Analysis Of Potential Sources And Formation Processes On Atmospheric NH_x And HONO Through The Stable Isotope

As the main reactive nitrogen components in the atmosphere,NH₃ and HONO play an important role in the atmospheric nitrogen cycle.Among,NH₃ is the most abundant alkaline gas in the atmosphere,which can easily neutralize acidic substances in the atmosphere to form secondary inorganic aerosols（SIAs）and lead haze process.While the concentrations of HONO,as one of the most important precursors of OH radicals,can directly affect the atmosphere oxidation ability.In addition,both the NH₃ dry/wet deposition and the HONO deposition on soil surface can participate in the process of microbial nitrification and denitrification,which are significant in the atmospheric nitrogen cycling by connecting the biosphere and the atmosphere.However,whether agricultural or non-agricultural sources are the dominant potential source of atmospheric NH_x（NH₃+p-NH₄⁺）is still controversial.Otherwise,high concentrations of atmospheric HONO from unknown sources have been observed during the daytime.Some studies point out that HONO from unknow source is directly emitted from soil,but there is lack of direct evidences.Therefore,potential sources and formation pathways of atmospheric NH_x and HONO were quantitatively analyzed through stable the isotope technology in this study:（1）We optimized the reaction parameters and established a rapid,accurate and suitable improved chemical method for the determination ofδ¹⁵N-NH_x ratio in atmospheric samples.Then used this improved chemical method to analyze theδ¹⁵N-NH₄⁺ratios in a haze event in the urban agglomeration of the North China Plain.The results showed that non-agricultural sources were the dominant sources of atmospheric NH_x in different cities（42%～85%）.However,during the extreme haze phase,the contribution rate of fossil fuel source decreased by 15%～20%,while livestock breeding source increased to 15%～32%.It indicated that agricultural sources（especially livestock breeding）could discharge considerable NH₃ to form SIAs-led haze event in urban cities through long-distance transmission of air mass.（2）The variation characteristics of p-NH₄⁺mass concentration andδ¹⁵N-NH₄⁺ratios during winter and summer in Dongshan,Suzhou indicated that the difference ofδ¹⁵N-NH₄⁺in the same season was caused by different contribution of various potential sources,while the difference ofδ¹⁵N-NH₄⁺between different seasons was led by the N isotope fractionation of ammonium-ammonia particle distribution.The source apportionment results shown that non-agricultural sources were still the dominant sources（59%～69%）in the suburb of the Yangtze River Delta.It pointed out that,NH₃ from non-agricultural sources was more likely to form p-NH₄⁺with other pollutant gases emitted together（such as SO₂ and NO_x）,while NH₃ from agricultural sources was easier to deposit to the ground,with weaker ability to form p-NH₄⁺.（3）Features of soil HONO mass concentration,nitrogen and oxygen isotopes(δ¹⁵N and?¹⁷O)under different soil conditions during different periods in the northern suburbs of Nanjing indicated that artificial fertilization could promote the release of HONO from soil to the atmosphere.The quantitative estimation results through?¹⁷O-HONO showed that the primary emission of soil-atmosphere exchange was the dominant formation pathway of soil HONO both in the non-fertilization period（>61%）and intensive-fertilization period（>95%）.Among,fertilization-related processes accounted for more than 76%of the primary emission.On the contrary,during the non-fertilization period,the heterogeneous reaction of NO₂ was also a crucial pathway with a relative fraction of 22%～37%.It indicated that soil could also releases large amounts of HONO into the atmosphere through secondary reactions occurring on its surface,especially during periods without frequently fertilization activities.（4）There was no significant seasonal variation in the atmospheric HONO mass concentrations in Nanjing.However,HONO mass concentrations had significant diurnal variation is significant（nighttime>daytime）.Meanwhile,Δ¹⁷O-HONO values andδ¹⁵N-HONO had no significant differences between daytime and nighttime in winter,but in summer（nighttime>daytime）.This indicated that the formation pathways of atmospheric HONO is closely related to the atmospheric photochemical cycling process.There were differences on the formation mechanism of daytime HONO and nighttime HONO.During the daytime,the primary emissions from motor vehicles or soil was the dominant formation pathway of atmospheric HONO（47±11%）.While during the nighttime,the hydrolysis reaction of NO₂ on wet surfaces was the dominant formation pathway of atmospheric HONO（49±4%）.The quantitative estimation throughΔ¹⁷O-HONO indicated that HONO from unknown source was related to the primary emission process.In addition,the contribution of HONO from primary emission in roadside was higher than that in ambient（more than 0%～20%）,suggesting that vehicles could emit large amounts of HONO directly,especially in urban areas.