Aerosol Composition, Physiochemical Properties, and Source Apportionment at a Forest Site in Taiwan

To investigate the interaction between local circulation and aerosol major chemical composition and hygroscopicity, a series of studies in Xitou Experimental Forest of National Taiwan University (23.40°N, 120.47°E, 1,178 m asl) in December 2018 was conducted. The isotopes of δ¹⁵N and δ¹⁸O from the filter samples were applied to identify the possible formation pathways. The single hygroscopicity parameter, κ, of aerosols between 9-437 nm in diameter was derived from the measurements of a cloud condensation nuclei counter (CCNc), an ultrafine condensation particle counter (UCPC) and a scanning mobility particle sizer (SMPS) using the κ-Köhler equation. Filter samples collected by a multi-orifice uniform deposit impactor (MOUDI) were applied to quantify the major aerosol composition based on the absorbance of selected functional groups (NH₄⁺, SO₄^2-, NO₃^-, elemental carbon) by a Fourier transform infrared spectroscopy with an attenuated total reflection accessory (FT-IR-ATR). The δ¹⁵N of particulate NH₄⁺ (p-NH₄⁺) and particulate NO₂^- or NO₃^- (p-NO_x^-) and the δ¹⁸O of p-NO_x^- were analyzed by an isotopic ratio mass spectroscopy (IR-MS) to infer the source and chemical pathway of aerosols. The mean κ value of aerosol is mostly between 0.07 and 0.22 during the field study period. The aerosol concentration shows a significant correlation with the local circulation, sea-land breeze combined with the mountain-valley circulation, and is significantly higher in the daytime than that in the nighttime. The foggy period has revealed a higher concentration of NH₄⁺, SO₄^2-, NO₃^-, and elemental carbon (or black carbon, BC), which may be caused by the lower boundary layer and weaker upward turbulent mixing during the foggy period. Aerosols containing NH₄⁺, SO₄^2- shifted to the larger size distribution during the foggy period and that is likely due to the hygroscopic growth of aerosols containing these functional groups at higher RH. The observed stable and high NO₃^- concentration of aerosol in the diameter of 0.56-1 µm during foggy periods is likely caused by the partition of HNO₃ in the aqueous phase under a basic condition or further stabilized by the dissolved ammonium to form particulate NO₃^-. The daily mass-weighted δ¹⁵N of p-NH₄⁺ is ranged from +3.7‰ to +16.3‰ and δ¹⁵N of p-NO_x^- from +1.5‰ to +5.2‰, indicating that p-NH₄⁺ and p-NO_x^- are likely contributed from anthropogenic sources such as coal-burning and traffic. The δ¹⁸O of p-NO_x^- is in the range of +70‰ to +80‰, similar to the result of southeast Asia in winter. The observed high δ¹⁸O might be contributed through the pathways of the oxidation of NO with O₃ to form NO₂, which is further oxidized by OH radicals to form HNO₃.