Research On The Impact Of Summer Precipitation On Urban Air Pollutants

Urban air pollution is a gradual environmental problem in the process of urbanization.Air pollutants have received widespread attention because of their significant impact on climate change,visibility,and human health.This study uses hourly air pollutant data from2015 to 2017 to study the spatial distribution and characteristics of different time scales of six air pollutants(PM_2.5,PM₁₀,NO₂,CO,SO₂,O₃)in Hefei;Using the precipitation data of Hefei City in the summer of 2015 to 2017,combined with the raindrop spectrum data,the characteristics of the change of atmospheric pollutant concentration in Hefei City during the summer precipitation were analyzed,and discussed in conjunction with typical cases.The main conclusions are as follows:1.The annual average concentrations of gaseous pollutants CO and SO₂ are on the whole decreasing.The average annual concentrations in 2017 were 0.87?g·m^-3 and 12.43?g·m^-3,respectively,which were significantly lower than those in 2015 and 2016.The concentration level of each season shows that the concentration of CO and SO₂ is the highest in winter,followed by spring and autumn,and the lowest in summer.The concentration of CO and SO₂in summer is 0.78?g·m^-3 and 10.49?g·m^-3,respectively,and the concentration distribution interval is more concentrated.The daily concentration of CO is characterized by a double-peak type.The peak value usually appears at noon and night from 21:00 to 22:00.The lowest value of the day's concentration generally appears in the afternoon.The daily SO₂concentration change shows a single peak type,and the summer SO₂ daily concentration is about It is 10?g·m^-3,which is obviously lower than other seasons.In spring and winter,the SO₂ concentration value is relatively high,and the concentration levels in the two seasons are close,the overall change in daily SO₂ concentration is small,and the peak concentration appears around 10:00 in the morning.And the duration of concentration fluctuation is shorter.2.The annual average concentration of O₃ and NO₂ in 2015-2017 is increasing year by year.From 2015 to 2017,the annual average concentration of O₃ in Hefei City was44.34?g·m^-3,59.75?g·m^-3,73.55?g·m^-3;NO₂ is 34.98?g·m^-3,43.38?g·m^-3,47.85?g·m^-3.During the four seasons,the concentration of O₃ is the highest in summer,followed by spring and autumn,and the lowest in winter.The average of summer and winter is 68.94?g·m^-3 and38.41?g·m^-3;NO₂ concentration is the lowest in summer and the highest in winter.The characteristic of daily concentration change of O₃ is single-peak type.There is a significant rise and fall process of daily concentration from 08:00 to 23:00.The peak value of O₃ daily concentration appears at 15:00.During other periods,the concentration of O₃ maintains a low level.The range of daily concentration of O₃ is reduced,but the average daily concentration is not significantly affected.The concentration of O₃ at night without precipitation is 55-60%of the time when precipitation occurs;the daily concentration of NO₂ is bimodal,with peaks at 09:00 and 22:00,the peak concentration at night was significantly higher than during the day,and the concentration of NO₂ at all times during summer precipitation was significantly lower.3.The average annual concentrations of PM_2.5 and PM₁₀ are 59.52?g·m^-3 and93.53?g·m^-3,respectively,which are decreasing year by year.The average concentrations of PM_2.5 in spring,summer,autumn and winter were 17.10?g·m^-3,10.19?g·m^-3,13.55?g·m^-3and 18.51?g·m^-3,respectively.The concentrations of PM₁₀ were 62.54?g·m^-3,38.10?g·m^-3,51.24?g·m^-3 and 94.15?g·m^-3.The concentrations of PM_2.5 and PM₁₀ in each season were winter>spring>autumn>summer;The spatial distribution of PM_2.5 and PM₁₀concentrations in each season in Hefei city is gradually decreasing from north to south,but it will be prominent in some areas.From the perspective of seasonal change,PM_2.5 and PM₁₀concentrations have a trend of northward regression in spring,and then continue to move southward,showing a cyclical change from North to south again.The daily concentration of PM_2.5 and PM₁₀ showed a typical two peak pattern,the peak concentration time was at 08:00a.m.and 21:00 p.m.,and the minimum concentration of PM_2.5 and PM₁₀ was at 17:00 and14:00 p.m.,respectively.The daily concentration of each season was significantly different,the same time concentration was winter>spring>autumn>summer,in which the concentration of PM_2.5 in summer was about 35-40%of that in winter,and PM₁₀ in summer was about 35-40%of that in summer 40-50%in winter.In summer,precipitation can significantly change the concentration level of air pollutants.PM_2.5 and PM₁₀ collide when precipitation occurs,and their concentration change characteristics are easily affected by the distribution of precipitation periods.At night,PM_2.5 and PM₁₀ will fall to relatively stable concentration.In the process of precipitation,it can significantly change the mass proportion of particles with different particle sizes in the daytime,and PM_2.5/PM₁₀ in different precipitation strips after sunset All of them are at the same level,which shows that when the particles fall to a stable concentration at night,the mass proportion of particles with different sizes in the atmosphere is also stable.4.Summer precipitation has a significant effect on PM_2.5 and PM₁₀.According to the distribution period of precipitation in the observation,when the precipitation occurs in the morning and evening,the PM_2.5 and PM₁₀ concentrations significantly reduce in the precipitation process,and the PM_2.5 and PM₁₀ mass concentrations will maintain at a lower level after the precipitation ends;while for the precipitation in the afternoon,because the PM_2.5 and PM₁₀ concentrations are in a downward trend in this period,the impact of precipitation on the concentration change is relatively insignificant.The results also show that the mass concentrations of PM₁₀ and PM_2.5 aerosols are different during precipitation.Similarly,the aerosol mass concentration changes in the range of 2.5-10?m and 1.0-2.5?m particle size are also different;the mass concentration structure of particles with different particle size changes significantly before and after precipitation time.When precipitation occurs,PM_2.5/PM₁₀ on the same day is significantly smaller than that without precipitation,and PM_2.5/PM₁₀ increases significantly during precipitation,especially PM_2.5/PM₁₀ at the beginning of precipitation is significantly smaller than that at the end of precipitation,that is to say,the removal effect of raindrops on PM_2.5-10 is more obvious during precipitation;when precipitation ends,PM_2.5/PM₁₀ decreases significantly and maintains a low level,which may be larger than that after precipitation ends The formation process of particle size particles.