Effects of anthropogenic heat on ozone air quality in a megacity

Anthropogenic heat released into the atmosphere in urban areas affects the characteristics/structures of boundary layer and local circulation and thus can affect local air quality. In this study, the effects of anthropogenic heat on ozone (O3) air quality in the Seoul metropolitan area, Republic of Korea, are examined using the Community Multiscale Air Quality (CMAQ) model coupled with the Weather Research and Forecasting (WRF) model. The release of anthropogenic heat is found to increase O3 concentration by 3.8 ppb in the urban area, and this effect is larger in the nighttime (5.3 ppb) than in the daytime (2.4 ppb). As stronger anthropogenic heat is released, the urban boundary layer becomes deeper and NOx concentration becomes lower due to the dilution. The decrease in NOx concentration causes the chemical loss of O3 by the reaction with NO to be reduced. In addition to the enhanced net chemical production of O3 in the deepened urban boundary layer, the strengthened urban-breeze circulation by anthropogenic heat contributes to an increase in O3 concentration in the urban area. The O3-rich air in the surroundings of Seoul, where biogenic volatile organic compound (BVOC) emissions are high and NOx emissions are low, is brought into the urban area by the urban breeze. Moreover, in the daytime, the advection of oxidant products of BVOCs by the urban breeze contributes to an enhanced chemical production of O3 in the urban area. Anthropogenic heat also modifies the sea/land-breeze circulations, which, in turn, is found to influence O3 concentration in the urban area.