Influence of Upslope Fog on Hygroscopicity and Chemical Composition of Aerosols at a Forest Site in Taiwan

This study investigated the influence of upslope fog formation on the chemical composition and single hygroscopicity parameter (κ) of rural aerosols. The compositions were monitored using a mini compact time-of-flight aerosol mass spectrometer (mini-C-ToF-AMS), and a scanning mobility particle sizer (SMPS) from Dec. 1st to Dec. 24th, 2018 at the Xitou forest site (23.40°N, 120.47°E, 1,178 m asl) in Taiwan. Ambient wet aerosol particles were collected by a 13-stage nano-MOUDI II impactor (micro-orifice uniform deposit impactors) and analyzed using a Fourier-transform infrared spectrometer with an attenuated total reflectance accessory (FTIR-ATR). The single hygroscopicity parameter (κ) of aerosols derived from the comparison of AMS pToF size distribution using the κ-Köhler equation and FTIR-ATR measurement. The moderate correlation (r = 0.73) between the oxidized oxygenated organic aerosol (OOA) and CO evidenced the upstream anthropogenic emission transport by sea/land breezes. The decreasing (aerosol mass)/CO ratio with decreasing visibility trends during in-fog periods at two dense foggy events indicated that the fog activation scavenging mechanism dominated the aerosol particle removal. The inconsistency of online real-time AMS and offline FTIR-ATR measurement for submicrometer particles indicated that the evaporation loss of HNO₃ or NH₄NO₃ particles during MOUDI filter sampling could lead to the unavailable κ retrieval for nitrate-containing particles at non-foggy daytime and the discrepancy of aerosol acidity. Similar κ ranges of organic carboxylic acid group particles (0.1 < κ_p-org < 0.3), ammonium-containing, and sulfate-containing particles (0.2 < κ_p-NH4 or κ_p-SO4< 0.5) but ambiguous nitrate-containing particles (0.4 < κ_p-NO3 < 0.6 or 0.6 < κ_p-NO3 < 0.8) were observed at foggy daytime, suggesting that ammonium sulfate and organic carboxylic acid compounds were more likely internal mixture particles with similar hygroscopicity and physicochemical mixing state influenced by upslope fog. However, the distinct κ ranges of sulfate-containing particles (0.5 < κ_p-SO4 < 0.7 or 0.6 < κ_p-SO4 < 0.8) and organic carboxylic acid group particles (0.1 < κ_p-org < 0.2) revealed the different chemical and physical properties of external mixture particles at non-foggy daytime.