The characteristics of ozone and related compounds in the boundary layer of the South China coast: Temporal and vertical variations during autumn season

We present measurements of several trace gases made at a subtropical coastal site in Hong Kong in October and November 1997. The gases include O3, CO, SO2, and NOx. The surface measurement data are compared with those from an aircraft study [Kok et al. J. Geophys. Res. 102 (D15) (1997) 19043-19057], and a subset of the latter is used to show the vertical distribution of the trace gases in the boundary layer. During the study period, averaged concentrations at the surface site for O3, CO, NOx, and SO2were 50, 298, 2.75, and 1.65 ppbv, respectively. Their atmospheric abundance and diurnal pattern are similar to those found in the 'polluted' rural areas in North America. The measured trace gases are fairly well mixed in the coastal boundary layer in the warm South China region. Large variability is indicated from the data. Examination of 10-day, isentropic back trajectories shows that the measured trace gases are influenced by maritime air masses, outflow of pollution-laden continental air, and the mixing of the two. The trajectories capture the contrasting chemical features of the large-scale air masses impacting on the study area. CO, NOxand SO2all show higher concentrations in the strong outflow of continental air, as expected, than those in the marine category. Compared with previously reported values for the western Pacific, the much higher levels found in the marine trajectories in our study suggest the impacts of regional and/or sub-regional emissions on the measured trace gases at the study site. The presence of abundant O3and other chemically active trace gases in the autumn season, coupled with high solar radiation and warm weather, suggests that the South China Sea is a photochemically active region important for studying the chemical transformation of pollutants emitted from the Asian continent.