Abstract
A study was conducted in a subtropical forest, Xitou, in central Taiwan to investigate chemical characteristics in fog/cloud water and aerosol particles. Specifically, selected carbohydrate species, including arabitol, mannitol and methyl-tetrols (methyl-threitol and methyl-erythritol), were quantified and used as molecular source tracers for bioaerosol (fungal spores) and secondary organic aerosols (SOA).The ambient concentrations of the fungi and SOA tracers in the subtropical forest were high, indicating that microbial activities and secondary aerosol formation were important processes in Xitou. In addition, inorganic ions were measured in order to determine the influence from other sources, such as agricultural and industrial activities, as well as the leaching from canopy. The large amounts of ammonium, nitrate and sulfate found in both fog water and aerosol, apparently caused the high measured acidity in the atmosphere. In addition, low-molecular weight organic acids, including acetic, formic and oxalic acid, gave information about the emission type, and accounted for a relatively large proportion of known organic species. Moreover, total organic carbon (TOC) and carbonaceous species (organic and elemental carbon) were measured, showing that on average more than 75% of organic carbon in fog water was water-soluble and the concentration of organic carbon in aerosol particles was rather high. Trace elements were detected in fog water as well, displaying high concentrations of S, Si, Al, Fe and some heavy metals. Besides the chemical analyses, physical measurements, e.g., SEM-EDX analysis and determination of total particulate matter (PM) mass were performed as well. SEM images and EDX spectra (scanning electron microscopy with energy dispersive X-ray) revealed aluminosilicate inorganic particles or mineral dust with K, Fe, Ca, Mn, Fe, Cl inclusions. Concentrations of PM mass in daytime were higher than in nighttime, implying the chemical aerosol components in Xitou were mainly of biogenic origin.As few studies have investigated the role which organic compounds play in cloud-aerosol interactions, especially in form of ambient measurements, and specifically in subtropical forests, the results from this study reveal new insights into the composition and sources of aerosol particles and fog/cloud water in such environments. Furthermore, different analytic methods were compared, providing more reliable results for this study, especially for bioaerosol characterization.