Characteristics Of PM_2.5 And Its Main Water-soluble Inorganic Ions In Beijing Under Different Air Pollution Processes

In the past few years,Beijing has experienced frequent heavy pollution incidents with PM_2.5 as the main pollutant,but now the air quality has generally improved,and the primary pollutants such as SO₂ have been significantly reduced,but PM_2.5 pollution still occurs occasionally in winter.As an important component of PM_2.5,water-soluble inorganic ions account for more than 30%of the mass concentration of fine particulate matter,which has a non-negligible impact on the environment,climate change and human health problems.It was of great significance to study the change characteristics of PM_2.5 and its water-soluble inorganic components under different air polluted processes and the formation mechanism of major water-soluble inorganic ions,which was of great significance for the formulation and prevention of air pollutants prevention and control countermeasures.In this study,a one-year observation experiment on water-soluble inorganic ion components in PM_2.5 was conducted in Beijing（October 2021-September 2022）,focusing on the analysis of the mass concentration characteristics and formation mechanism of water-soluble inorganic ions in different polluted processes,and the main conclusions were as follows:（1）During the observation period,online continuous observations of water-soluble inorganic ions of fine particulate matter in the atmosphere were performed using an on-line ion chromatography monitor,and these nine inorganic ions were analyzed using an ion chromatograph.Throughout the observation period,the average mass concentration of PM_2.5 in the atmosphere of Beijing was 30.5μg/m³,which did not exceed the secondary standard（35μg/m³）of China’s national air quality standard（GB3095-2012）,and the average mass concentration of water-soluble inorganic ions in PM_2.5 was 10.6μg/m³;The mass concentration order of water-soluble inorganic ions was:NO₃^->NH₄⁺>SO₄^2->Cl^->Ca²⁺>Na⁺>Mg²⁺>K⁺.The average mass concentrations of water-soluble inorganic ions in spring,summer,autumn and winter were 14.8μg/m³,8.5μg/m³,16.0μg/m³ and 13.5μg/m³,respectively,with high mass concentrations in autumn and spring and low in summer.（2）Combined with meteorological data（wind speed,wind direction,relative humidity,temperature）,it was found that the the processed of pollution were often accompanied by lower wind speed,south or southwest wind,lower temperature and higher relative humidity.The observed data of the whole heating season（November2021 to March 2022）were divided into cleaning period and pollution period,the total water-soluble inorganic ions accounted for 38.2%of the mass concentration of PM_2.5during the clean period,while the polluted period accounted for 61.4%of the mass concentration of PM_2.5,and during the Winter Olympic and Paralympic Winter Games,water-soluble inorganic ions accounted for 40.6%and 38.1%of the mass concentration of PM_2.5,respectively.（3）Both sulfur oxide ratio（SOR）and nitrogen oxide ratio（NOR）have a strong relationship with relative humidity.Especially at RH>60%,SOR increases dramatically with RH increase,which means that RH is an important factor affecting sulfate formation.The ratio of PM_2.5/CO in the polluted period of Beijing was 3.3 times that of the clean period,which indicated that secondary transformation processes such as sulfate,nitrate and ammonium（SNA）were significantly enhanced during polluted periods.The formation mechanism of sulfate and nitrate at high relative humidity was discussed,and the value of NOR increased with the decrease of O₃ concentration and the increase of RH at night,indicating that heterogeneous hydrolysis of N₂O₅ may contribute to the formation of HNO₃ under nocturnal high humidity conditions.