Seasonal Relationships Between Tropospheric Ozone and Its Precursors over East Asia

Satellite data of ozone with its various precursors and global reanalysis products of vapor throughout 2010 and 2012 are used to evaluate spatial-temporal variations of tropospheric ozone and its various precursors over East Asia with their correlations over East China. Northern and southern regions of East China are treated separately in order to address Asian Summer Monsoon's different influences on these two regions. It shows that the tropospheric column densities of NO₂ and CO are high in winter, while low in summer. The tropospheric column density of ozone reaches its maximum and minimum in summer and winter, respectively. Correlations between ozone and NO₂ varies similarly with seasons change, appearing positive in summer and autumn, while negative in winter and spring over both regions. There are significantly positive correlations between ozone and NO₂ over both northern and southern regions during summer, which demonstrates due to the strong sunlight, NO₂ appears short life-time and declined densities, thus the NO_x-involved photochemical cycles produce ozone actively in summer. The negative correlations during winter result from the depression of ozone photochemical cycles due to long life-time and high densities of NO₂ over most regions, especially in the northern region with much more heating emissions, leading to ozone depletion by NO. Owing to the transport of polluted air masses to the downwind directions, positive correlations between ozone and CO reach the maximum in summer and autumn over the northern region and summer over the southern region. There are slightly negative correlations between ozone and CO over the northern region in winter and over the southern region in summer, because CO reaches maximum companioned with the enhancement of NO that has the titration effect to deplete ozone. In addition, stratospheric intrusion might also cause inverse correlations between ozone and CO. In most time over northern and southern regions, correlations between ozone and vapor appear highly positive, while they appear highly negative in summer over the southern region and in winter over the northern region. Although southwesterly vapor is usually companioned by pollutant transport and photochemical reaction increasing, air masses from the sea bring sufficient vapor and little ozone in summer over the southern region, and vapor in clean marine air masses may deplete ozone as well. The inverse tracing relationships between ozone and vapor possibly result from stable weather condition variations. In both regions, correlations become more significant under slightly pollution conditions, however, they are insignificant in the severely polluted and background regions, indicating that due to enhanced NO, ozone is depleted through titration effects over severely polluted regions, whereas ozone elevates with the weaker NO titration over the downwind slightly polluted regions.