.1 and 1.7 μm in four samples, when fog occurred at <i>T</i> > 0 °C, and in two samples, at <i>T</i> > −3 °C. The MMAD of NH<sub>4</sub><sup>+</sup> in this size range was also observed in various atmospheric environments, as discussed in the literature. In the other regime, the MMAD was between 2.8 and 3.4 μm in four samples when fog occurred at <i>T</i> < −4 °C, a phenomenon that was first observed in this study. The MMAD was not related to chemical composition and concentration of ammonium salts. Further investigations are needed in order to fully understand the cause of the MMAD. The larger supermicron mode of ammonium salts aerosol observed at <i>T</i> < −4 °C has added new knowledge on the size distributions and chemical compositions of fog-processed aerosols under various ambient conditions.

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