Pollution Characteristics Of PM_2.5 And Its Secondary Components In Handan City From 2016 To 2018

Secondary components were produced by gaseous pollutants through complex atmospheric chemical reactions.It was of great significance to study the secondary components for the coordinated control of compound pollutants.In recent years,the comprehensive implementation of"coal to electricity,coal to gas"transformation project in Hebei Province,the changed of energy structure maked a significant change in emissions,secondary components accounted for more and more in the composition of the atmosphere.This study studied the characteristics of secondary components in the interannual variation,seasonal variation,different pollution levels and heavy pollution process in winter of Handan city from 2016 to 2018.Relavant results showed below:From 2016 to 2018,the number of days when the daily average PM_2.5concentration in Handan City exceeded the national average secondary standard?75?g/m³?was decreasing year by year,but the mass concentration of PM_2.5?101.9?g/m³?in 2018 was 2.9 times of the national average secondary standard?35?g/m³?,indicating that the PM_2.5 pollution in Handan city was still serious.The contribution of secondary components to PM_2.5 in 2016-2018 was 51.7%,66.7%and 61.7%respectively,and the proportion of secondary components increased.In 2016-2018,the highest PM_2.5concentration appeared in winter,the seasonal variation of secondary organic aerosol?SOA?was larger,and the contribution of secondary inorganic component?SNA?to PM_2.5 was higher in summer and autumn.Gaseous pollutants provide the material basis for the formation of secondary components.Meteorological factors affect the transport and transformation of secondary components by changing the oxidation of the atmosphere.With the increase of pollution level,the SNA and SOA of heavy pollution period?HP?in 2016-2018 are 3-5 times and2.9-4.8 times of clean period?CP?SNA and SOA.The increase of nitrogen conversion rate?NOR?and sulfur conversion rate?SOR?indicated that the transformation of NO₂and SO₂ from gas phase to particulate phase was enhanced,and the mean value of NO₂/SO₂ was greater than 1.The emission of nitrogen in the atmosphere was higher than that of sulfur.With the increase of aerosol liquid water content?ALWC?,nitrate was mainly homogeneous reaction,while sulfate was mainly heterogeneous reaction.RH and ALWC had poor correlation with SOA,and heterogeneous reaction had little influence on the formation of SOA.In the winter of 2016-2018,the secondary components accounted for 54%,60%and 68%of the chemical components respectively,and the proportion of secondary components showed an increasing trend year by year.When the temperature was lower than 2?,the ratio of SOC/OC decreased with the increase of temperature.Low temperature was conducive to the adsorption and condensation of VOCs on particles.Both SOR and NOR increased with the increase of RH,indicating that heterogeneous reaction played an important role in the formation of sulfate and nitrate;SOC/OC increased with the increase of O₃ concentration,and O₃ promoted the formation of SOA.The heterogeneous hydrolysis of N₂O₅ at high RH contributes significantly to the nocturnal atmospheric nitrate,and the heterogeneous reaction of NO₂ on the surface of mineral aerosols may be responsible for the diurnal variation of nitrate.