Abstract
This study used a closed-circuit boundary-layer wind tunnel, with a series of instruments including SO2, NO2, O3, NH3, and PM2.5 monitors and an Aerodyne Aerosol Mass Spectrometer (AMS), to explore the effects of relative humidity (RH) on properties of ambient fine particles (PM2.5 and PM1) in the northern suburbs of Nanjing. The results showed under all conditions the PM1 concentration were influenced largely by organic matter, and meanwhile, the contribution of sulfate decreased. Under non-humidification conditions, the mass concentrations of fine particles increased with the increase of RH. The mass concentration of PM2.5 was not influenced by the SO2 concentration and changed in a manner similar to those of the NO2 and NH3 concentrations at higher RH; however, the mass concentration varied inversely with changes of O3 concentration. The mass concentration and mass fraction of sulfate and nitrate changed in different manners as RH varied. CxHy+ ions took up the highest percentage of organic matter in the AMS spectra, indicating organics in a large part might be emitted as primary organic aerosol (POA), and the fine particles were overall alkaline. With atomized humidification (at constant temperature), the mass concentrations of all fine particles showed an explosive increase, with PM2.5 levels rising at an average rate of 11.9 μg/m3·min. The increase in the fine particle concentration was primarily due to increases in the PM1-2.5 level. The percentage of organic matter increased with RH, with CxHyO+ and CxHyOz+ ions becoming the major ion groups indicating the dominance of secondary organic aerosol (SOA) species, facilitated by high humidity. The SO42−/SO2 ratio increased with RH, indicating that sulfate was formed at high humidity rapidly via an aqueous-phase oxidation of SO2 and results show that O3 might become an important oxidant. The fine particles were acidic, and the increased O/C ratio also indicates the SOA contribution.
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