Photochemical and transport effects on NOx and ozone concentrations in Seoul during the 22 July 2009 solar eclipse

The temporal and spatial variations of NOx and O3 concentrations in Seoul during the 22 July 2009 solar eclipse are investigated. During the solar eclipse, observation data at 25 stations in Seoul show increases of NO2 concentrations and decreases of O3 and NO concentrations due to weakened NO2 photolysis in the presence of O3 titration by NO. O3 concentration averaged over Seoul is estimated to be reduced by 45% due to the solar eclipse about an hour after the solar eclipse maximum. The maximum reductions of O3 concentrations downwind of NOx source area are higher and appear later than those in the downtown area. An analysis of photostationary state (PSS) between NO, NO2, and O3 characterizes the downwind area as a region with high NO2/NO and low O3, whereas NO2/NO is low in the downtown area. A large deviation from the PSS between NO, NO2, and O3 in the downwind area strongly implies that, in addition to photochemical effects, the effects of transport by winds strengthen an ozone reduction potential in the downwind area during the solar eclipse.