Characteristics Analysis And Numerical Study Of Tropospheric Ozone Over East Asia

Ozone is an important constituent of atmosphere and plays a key role in global climate change and atmospheric environment.Tropospheric ozone is not only a main greenhouse gas,but also a secondary pollutant.It has been known to the public because of extensive impacts on air quality,climate and human health.With rapid economic development,photochemical pollution over East Asia becomes more and more serious.Investigation on tropospheric ozone will be benefit to reveal the trend of surface ozone and tropospheric column ozone(TCO),understanding of interaction between climate change and air pollution.In this study,with the use of observation data and model simulation,firstly,the inter-annual and seasonal variability of tropospheric ozone were analyzed over East Asia.Then,simulation performances of a regional climate chemistry model(RegCM4+Chem)were evaluated.Finally,effects of heat wave and East Asian Summer Monsoon on ozone and the main influence factors were revealed using RegCM4+Chem and process analysis.Firstly,analysis of surface ozone and TCO indicated that concentration of yearly average surface ozone is about 41.4 ppb over East Asia during 2012.From 2000 to 2012,East Asia surface ozone showed an average upward trend of+0.56 ppb yr-1 and the maximum growth rate is+1.53 ppb yr-1.Surface ozone showed an apparent seasonal cycle with maximum amplitude of 45 ppb,the highest value occurred in April or May and the second highest value mostly happened in September or October,while ozone level in summer and winter was the lowest.Compared results of three types of observation stations,at urban sites,nitrogen oxide level(NOx)was the highest while ozone was the lowest.For remote sites,ozone value was the highest while nitrogen oxide level was the lowest.Values of ozone and NOx at rural sites were situated between urban and remote levels.Annual mean level of TCO over East Asia was 32.2 Dobson Unit(DU).TCO value over Tibet Plateau was the lowest while the highest value located in eastern China.From south to north,TCO showed an obvious positive gradient.Seasonal cycle of TCO was distinctly with maximum in June and minimum in November.High value center of TCO shifted from south to north during March to August.Secondly,evaluation of RegCM4+Chem demonstrated that the model can accurately simulate meteorological field,surface ozone levels and its seasonal cycle over East Asia.Compared the observation and simulation,average of the absolute deviations was 3.5 ppb and the relative deviation was 9.6%.Correlation coefficient of seasonal variation between simulation and observation was 0.72.The study of heat wave revealed that surface air temperature and ozone value showed a consistent variation and a significant positive correlation.Comparing with the same period in other years,temperature and ozone value increased 2.67℃ and 3.91 ppb separately in June 2010.The temperature and ozone were 14.2%and 15.1%higher than the average during the heat wave,respectively.Increasing the rate of chemical reaction and weakening of dry deposition process are the key factors that contribute to raising ground ozone levels during heatwave.Finally,ozone variance analysis between strong and weak East Asian summer monsoon(EASM)proved that anomalies of EASM have a remarkable impact on spatial distribution of low layer ozone.Ozone difference range between strong EASM and weak EASM was-6.5～6.9 ppb from May to August with the biggest difference in August.Average difference of 4 months ranged from-3 ppb to 3.8 ppb,it accounted for-10%to 12.5%of the average ozone over the area.Process study indicated advection and chemistry reaction acted main role in the influence of EASM on ozone.Via wind,circulation fields,total cloud fractions and net downward solar radiation flux,anomalies of EASM influenced advection and chemical reaction process of ozone and eventually lead to the differences of ozone spatial distribution at lower atmospheric layer.