Research On The Characteristics Of Precipitation In Chengdu And Its Effect On Pollutant Removal

The source controls of air pollutants have had an effect in recent years,but atmospheric conditions still have an impact on air pollution.Studying the relation between atmospheric conditions and pollutants would help to develop and implement programs of air pollution control.Based on hourly data of six primary pollutants(SO₂?NO₂?CO?O₃?PM_2.5?PM₁₀)from 16 environmental monitoring stations in 2016-2018and hourly precipitation data of 16 automatic stations near to the environmental monitoring stations,statistical method and Inverse Distance Weighting are used to analyze the spatial-temporal characteristics of precipitation and pollutants in Chengdu.Removal rate which is as an index and typical precipitation events are also used to quantify the cleaning effect.Not only precipitation,but the differences of cleaning effect of different rainfall intensity,duration,and day and night are also considered.The main results are as follows.(1)The concentration of SO₂ which is related to industrial sources decreases from northeast to southwest;Because of the vehicles and population,the highest concentrations of NO₂,CO,PM_2.5 and PM₁₀ are located in the middle part of Chengdu and reduce to the surrounding;The high concentration of O₃is in the suburbs with lush vegetation,meanwhile the low concentration is in the middle part of Chengdu.The daily variations of SO₂,CO,NO₂,PM_2.5,PM₁₀ present bimodal pattern because of the daily variations of air pressure and human activities.The peaks are at midnight and 9-10am.The daily variation of O₃ is a single peak with the highest value at 4pm.(2)The highest values of annual daily precipitation,annual night rain,precipitation intensity,rain days and night rain days are in the northwest and southwest of Chengdu.The night rain ratio is above 60%,and the precipitation mainly occurs after midnight.(3)Basal concentration represents pollutant concentration before precipitation.The removal rate is close to zero while the basal concentration is too low.Therefore,the precipitation events with the basal concentration of SO₂,NO₂,CO,O₃,PM_2.5,PM₁₀ above 10?g/m³,35?g/m³,0.6 mg/m³,35?g/m³,25?g/m³,35?g/m³are chosen to analyze the cleaning effect.There is little cleaning effect of light rain on pollutants.The cleaning effects of the amount of precipitation on SO₂,NO₂,PM_2.5,PM₁₀ are as follows:intense fall>heavy rain>moderate rain>spite>light rain;Cleaning effects on O₃ are as follows:intense fall>heavy rain>moderate rain and below;There is small difference between cleaning effects on CO in different daily precipitation.Because of the downward transport of O₃,The cleaning effect on O₃ decreases with the precipitation intensity.The precipitation cleaning effects on other pollutants are proportional to the precipitation intensity.(4)Because of the difference of basal concentration,the removal rates of diurnal rainfall on SO₂,CO and particulate matter are higher than the removal rates of night rainfall,while the removal rates of night rainfall on NO₂ are higher than the removal rates of diurnal rainfall.The removal rates on pollutants(except O₃)are highest in winter,next in spring;the cleaning effects are unapparent with low basal concentration in summer and autumn.(5)There are obvious cleaning effects on particulate matters in the 3^rd?12^thhours of precipitation.There are obvious cleaning effects of gaseous pollutants within the first five hours of precipitation.The high cleaning effects of persistent precipitation on pollutants mostly occur in first two days of rainfall with the maximum reduction of pollutant concentration.Started from the third day,the cleaning effects almost disappear.(6)Research of typical precipitation events has shown that removal rates do relate to rainfall intensity,time of duration and basal concentration.The higher the rainfall intensity is,the higher the removal rate is.As precipitation time increases,the pollutant concentrations and removal rates keep decreasing.