Supermicron modes of ammonium ions related to fog in rural atmosphere

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.