Nighttime chemistry at a high altitude site above Hong Kong

Nighttime reactions of nitrogen oxides influence ozone, volatile organic compounds, and aerosol and are thus important to the understanding of regional air quality. Despite large emissions and rapid recent growth of nitrogen oxide concentrations, there are few studies of nighttime chemistry in China. Here we present measurements of nighttime nitrogen oxides, NO3and N2O5, from a coastal mountaintop site in Hong Kong adjacent to the megacities of the Pearl River Delta region. This is the first study of nighttime chemistry from a site within the residual layer in China. Key findings include the following. First, highly concentrated urban NOxoutflow from the Pearl River Delta region was sampled infrequently at night, with N2O5mixing ratios up to 8 ppbv (1 min average) or 12 ppbv (1 s average) in nighttime aged air masses. Second, the average N2O5uptake coefficient was determined from a best fit to the available steady state lifetime data as γ(N2O5) = 0.014 ± 0.007. Although this determination is uncertain due to the difficulty of separating N2O5losses from those of NO3, this value is in the range of previous residual layer determinations of N2O5uptake coefficients in polluted air in North America. Third, there was a significant contribution of biogenic hydrocarbons to NO3loss inferred from canister samples taken during daytime. Finally, daytime N2O5mixing ratios were in accord with their predicted photochemical steady state. Heterogeneous uptake of N2O5in fog is determined to be an important production mechanism for soluble nitrate, even during daytime.