Quantitative influences of interannual variations in meteorological factors on surface ozone concentration in the hot summer of 2018 in Japan

In this study, the high surface O3 episodes in the hot summer in Japan in July 2018 are studied using two models, NHM-Chem and ASUCA-Chem. The model performances are similar to each other with both models tending to overestimate the observed surface O3 by-5-10 ppbv. The meteorological factors causing the interannual changes of surface O3 are investigated through sensitivity tests of NHM-Chem using meteorological fields of different years, i.e., July 2007 and 2012. July 2007 and 2012 represent cold and normal summers, respectively, with respect to the monthly mean surface temperature in Japan. The hot summer provides favorable conditions for the O3 formation, such as a higher temperature and a higher solar radiation. However, the monthly mean surface wind speed in July 2018 is greater than that of the other periods, causing a negative effect on the monthly surface O3 concentrations because of faster dilution and diffusion in certain locations. Over the Kanto plain, which is the most populated region of Japan, the monthly mean surface temperature is-2 K higher than the normal values; however, both observation and simulation demonstrate that the monthly mean surface O3 is lower than the normal temperature summer (July 2012) by 4-6 ppbv. The sensitivity tests indicate that the enhanced biogenic volatile organic compound emissions increase surface O3 by 4-5 ppbv, but the wind field changes decrease surface O3 by 9-10 ppbv. Heat wave is associated with high surface O3 episodes; however, the monthly mean value is a mixed result of many other meteorological events in addition to the heat wave. The interannual changes in the monthly surface O3 can vary depending on magnitude of the positive and negative effects at each location.