Characteristics Analysis And Numerical Simulation Of Atmospheric Visibility In Nanjing

Atmospheric visibility can reflect the quality of atmospheric environment.The low visibility events affect human production activities and life.Recently,with the rapid economic development and more serious air pollution,reduced visibility becomes important atmospheric environment issues.Nanjing where low visibility occurred more and more frequently has been one of the hazy cities.Therefore,analysis and numerical simulation of atmospheric visibility in Nanjing has important theoretical and practical value.In this study,the atmospheric visibility and its main influencing factors was analyzed with the observations.Experiments were designed to study the relationship between the chemical compositions of fine particulate matter and visibility,and localized visibility calculation scheme was established.A case of low visibility in Nanjing was simulated with the regional air quality model WRF-Chem.Research work and conclusions are as follows.Firstly,the spatial and temporal characteristics of atmospheric visibility and its influence factors in Nanjing were investigated.The mean visibility was 10.14km in urban Nanjing,and 9.64km in suburb Nanjing during 2011-2013.Atmospheric visibility exhibited a significant diurnal variation with the minimum at 7:00 and the maximum at 15:00-16:00.The highest visibility was in summer,and the lowest visibility was in winter.Using the method of cumulative percentile,the urban annual visibility was 11.5km?13.0km?13.5km,and the suburb was 12.3km?11.1km?11.5km.The correlation coefficient between RH(relative humidity),temperature,wind speed,sea level pressure and the atmospheric visibility were 0.66,0.21,0.17,-0.01,respectively.Otherwise,the correlation coefficient of atmospheric visibility with SO2,NO2,PM10,PM2.5 and O3 were-0.03,-0.31,-0.33,-0.50,0.22.RH and PM2.5 were important factors of visibility.With higher RH and mass concentration of PM2.5,the visibility becomes lower.Secondly,the characteristics of the chemical composition in fine particles and the relationship between the composition and visibility were studied.The average mass concentration of fine particles was highest in autumn(1 17.2?g/m3),and lowest in summer(31.6?g/m3).The ratio of PM2.1/PM10 was biggest in winter(0.68)and smallest in spring(0.30).Correspondingly,the lowest average visibility was in winter(5.2km),and the highest was in summer(18.5km).It indicated that the fine particles were abundant and the visibility was worst in winter.The average concentration of water-soluble ions in PM2.1 was 30.6?g/m3,accounting for 42%.TC(total carbon)constituted 24%of PM2.1.The most composition of water-soluble ions was SO42-(16.6%),followed by NO3-(6.5%),NH4+(5.7%).The concentration of OC and EC was 15.4?g/m3,3.2?g/m3,respectively.Among the chemical composition,NO3-,SO42-,NH4+,TC were strongly correlated with visibility.The correlation coefficients were-0.80,-0.53,-0.71,-0.69.The chemical compositions and atmospheric visibility was fitted with different RH in every season.From this,the localized visibility calculation scheme was established.Finally,a case with low visibility in Nanjing was analyzed and simulated by WRF-Chem with localized visibility calculation scheme.Results showed that the lowest visibility was in the middle of Nanjing,and the highest was in the west area.The site of the lowest monthly average visibility(2.95km)was in Jiangning,and the highest(4.02km)was in Pukou.The diurnal visibility variation was the worst in the morning and the best in the afternoon.The chemical composition affecting the visibility most was NO3-.The spatial characteristic of the visibility simulated by IMPROVE was similar with the results from localization scheme.Compared with the observation data,the visibility simulated by localization scheme was lower with absolute error of-3.28?-0.90km in 7 sites of Nanjing;while the results of IMPROVE were higher with absolute error of 0.50?-4.92km.The normalized mean deviation was-50%?18%from localization scheme and 8%-99%from IMPROVE.The accurate ratio of the low visibility(?5km)simulated by localization scheme was higher than that simulated by IMPROVE.In general,the visibility from localization scheme was better than IMPROVE compared to observations.Localization scheme is helpful to improve the forecast of visibility.