Abstract
Abstract. 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 NH4+ was observed. The supermicron modes were likely associated with fog events since they were absent on non-fog days. The supermicron mode of NH4+ 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 NH4+ 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 NH3-poor conditions or a mixture of ammonium nitrate and ammonium sulfate aerosols in NH3-rich conditions. Interstitial aerosols and fog droplets presented during fog events likely yielded a minor direct contribution to the measured NH4+. The mass median aerodynamic diameter (MMAD) of the 1–4 μm mode of NH4+ strongly depended on ambient temperature (T) 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 T > 0 °C, and in two samples, at T > −3 °C. The MMAD of NH4+ 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 T < −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 T < −4 °C has added new knowledge on the size distributions and chemical compositions of fog-processed aerosols under various ambient conditions.
Highlights
The purpose of the present study is to identify fogprocessed aerosols from a suite of field measurements made at seven inland rural sites and one coastal rural site in Canada (Zhang et al, 2008a, b) and to investigate the size distributions, neutralization extents, and chemical compositions of these aerosols under various ambient conditions
Supermicron modes of ammonium salts were absent in 158 non-fog samples where ammonium salt aerosols were dominantly distributed in the submicron particle size
Submicron modes of ammonium salt aerosols in the atmosphere have been well characterized in the literature, i.e., the 0.2 ± 0.1 μm mode due to gas-particle condensation, the 0.4 ± 0.1 μm mode due to either primary emissions or hygroscopic growth of 0.2 μm mode particles together with chemical reactions, and the 0.7 ± 0.2 μm mode due to in-cloud processing of aerosols (Ondov and Wexler, 1998)
Summary
Fog events can modify size distributions and chemical compositions of preexisting atmospheric aerosols and modify their optical properties (Ondov and Wexler, 1998; Moore et al, 2004; Fahey et al, 2005; Sun et al, 2006; Aikawa et al, 2007; Herckes et al, 2007; Biswas et al, 2008; Collett Jr., et al, 2008; Dall’Osto et al, 2009; Kaul et al, 2011; Rehbein, et al, 2011; Yu et al, 2011). Fog droplets evolve into atmospheric aerosol particles with modified physical and chemical properties and the process has been shown to take a short amount of time, from dozens of minutes to less than an hour (Ming and Russell, 2004; Fahey et al, 2005; Dall’Osto et al, 2009; Yao et al, 2011).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
