The influence of upslope fog on hygroscopicity and chemical composition of aerosols at a forest site in Taiwan

Abstract

This study investigated the influence of upslope fog formation on the chemical composition and single hygroscopicity parameter (κ) of rural aerosols from Dec. 1st to Dec. 24th, 2018 at the Xitou forest site (23.67°N, 120.80°E, 1178 m above sea level (a.s.l.)) in central Taiwan. The aerosol chemical compositions were monitored using a mini compact time-of-flight aerosol mass spectrometer (mini-C-ToF-AMS), and the ambient aerosol particles were collected by a 13-stage micro-orifice uniform deposit impactor (MOUDI) for a Fourier-transform infrared spectrometer with an attenuated total reflectance accessory (FTIR-ATR) analysis. κ of aerosols was derived from the comparison of AMS pToF size distribution and FTIR-ATR measurement using the κ-Köhler equation. Our results show that the moderate correlation between OOA and CO provided evidence of upstream anthropogenic emission and time-lagged aged secondary organic aerosol transport by the daytime sea breeze and valley wind. Larger particles (Dva ~800 nm–1.0 μm) had a higher fog scavenging removal efficiency (62% for organic and 81% for nitrate, respectively), and smaller particles (Dva ~100 nm–700 nm) could remain growth through gas-to-aqueous partition. The submicrometer nitrate particles were likely significantly formed via gas-particle partitioning as HNO3(aq) or NH4NO3 while the larger micrometer-sized sea salt-like nitrate particles might be formed via heterogeneous reaction of gaseous HNO3 with deliquesced sea salt particles. The inconsistency of submicrometer nitrate between real-time AMS and offline FTIR-ATR measurements indicates that the evaporation loss of HNO3 or NH4NO3 during MOUDI filter sampling could lead to the unavailable κp−NO3 (κ for nitrate-containing particles) retrieval. The average κp-org (organics-containing particles) was 0.19 ± 0.12 (r2 ≥ 0.8) and nearly overlapped with κp−SO4 (0.22 ± 0.08) (sulfate-containing particles) during misty daytime, indicating that aerosols were more likely internally mixed particles to have similar hygroscopicity and physical mixing state property.