Characteristics And Evolution Of Nitrogen/sulfur Compound Pollution In Typical Northern Cities

In order to study the pollution characteristics and evolution rules of particulate and gaseous nitrogen/sulfur pollutants of a typical city in North China,the hourly data of water-soluble ions and gaseous precursors monitored by online ion chromatography in the heating period of 2017-2019 in Environmental monitoring station of Dezhou were statistically analyzed.Combined with the meteorological data and the conventional ambient air quality data at this site,the pollution characteristics of the annual,monthly and daily changes of nitrogen/sulfur compounds in the air of Dezhou City during the heating season were analyzed,and the evolution law of nitrogen/sulfur compounds under different air pollution levels were discussed.The main conclusions are as follows:1.Pollution characteristics of PM_2.5.5 in Dezhou CityIn the heating period of 2017-2019,the average mass concentration of PM_2.5.5 was81.46±51.38,85.36±49.63 and 80.59±49.56μg/m³,respectively,which indicated that the PM_2.5.5 concentration rose fiest and then declined in 2019,which may be closely related to the decline of industrial and human activities during the prevention and control of novel coronavirus pneumonia.The average value of the atmospheric extinction coefficient(σ_ext)during 2017-2019 heating period was 424±270,495±282,461±296 Mm^-1,respectively,and the correlation coefficient（R²）of the fitting straight line between the mass concentration of PM_2.5.5 and the extinction coefficient in the heating period of 2017-2019 was above 0.95,which showed that the fine particles in the atmosphere had a great impact on the atmospheric extinction.The PM_2.5.5 mass concentration of each heating period increased significantly in December,which may be due to a large number of pollutants emitted by heating in winter.The PM_2.5.5 mass concentration decreased significantly in February during the 2019 heating period,which may be related to the strict traffic control that began after the Spring Festival,resulting in a reduction of the primary pollutants emitted by motor vehicles.Months with high PM_2.5.5 concentration were generally unfavorable meteorological conditions of calm and high humidity,and the weather conditions in 2019 are particularly unfavorable due to the increase in relative humidity during the heating period period.2.Pollution characteristics of nitrogen/sulfur compounds in Dezhou CityIn the heating period of 2017-2019,the order of mass concentration of gaseous precursors in Dezhou City was NO₂>SO₂>NH₃,and the change range of which was34.90-44.82,12.75-27.66 and 12.22-17.92μg/m³,respectively;The concentration of NO₂ and NH₃ showed the trend of rising in 2018 and then declining in 2019,while SO₂ showed the trend of continuous declining;the concentration order of the concentration of secondary ions was NO₃^->SO₄^2->NH₄⁺,which showed an obvious rising trend,the range of which was 18.70-23.71,12.31-15.05 and 9.57-13.01μg/m³.The sum of NO₃^-,SO₄^2-,and NH₄⁺was defined as SNA,and SNA/PM_2.5.5 exceeded 0.5in three-year heating period,indicating that secondary ions were an increasingly important component of PM_2.5.NO₃^-/SO₄^2-was significantly higher than the average level of domestic cities,suggesting that the main sources of air pollution in Dezhou City were mobile sources.The concentrations of NO₃^-,SO₂,SO₄^2-and NH₄⁺were significantly higher from November to February of the next year due to the impact of winter heating;The concentration of NO₂ kept a high value from November to January of the next year,and had a significant downward trend in February,which may be related to the reduction of NO₂ emissions from motor vehicles during the Spring Festival.The concentration of NH₃ reached a high value in March in each heating period,which indicated that NH₃ was greatly affected by agricultural production activities and less affected by heating in winter.The average hourly mass showed that the concentrations of SO₂,SO₄^2-,NH₃ and NH₄⁺all showed peaks in daytime（08:00-17:00）,which was related to the large amount of NH₃,SO₂ and NO₂emitted in daytime.The concentration of NO₂ showed a concave distribution,its high concentration at night may be related to the reaction between NO and O₃ or between NO and atmospheric free radicals.There were also obvious three peak changes for NO₂,which were greatly affected by vehicle exhaust emissions and the heterogeneous transformation of NO₂ at night.The proportion of serious pollution and serious pollution in the three-year heating period was 10.49%,7.65%and 17.93%respectively,and PM_2.5.5 was the primary pollutant in most of the polluted weather.It can be seen that the air situation in the heating period of Dezhou City was not optimistic,and the pollution of fine particles was serious.Under the heavy pollution weather,the secondary ions and gaseous precursors increased significantly,and NO₂ and NO₃^-increased rapidly;In addition,the concentration of NH₃ and NH₄⁺also had a significant growth trend under heavily polluted weather,indicating that atmospheric ammonia emissions were also worthy of attention.When air pollution intensified,it was often accompanied by a decrease in wind speed and temperature,and an increase in humidity.3.Correlation between nitrogen/sulfur compoundsIn the heating period of 2017-2019 in Dezhou,there was a significant correlation between NH₄⁺and NH₃,NO₃^-and NO₂.SO₂ and SO₄^2-only have a significant correlation in the heating period of 2017,which showed that the formation and transformation of SO₄²was complex.SO₄^2-,NO₃^-and NH₄⁺have significant correlation（r>0.80,P<0.01）,among which NH₄⁺and NO₃^-have the strongest correlation,all above 0.9,indicating that the formation mechanism of secondary ions was relatively similar.There is a significant correlation between NO₂ and SO₂,which showed that they have the similar emission sources,but there was no significant correlation between NH₃ and NO₂,or NH₃ and SO₂,which showed that NH₃ came from different emissions sources.4.The evolution law of sulfur-containing pollutantsFrom 2017 to 2019,the average value of sulfur oxidation rate（SOR）in this area was 0.22±0.10,0.28±0.16 and 0.42±0.17 respectively,which indicated that the sulfur oxidation rate in winter in Dezhou City was increasing year by year.With the aggravation of air pollution level,the SOR value of heating period in Dezhou increased significantly,mainly from the secondary conversion in the daytime.SO₄^2-mainly came from SO₂ in this area,which was greatly affected by humidity,and the degree of influence was increasing year by year,indicating that there may be strong liquid phase transformation.In addition,SO₄^2-was less affected by atmospheric acidity and alkalinity,was more affected by aerosol water,and the degree of influence was increasing,and it also had an increasing effect on atmospheric extinction.Through the analysis of the sources of sulfate formation,it can be seen that the main source of sulfate in heating period in this area is secondary generation source,which accounted for more than 80%of the total,and the proportion increases obviously under the condition of heavy pollution weather quality.The proportion of secondary source generation and SOR showed an exponential distribution.It can be seen that with the increase of the proportion of secondary source,the SOR changed slowly at the beginning,and then increased sharply.5.The evolution law of NO₂ and NO₃^-From 2017 to 2019,the mean value of NOR in the heating period in this area was0.30±0.20,0.24±0.32 and 0.32±0.14,respectively,that is to say,the nitrogen oxidation rate in Dezhou urban area showed a trend of slightly decreasing first and then rising.The change of air quality had a great influence on the transformation of nitrogen pollutants in the air particles in this area,with a significant decrease of F_HNO3and NO₂/NO,indicating that the transformation of air particles was relatively strong,and the contribution of vehicle exhaust to heavy pollution weather increased.The increase of NO₃^-mainly came from the transformation of NO₂ during the day,and the influence degree of temperature is increasing year by year,and the influence degree of humidity is decreasing year by year.In addition,NO₃^-was less affected by atmospheric acidity and alkalinity,and was more affected by aerosol water,and the degree of influence was increasing,and it also affected atmospheric extinction.The comparison of NO₂ formation pathways showed that NO and O₃ are the main pathways of NO₂ formation in the atmospheric particles of Dezhou City during the day,but there are obvious periodal changed at night,and the reaction of NO and free radicals will also lead to the high value of NO₂ at night.6.The evolution law of NH₃ and NH₄⁺The mean value of atmospheric NHR in the urban area was 0.41±0.13,0.40±0.12and 0.49±0.14 in the heating periods,respectively,which indicated that the secondary oxidation capacity of ammonium salt in the area showed a trend of slightly decreasing first and then increasing.In addition,NH₄⁺was less affected by atmospheric acidity and alkalinity,and was more affected by aerosol water,and the degree of influence was increasing,and it also affected atmospheric extinction.The mean value of the excess ammonia index（F_N）in the three-year heating period is 0.69,1.05 and 0.74,respectively.It showed that the atmosphere of Dezhou was in the ammonia rich environment in the heating period.The main forms of NH₄⁺were NH₄NO₃ and（NH₄）₂SO₄,which were consistent with the conclusion of quantum chemical molecular simulation.