Numerical modeling of a continuous photochemical pollution episode in Hong Kong using WRF–chem

A continuous and heavy photochemical pollution episode when Typhoon Nari occurred during Sep. 14–19, 2001 in Hong Kong was studied by use of the new generation of regional air quality model WRF–chem V2.1, in which the meteorological model (WRF) and chemical model (chem) are fully coupled on line. In this episode, the regional air quality monitoring stations in Hong Kong recorded high concentrations of O3, CO and NOx for six days, the maximum concentration of O3 reached 191 ppb, and the atmospheric visibility dropped to 1.8 km. Investigations showed that WRF–chem can basically simulate this continuous photochemical pollution episode, with the best correlation coefficient of the simulated and the observed O3 concentrations being 0.84. The production and loss of O3 were significant in this episode, and the largest rates were 22.7 and −17.6 ppb h−1, respectively. The simulated meteorological conditions showed high temperature, lower relative humidity, strong solar radiation, northerly airstream, and stable boundary layer structure in Hong Kong during the episode, these weather factors were conducive to formation and maintenance of photochemical pollution. It was also clear that the vertical transport played an important role in this episode. As controlled by downdraft at the outside of Typhoon system with more stable atmosphere, air pollutants were limited and accumulated in the low level of boundary layer, leading to high O3 concentration from photochemical formation.