A Simulation Study Of The Effect Of Lake-air Exchange On High Ozone Concentration Events

Lake Taihu basin is one of the most developed in economy and industry and most densely populated areas in China.With the increase of NO_x and VOC_s emissions and the frequent occurrence of high-temperature heat waves,ozone pollution has become increasingly severe,requiring more research and attention.The local circulation generated by the thermal difference between lake and land complicates the research on air pollution,especially in the conditions with high temperature and lite wind.Under this circumstance,accurately reproducing the lake dynamic and lake-atmosphere exchange is of great significance to the research and prediction of air pollution.The impacts of complex local circulation on ozone distribution and chemical reaction,when lake,city or mountain exists,have become the challenging question in the current simulation and prediction of air pollution.In this article,parameters in Community Land Model 4.5(CLM4.5)lake model in Weather Research and Forecasting model coupled with Chemistry model was modified.The performance of this model on the simulation of meteorological variables and ozone concentration is evaluated first,and then,the impacts of lake-atmosphere exchange on ozone simulation and the contributions of Lake Taihu(LT)to ozone high-concentration event are analyzed.The results indicate that:(1)The improved CLM4.5 lake model can better simulate the characteristics of large diurnal temperature variations,high water temperature during the day,and low water temperature at night in the shallow lake.Meteorological variables on the lake have the most obvious improvement with the increasement in lake surface temperature of about 8?.Simulated 2m air temperature over the lake surface also increases by 3.31?.The improvement of water temperature simulation modifies the local circulation,and further improves the simulation of wind speed.The simulation results of the meteorological variables on the land have also been improved accordingly.The improvement of the simulation of meteorological variables refreshes the characteristics of the local circulation,which also makes the simulation results of ozone concentration improved.The average RMSE decreases from 55.40?g m^-3 in NL to 47.97?g m^-3 in Lake-improved case.(2)Lake-atmosphere exchange simulated by CLM4.5 lake model and Noah scheme significantly change the characteristics of local circulation.As a result,the vertical and horizontal advection directly impacts distribution of ozone around and over the lake in early morning,the difference in ozone concentration is 20-60?g m^-3.Location of the lake breeze front and development of the thermal inner boundary layer(TIBL)around midday are also highly sensitive to the lake-atmosphere exchange.In addition,difference in the local circulation patterns leads to distinct spatial distributions of NO_x,an ozone precursor,in regions downwind of their emission sources,especially along the Yangtze River.In response,ozone concentration in this region is drastically changed with maximum value of about 80?g m^-3 due to the combined actions of transport and chemical reactions.(3)Under the influence of lake breeze,the ozone on the lake surface will gradually be transported to the surrounding land,and the ozone on the land will be blown further downwind.Due to the formation of TIBL,the area within 5-15 km from LT formed a low-value zone of near-ground ozone at 13:00-15:00.Although the lake breeze circulation triggered by the thermal difference between LT and the surrounding land can reduce the ozone concentration in the surrounding area in the afternoon.However,when LT exists,ozone and its precursors accumulates over lake surface under the influence of land breeze in the night,and the nature of the lake is conducive to the photochemical generation of ozone,causing the increase of ozone concentration over lake surface.Therefore,LT will still make a positive contribution to the high ozone concentration incident around the midday.Local circulation induced by the surface temperature difference between water and land surfaces complicates the study of air pollution,especially under high-temperature and light-wind synoptic condition,where local circulation is a dominant controller.Furthermore,under such circumstances,accurately simulating the lake dynamics and lake-atmosphere interactions is critical for the analysis and prediction of air pollution.