Impact of synoptic weather patterns on spatio-temporal variation in surface O3 levels in Hong Kong during 1999–2011

Potential impacts of circulation patterns on surface ozone (O3) concentrations over Hong Kong were investigated for the time period of 1999–2011. Synoptic weathers during the study period were classified into seven typical patterns using a semi-objective weather typing technique. Temporal and spatial variations of O3 and total oxidant (Ox = O3 + NO2) in Hong Kong were found to be closely connected with weather/circulation patterns. The highest O3 concentrations (25 ppbv in average) among the 7 categories were found to be associated with the influence of Northwest Pacific typhoons, whereas the lowest average concentrations (13 pbbv) were linked to southerly flow introduced by summer monsoons. Ozone episodes with hourly mixing ratios exceeding 120 ppbv were found to be caused primarily by regional transport under influence of the tropical cyclone and by photochemical reactions upon prevalence of anti-cyclonic circulation. Taking into account interannual variabilities in frequency and intensity of circulation patterns, a reconstructed time series of O3 captured up to 50% of the observed interannual variability and 36% of the increasing trend. The study highlights an important linkage between weather/climate and air quality.