Increase of Ozone Concentration in an Inland Basin During the Period of Nocturnal Thermal High

Since ground level ozone concentrations in the basin on one day before the occurrence of unusually high air temperature with nocturnal thermal high showed a typical urban type of a maximum ozone concentration at 1300 LST and a minimum at night. However, a maximum ozone concentration under extremely high air temperature of 39.2 °C was detected at 1700 LST or 1800 LST at two environmental monitoring sites, which was 4 or 5 hr delayed from the typical occurrence time, 1300 LST. Its maximum value showed about 50 or 70% increase of the concentration more than the typical maximum value and its concentration gradually decreased until 2100 LST. After 1200 LST until 1800 LST, air temperature was maintained over 35 °C and the high temperature made a great contribution to the increase of O₃ for several hours. The deviated occurrence time of a maximum ozone concentration is mainly attributed to meteorological and topographic effects – shifted occurrence time of maximum air temperature, shrunken atmospheric boundary layer depth and wind. While daytime O₃ concentration due to photochemical production of O₃ from NO₂ increased, NO₂ concentration decreased, with their reverse respondent patterns each night. A secondary maximum concentration of O₃ at 2300 LST or 2400 LST is due to a much shallower depth of nocturnal surface inversion layer with daytime producing more O₃ than that of the daytime convective boundary layer, resulting in the increase of ozone concentration, though the reduction of ozone occurred under the reversal process of O₃ into NO₂.