Organic acids in the atmosphere and bulk deposition of Hong Kong

Abstract

This study focused on the air pollution of atmosphere of Hong Kong and its surroundings. The sources and factors affecting the variation in concentrations of organic acids were of interest because no previous studies were performed before in Hong Kong. Since organic acids are ubiquitous, bulk deposition, aerosol and gaseous phase samples were collected and analyzed for the organic acids. The monitoring program of the concentrations of organic acids in bulk deposition was the most important experimental area in this work. 57 daily bulk deposition samples were collected and analyzed using ion chromatography at the location of City University of Hong Kong (CityU) from May 1999 to May 2000. The volume-weighted mean (VWM) concentrations for acetate, formate, propanoate and oxalate were 4.5, 6.1, 0.4 and 1.4 ueqv L" respectively. The VWM pH was determined as 4.7. For bulk samples with pH<5.0, the maximum acidity contribution by acetic and formic acids were 14.2 % and 17.3 % respectively. High organic acid levels were found in the dry season. The synoptic weather system types approaching cyclone (AC) and cold front (CF) also had higher levels than the types trough (T) and cyclone (C). Weak correlations were found between the organic acids with the inorganic anions. The possible sources of organic acids are not determined from the bulk deposition results, so that simultaneous aerosol and gas phase samplings using the impinger tube sampling technique were performed next. Since traffic emission was one of the possible sources in the Hong Kong environment, Central district (busy urban area) and CityU (semi-residential) were chosen 3s sites for the comparison of the levels of organic acids. The unique conditions with tall buildings and high vehicle loading (about 1000 vehicles per hour) at the Central sampling site showed higher levels of organic acids in aerosol and the gaseous phase. While for the CityU sampling site, the concentrations of organic acids in the gas phase changed with the windspeed. The study of the variations of major pollutant gas concentrations with different meteorological parameters provided the background of Hong Kong ambient air pollution in this work. Types AC, CF and northeasterly monsoon (NE) resulted in high levels of pollutant gases. To compare the Hong Kong environment and its surroundings, aerosol (collected using a nine-stage cascade impactor), dry deposition and daily bulk depositions were collected in Hong Kong and Shenzhen. Mass median diameters, dry deposition fluxes and dry deposition velocities for different ionic species were estimated. The Hong Kong bulk and dry deposition samples were affected significantly by maritime airmasses, while the Shenzhen samples were more crustal in nature.