Nighttime Chemistry at a High Elevation Site above Hong Kong: Implications for Regional Air Quality
Abstract:Nighttime chemical processes of nitrogen oxides, including reactions of the nitrate radical (NO3) and dinitrogen pentoxide (N2O5), are important to numerous tropospheric chemical cycles, including the removal of NOx, the oxidation of biogenic hydrocarbons, and the heterogeneous activation of halogen species. These cycles influence regional ozone and aerosol pollution but remain uncertain. Although nitrogen oxide levels are highest in urban areas, nighttime chemistry is notoriously difficult to study in these regions due to surface level buildup of NO and consequent titration of O3, which suppress nighttime chemical reactions. Tai Mo Shan (TMS) is a high elevation site (950 m) situated between the cities of Hong Kong and Shenzhen, each of which has a population of approximately 7 million. Both lie within the Pearl River Delta, a regional megacity of population 45 million. TMS is ideally suited for the study of nighttime chemistry within a megacity because it is situated within the residual boundary layer at night and is influenced by regional pollution in the absence of strong local effects.
During the fall season, the Pearl River Delta region and Hong Kong experience peak ozone due to meteorological pattern that brings polluted continental outflow over the coastal region of southern China. During November and December 2013, a small-scale field study was conducted at the TMS summit to investigate nighttime chemistry. Chemical instrumentation included cavity ring-down and mass spectrometric instruments for NO3, N2O5 and ClNO2, as well as instrumentation for measurement of NOx, NOy, O3, VOCs, aerosols, other trace gases and meteorological data. Regular late afternoon and evening outflow events from mainland China were observed at this site, including one event with extreme (12 ppbv) levels of N2O5. This presentation will give an overview of the campaign, the atmospheric chemical data and its relationship to meteorological regimes. It will also examine budgets for nighttime production and loss of NO3 and N2O5 and the implications for air quality in this region.