Abstract:

Based on the  observation data of acid rain in Wuxi during 2008-2011,we analyzed the temporal characteristics of acid rain,the relationship with meteorological conditions and the characteristics of 1500 m moving path to investigate the effect of diffusion of acid precursor on acid rain with power spectrum analysis,backward trajectory and clustering analysis.The results showed that the annual value of pH appeared an increasing trend in Wuxi.The frequency of strong acid rain decreased year by year,while that of weak acid rain increased.The frequency in summer and autumn was low while that in spring was high.The acidity of rain was weak in June and July while strong in September.The largevalue and lowvalue months of strong acid rain frequency were March and July,respectively.The largevalue and lowvalue months of weak acid rain frequency were August and September,respectively.The monthly pH and the frequencies of weak acid rain and strong acid rain had different periodic change.We found that the monthly pH had lowfrequency variation of 56 months.We also found that the frequencies of weak and strong acid rain had highfrequency variation of 2.1 and 2.5.The acidity of rain increased first and then decreased with the increasing precipitation,because the concentration of acid substance was high at the initial stage of raining while it was lower with raining.But the relationship between the precipitation of summer and the pH of acid rain was not significant while its significant in spring,autumn and winter.There was significant negative correlation between the relative humidity of spring and the value of pH because the ascending of relative humidity strengthened the hydration of acid ions and the acidity of rain was higher with the absorption and collision of rain.There was significant positive correlation between atmospheric temperature of summer and the value of pH because hyperthermia helps vertical convection and the concentration of acid precursor became lower with horizontal migration of 1500 m.The simulations of backward trajectories showed that the trajectories of 1 500 m in different seasons are different,but Jiangsu,Zhejiang,Jiangsu are the main sources of acid precursor.These domains were polluted severely and they were typical highvalue regions of   and  .The paths of simulations of backward trajectories were short relatively and the air mass moves slow in these regions.It did not help the dilution and diffusion of pollutants,but the effect of vapor transportation eased the acidity of rain from Western Pacific in summer and autumn.In general, there is relationship between rain in Wuxi and acid precursor of Jiangsu,Zhejiang and Jiangxi in spring,summer and autumn.The trajectory of transportation on pollutants is long and it helps the dilution and diffusion of pollutants,so the pH of the rain in winter is larger than the pH in spring