Distribution Characteristics And Formation Mechnism Of Inorganic Nitrogen And Microbiome In PM_2.5 In A Northern China City

As a Northern China city,urban Harbin was affected by PM_2.5pollution,which has almost become the main environmental issue during the heating seasons.However,many key points such as the conversion pathway and source contributions of inorganic nitrogen precursors,and the characteristics of the microbiome are still unclear.Thus,it is necessary to conduct further study of PM_2.5and its components.It’s worth noting that the average annual concentration of PM_2.5in Harbin rebounded in 2017,so PM_2.5pollution in that year deserves more attention.Based on this,this study collected PM_2.5samples from May 2017 to April 2018,analyzed PM_2.5pollution characteristics and the influences posed by potential factors,explored gas-aerosol conversion process and emission source contributions of precursors,explored the bioaerosol and the influence posed by environmental factors,so as to provide theoretical bases for further air pollution control.The analysis of PM_2.5pollution character and its influence factors in urban Harbin during the study period showed PM_2.5concentrations in the samples collected during heating season（177.52±151.76μg/m³）were significantly higher than that during non-heating season（92.57±68.08μg/m³）.The pollution episodes with PM_2.5concentrations higher than 150μg/m³mainly occurred in the heating season,among which the special pollution episode from late October to early November was the most significant with long duration and extremely high PM_2.5concentrations.Source apportionment of PM_2.5showed that secondary aerosol provided high contributions both during heating season（28%）and non-heating season（14%）.Among the primary emission sources,traffic emission（29%）and coal combustion（28%）contributed the highest to PM_2.5concentration during non-heating season,while biomass combustion（35%）contributed the highest during heating season.With the elevation of PM_2.5pollution levels,the contribution of secondary aerosol and biomass burning increased by 9%and 37%.The pollution episodes in spring and autumn were mainly affected by biomass burning with markedly increased concentrations of K⁺and Cl^-fractions;the formation of pollution episodes in winter was significantly affected by static weather;while in spring episodes,mineral dust accounted for the highest proportion（25～50%）of PM_2.5components,and the episodes were significantly affected by the long-distance transmission from the northwestern region of Harbin.Inorganic nitrogen components are markedly formed under low temperature and high relative humidity during heating season,their production is significantly higher than that of sulfate.During heating season,ammonium was prone to remain solid phase at low temperatures.Besides,during the heating season,the rich ammonia condition is conducive to the conversion of NO_xto NO₃^-through the homogeneous gas-phase reaction,and a high level of atmospheric relative humidity（>60%）is conducive to the formation of NO₃^-through heterogeneous hydrolysis.Based on the measurement of isotope signatures(δ¹⁵N-NH₄⁺,δ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-)and the introduction of the Bayesian mixing model,this study assessed the conversion pathways and potential source contributions of NO_xand NH₃.The results showed that NO₂+OH pathway and O₃+N₂O₅pathway respectively dominated the gas-to-particle conversion of NO_xto NO₃^-during non-heating season and heating season.The potential sources of NO_xin order of importance were coal combustion（55.49%±23.06%）,biomass combustion（17.65%±7.53%）,traffic source（16.05%±7.49%）,and biomass soil emission（10.81%±8.88%）.The neutralization degree（non-heating:0.73;heating:2.18）and conversion rate（non-heating:0.13;heating:0.38）of the gas-to-particle conversion of NH₃to NH₄⁺in the non-heating season were low,while increased in heating season,which is proved to be conducive for equilibrium reaction andδ¹⁵N enriched in NH₄⁺.Source apportionment analysis showed that fertilizer application dominated the source contribution of NH₃in clean days（30.70%）,and its contribution decreased as the elevation of PM_2.5pollution level,while biomass burning contribution and waste treatment contribution increased synchronously,ascended as the main contributors in heavily polluted days（29.63%and 36.51%）.Fossil fuel combustion（21.32%）and NH₃slip（23.56%）contributed most to NH₃emission in non-heating season,while waste treatment（41.28%）contributed most to NH₃in heating season.The effects of decreased inorganic nitrogen on PM_2.5were analyzed by ISORROPIA-II model,and the results showed that the p H value,particulate water content,and total ion concentration of PM_2.5were more sensitive to the decrease of NH₄⁺than NO₃^-.The microbiome diversity and ARGs abundance analysis showed that the dominant bacteria genus in inhalable particulate samples were mainly Gram-negative bacteria.There were no significant ANOVA differences in flora abundance,flora diversity,pathogen sequences,and ARGs abundance between the sample from polluted and clean days,and between the sample from PM_2.5and PM₁₀samples.However,more fungal taxonomies,pathogen species,and ARGs sub-types were detected in the samples from polluted days.The bacterial pathogens with the highest detection rate were Shigella.The resistance genes with the highest detection rates were tet W,int I 1,sul1,and erm B.The environmental factors that can significantly influence bacterial and fungal communities were RH,Ca²⁺,Mg²⁺,SO₄^2-,SO₂,NO₂,NO₃^-,etc.Ca²⁺and Mg²⁺were negatively correlated with most bacterial genera but positively correlated with most fungal genera.SO₄^2-,NO₃^-,NO₂,and SO₂were positively correlated with Sphingomonas,Asticcacaulis,Lactobacillus,Brevundimonas,Shigella,Methylobacterium,Sanghuangporus,Saccharomycodes,and Hanseniaspora,while negatively correlated with other bacteria and fungi genus.Bacterial genera were negatively correlated with most target ARGs subtypes except tet W and blactx-M1,implying dominant resistance to tetracycline andβ-lactam antibiotics,and intolerance to other antibiotics.The significant positive correlation between int I 1 and sul 1 indicates int I 1 played significant roles in promoting the spread and pollution of sul 1 in the atmospheric environment.