Analysis Of The Characteristics Of Atmospheric Particulate Matter Pollution In Baoding City And Research On The Threshold Of Visibility Control

Located in the North China Plain,the Beijing-Tianjin-Hebei region is not only the political and cultural center,but also an important economic growth driver of China,and its atmospheric pollution is arousing people's awareness.Rapid development of national economy,the acceleration of industrialization and urbanization had brought about frequent particulate pollution in recent years with apparent visibility deterioration.Visibility is a direct and visual indicator of air quality.Improving visibility and achieving the"blue sky"target is one of the main purposes of air pollution prevention and control,so it is of great importance to establish the quantitative correlation between visibility and particulate matter mass concentration.Baoding,a typical polluted city in the Beijing-Tianjin-Hebei region,was selected to analyze the concentration variation in particulate matters chemical component,light extinction and visibility changes in this paper.Moreover,theoretical thresholds of particulate matters and visibility were studied by means of data normalization and the method of maximum curvature.The main conclusions are as follows:(1)During 2013-2017,the total average of PM_2.5 and PM₁₀ mass concentrations were 107±84?g·m^-3and 178±115?g·m^-3 respectively,and the annual averages had decreased 43?g·m^-3and 84?g·m^-3 respectively.The annual average concentration of total PM_2.5 components were in a distinct declining trend,and were 51±34?g·m^-3(water-soluble inorganic ions),28±23?g·m^-3(carbonaceous aerosols),and 4126±1975 ng·m^-3(inorganic elements),respectively during 2014-2017.Contributed 82.9%to the concentration of the total ions,NH₄⁺,NO₃^-and SO₄^2-(SNA)were the dominant components,and became the main pollutants in fine particle pollution.(2)Based on the IMPROVE algorithm,the seasonal average light extinction coefficients(b_ext)were 373.8±233.6 Mm^-1,405.3±300.1 Mm^-1,554.3±378.2 Mm^-1,1005.2±750.3 Mm^-1,respectively in spring,summer,autumn and winter.Average reconstructed PM_2.5 mass concentration was 93±69?g·m^-3,with a total of 656.5±479.0 Mm^-1for annual average b_ext during observation period.In addition,Vb_sca and Rb_scawere compared to evaluate performance of the IMPROVE algorithm,with high correlation coefficients of 0.84.The Vb_sca of high values were underestimated and low values were overestimated through comparison with the one-to-one line.Especially when Rb_sca>1123 Mm^-1(corresponding to vis<2.0 km),Vb_sca was underestimated by17.6%.PM_2.5 mass concentration and relative humidity also caused impact on the estimation.It was proved that,high PM_2.5 mass concentration or high relative humidity tends to cause underestimation in Vb_sca.(3)Both PM_2.5 and visibility are important parameters to measure the status of air pollution,yet they represent different physical significance.This work firstly established a new normalization approach for the relationship between PM_2.5 and visibility in heavy pollutant region,which realized a mathematical calculation for two different physical parameters based on non-dimensional data process(also called Min-Max normalization).The normalization relationship showed a power function(y=ax^b)with high correlation coefficients.The inflection points of the maximum power function curvature composed the theoretical threshold range of PM_2.5 for improving or deteriorating the visibility.In the case city of north China,the threshold range of PM_2.5was btween 112-121?g·m^-3 corresponding to the visibility range of 5.9-4.5 km.Within the threshold range,the mean concentration of main pollutant components is 106?g·m^-3.The mean extinction coefficient,b_ext 743.5 Mm^-1,was mainly contributed by organic matter(34%),ammonium nitrate(19%),ammonium sulfate(18%)and elemental carbon(13%).When PM_2.5 was below the range,visibility increased significantly with slight decline of PM_2.5.When PM_2.5 exceeded the range,visibility was seriously deteriorated due to the saturated extinction contribution of the main pollutants,and was no longer sensitive to PM_2.5 increment.The new normalization approach has theoretically revealed the significance of thresholds on how to control PM_2.5 for improving the visibility at different pollutant stages.