Characterization of photochemical pollution at different elevations in mountainous areas in Hong Kong

Abstract. To advance our understanding on the factors that affect photochemical pollution at different elevations in mountainous areas, concurrent systematic field measurements (September to November 2010) were conducted at a mountain site and at an urban site at the foot of the mountain in Hong Kong. The mixing ratios of air pollutants were greater at the foot of the mountain (i.e., Tsuen Wan urban site, TW) than near the summit (i.e., Tai Mao Shan mountain site, TMS), expect for ozone. In total, only one O₃ episode day was observed at TW, whereas twenty-one (21) O₃ episode days were observed at TMS. The discrepancy of O₃ at the two sites was attributed to the mixed effects of NO titration, vertical meteorological conditions, regional transport and mesoscale circulations. The lower NO levels at TMS and the smaller differences of "oxidant" O_x (O₃ + NO₂) between the two sites suggested that variations of O₃ at the two sites were partly attributed to different degree of NO titration. In addition, analysis of vertical structure of meteorological variables revealed that the inversion layer at the range of altitudes of 500–1000 m might be another factor that caused the high O₃ levels at TMS. Furthermore, analyses of the wind fields, the levels of air pollutants in different air flows, ratios of different trace gases and the correlation between variability and the lifetime of VOCs (volatile organic compounds) indicated that high O₃ concentrations at TMS were somewhat influenced by regional air masses from the highly polluted Pearl River delta (PRD) region. In particular, the diurnal profiles and correlations of gaseous pollutants suggested influence of mesoscale circulations, which is confirmed using the Master Chemical Mechanism moving box model (Mbox) and the Weather Research and Forecasting (WRF) model. By investigating the correlations of observed O₃ and NO_x* and the relationships of O₃ and its precursors by an observation-based model (OBM), as well as the ratios of VOC/NO_x, it was concluded that photochemical O₃ formation at TMS was mostly influenced by VOCs, with measurable impact of NO_x, while O₃ production at TW was generally limited by the concentrations of VOCs. This is the first report of the comprehensive analysis on the data of photochemical pollution obtained from concurrent measurements in mountainous areas in the PRD region.