A study of the photochemical characteristics in coastal Hong Kong with a box model

Abstract

A zero-dimensional time dependent photochemical box model was developed to study the photochemical characteristics in coastal Hong Kong. The model adopts the modified Carbon Bond IV photochemical mechanism and uses the observed data to constrain the model simulation. The performance of different modified Carbon-bond IV mechanisms have been evaluated for the conditions typical of background of land and marine environments. The results of intercomparison suggest that updating O3 and H2O2 photolysis rates, adding the reactions of HO2 +XO2 and HO2+OH, and explicit treatment of methane chemistry will improve the performance of original Carbon Bond IV mechanism for background atmosphere simulation. The modified Carbon-Bond IV mechanism (VHTM41), which includes all the modifications, has the best performance for background atmosphere simulation. The comparison of observation data in coastal Hong Kong with another regional background station in Taiwan indicates that concentrations of NO, CO, and O3 for the autumn cases can be regarded as regional concentrations. For the summer cases, while the concentrations of CO and O3 represent the regional background values, the NO levels may be influenced by local sources, causing higher NO at the measurement site. Based on the observed concentrations, the net ozone production rates are 31.9 ppbv/day in summer and 22.9 ppbv/day in autumn. These results suggest that ozone chemistry in coastal Hong Kong is characterized by net photochemical ozone production. The photochemical ozone production rate is dominated by HO2+NO in both seasons. The photochemical ozone loss rate is dominated by NO2+OH for summer because of the high NO concentration. The photochemical ozone loss rate is dominated by O1D+H2O for autumn season. The higher concentrations of odd hydrogen are predicted in autumn cases as compared to those in summer case due to the higher NO concentrations in summer case. The results from the uncertainty analysis indicate that photochemical ozone production rates have a 1o uncertainty of +-21% for clear sky, photochemical loss rates have a 1o uncertainty of +-9%, and net photochemical ozone production rates have a 1o uncertainty of +-23%. Net photochemical ozone production depends on the uncertainties associated with measurements of NO and to a lesser degree the measurements of NMHCs, CO, and ozone.