Relative enhancements of ozone, carbon monoxide, nitrogen dioxide, and aerosols at the Earth´s surface during Asian dust episodes in spring

Time series of ozone, CO, and NO₂ surface mixing ratios and of aerosol density have been derived using ultraviolet photometry, infrared absorption spectrometry, gas filter correlation radiometry, and beta-ray absorption, respectively, at Busan, Korea (35.23°N, 129.07°E) since 1992. Enhancements of ozone above 100 ppbv were observed during an Asian dust event on 1 May 1999. A secondary ozone maximum observed at the surface was expected as the result of invasion of air from the free troposphere due to the jet stream. Sudden increases in CO, NO₂ and aerosols were caused by the combination of the deepening cut-off low connecting with a surface cyclone, and the slow-moving surface anticyclone located to the south of the upper trough over Korea. The elevated mixing ratios of these gases and aerosols were the result of chemical reactions and stratosphere-troposphere exchange due to tropopause folding in the rear of the jet streak under these weather conditions. In the presence of sufficiently high concentrations of NO_x, the enhancement of ozone was due to the catalytic reaction of CO that originates in fossil fuel and biomass burning emissions from China, together with ozone precursors such as VOCs that occur in Busan, Korea. In addition, transport of ozone was determined not only by the invasion of stratospheric ozone followed by tropopause folding of the deep cut-off low associated with the movement of total ozone, but also by long-range horizontal transport from China and from an anticyclone located to the south of Korea. The relative importance of chemical reactions and transport of these gases in the urban area was analyzed using an urban airshed model (UAM) that included biogenic and anthropogenic emissions of VOCs and other gases. It showed that we could estimate the net flux of the ozone mixing ratio due to both long-range advection and free troposphere-boundary layer exchange in connection with stratosphere-troposphere exchange during Asian dust events in springtime.