Isotope Based Source Apportionment Of Atmospheric Nitrate And Its Application To Source Apportionment Of PM_2.5 In The Pearl River Delta

In recent years,haze pollution has occurred frequently in China,which has had an important impact on human health and regional environmental quality.At present,fine particulate matter(PM2.5)and ozone(O3)are the main air pollutants in China.The coordinated control strategy of PM2.5 and O3 is a comprehensive prevention and control strategy to improve air quality during the "fourteenth five year plan" period.Nitrogen oxides(NOx),as the main precursor of O3,can directly affect the formation of O3;At the same time,NOx can generate nitrate ions through chemical oxidation and participate in the liquid-phase catalytic reaction of SO2,which is the key component for the rapid increase of haze.After NOx is converted into nitrate(NO3-),its isotopic characteristics can reflect its source characteristics and formation process.The source and oxidation path of atmospheric NOx can be revealed by measuring the nitrogen and oxygen isotopes of nitrate.In view of the pollution situation in the Pearl River Delta,this study takes the atmospheric fine particle samples collected at several typical sampling points in the Pearl River Delta as an example,and applies the improved Bayesian model based on 15 N and 18 O to quantitatively analyze the sources,transformation pathways and influencing factors of atmospheric nitrate in the Pearl River Delta,as well as the daily variation characteristics of nitrate sources and generation pathways during the generation and elimination of haze,and the differences of nitrate sources and oxidation pathways on particles with different sizes in haze and non haze periods;In addition,the spatial variation characteristics of NO3-sources in the Pearl River Delta are analyzed on the basis of four site air monitoring and combined with potential source contribution factor analysis(PSCF).Finally,based on the isotope(15N+18O)combined PMF model,the primary source of PM2.5 in the Pearl River Delta is re-analyzed.These provide scientific data support for the improvement of air quality in the Pearl River Delta region.The main results are as follows:Firstly,Heshan regional station,a representative station in the Pearl River Delta,has an atmospheric NO3-concentration of 5.4 ± 4.8 μg/m3 from 2013 to 2014,accounting for 6.0 ± 3.3% of PM2.5.NO3-concentration showed a high level in cold season(7.4 ± 5.0 μg/m3),low in warm season(2.5 ± 2.6 μg/m3).The average value ofδ15N-NO3-is 7.5 ± 3.3 ‰,which is affected by NOx source and environmental parameters,δ15N-NO3-is the lowest in spring and the highest in winter.The average value of δ18O-NO3-is 62.4 ± 14.0 ‰,and the variation range is 21.0 ‰ ~90.1 ‰.The lowest value appears in spring,mainly because HO2 participates in the oxidation reaction;The highest value appears in winter,which is mainly affected by cold and dry weather,and the proportion of NO2+OH gas phase reaction is low.Montecarlo simulation results show that the annual · OH generation pathway is dominant,and its relative contribution rates in spring,summer,autumn and winter are 72 ± 18%,76 ±16%,63 ± 17% and 39 ± 15% respectively.The results of the improved Bayesian model show that coal combustion(50.1 ± 13.8%)is the main source of NO3-and PSCF analysis shows that the central and southern regions of Guangdong are the main potential source areas of NO3-coal sources.Secondly,in Guangzhou,a representative city site in the Pearl River Delta,the average concentration of PM2.5 during the formation of haze in spring 2016 was 136 ±42.8 μg/m3,the average WSI concentration of water-soluble total ion is 8.9 ± 11.7 μg/m3.In the process of haze formation from clean weather to moderate pollution to severe pollution,the relative WSI abundances of NO3-,Cl-,NH4+ showed an increase trend,indicating that NH4NO3 and NH4 Cl particles were the main contribution loads for haze formation in the Pearl River Delta.The average value of δ15N-NO3-and δ18O-NO3-is8.4 ± 4.6 ‰ and 72.7 ± 10.8 ‰,respectively.In the process of haze formation,δ15NNO3-shows an upward trend(2.2 ‰,7.6 ± 4.6 ‰ and 10.9 ± 3.7 ‰).There was no significant difference in δ18O-NO3-(72.4 ‰,72.4 ± 12.6 ‰ and 73.4 ± 8.9 ‰),indicating that the contribution of coal-fired sources increased during the haze period and there was no significant change in the formation mechanism of NO3-.Daytime NO3-concentration(14.0 ± 6.3 μg/m3)is higher than that at night(11.6 ± 5.8 μg/m3).The ratio of NO3-/PM2.5 also showed the same diurnal variation,with an average value of9.3 ± 2.1% in the daytime and 8.3 ± 3.0% at night.The higher ratio of NO3-/PM2.5 in daytime indicates that the increase of NO3-comes from the increase of NO3-generation rate in gas phase reaction under high temperature and low humidity in daytime.Affected by nitrogen and oxygen isotope fractionation,daytime δ15N-NO3-value(10.7 ± 4.4 ‰)is higher than that at night(6.0 ± 3.7 ‰),and at daytime δ18O-NO3-value(66.5 ±10.8 ‰)is lower than that at night(78.8 ± 6.9 ‰).The ratio of OH oxidation pathway was as high as 0.58 ± 0.06 during the day,and the ratio of N2O5 formation pathway was as high as 0.64 ± 0.06 at night;BB,CC,MS and SS contributed 17.9 ± 0.7%,59.0 ±3.3%,14.6 ± 0.8% and 8.4 ± 1.9%,and 23.0 ± 4.5%,46.3 ± 13.2%,19.8 ± 4.9% and10.9 ± 3.9% to NO3-in Guangzhou during the day and at night,respectively.During the generation and elimination of haze in Guangzhou in spring 2016,the NO3-distribution in six segment particles showed that the formation of NO3-during haze was mainly the homogeneous reaction of HNO3 and NH3(89.8%),and the formation of NO3-during non haze was mainly the heterogeneous reaction of NO2(59.6%).Thirdly,from 2020 to 2021,the spatial variation characteristics of NO3-sources at the four sampling points in the Pearl River Delta show that the contribution of coal sources is the lowest at BOLUO background point,but the proportion of soil microbial sources is the highest in the four places,which is consistent with the characteristics of the background point.The time variation results of NO3-sources at Guangzhou station and Heshan station show that NO3-pollution has been significantly improved,indicating that the environmental pollution control in Guangdong Province has been effective in recent years,but the order of contribution of pollution sources has basically not changed.PSCF analysis covering the whole Pearl River Delta shows that CC and BB of NO3-in the Pearl River Delta are mainly distributed in the eastern part of the Pearl River Delta,and MS and SS are mainly distributed in the ocean and surrounding urban areas.Although the contribution rate of NO3-pollution sources in the Pearl River Delta is different in space,the distribution of its main source areas is basically the same.The contribution ratio of NO3-different sources based on PMF model of Heshan station shows that CC,MS and bb+ss contribute 28.3%,27.9% and 39.1% respectively,just within the contribution range of Bayesian model analysis(28.2%~47.2%,19.47%~26%and 34.8%~44.4%),indicating the accuracy of Bayesian model source analysis of NO3-/NOx.Finally,the source apportionment of PM2.5 by PMF model showed that biomass combustion,sea salt,soil dust,ship emissions,industrial processes,coal burning and secondary aerosol account for 19.0%,4.3%,13.8%,7.6%,12.5%,12.7% and 30.0% of PM2.5,respectively.Based on stable isotopes of oxygen and nitrogen(δ15N-NO3-andδ18O-NO3-),the Bayesian mixing model was performed to apportion the source of NO3-to coal combustion,traffic emission and biogenic source.Then the secondary aerosol source was subdivided into three sources according to the discrepancy in source apportionment of NO3-between PMF and Bayesian mixing model results.The results showed that coal-related sources(36.5%),biofuel sources(29.4%),soil dust + sea salt(18.1%)and mobile sources(13.2%)were the main sources of PM2.5 in Pearl River Delta.The model performance was also estimated by 14C-TC of seven samples.After secondary re-distribution,the relative error between the modern carbon proportion of TC in PMF model results and the modern carbon proportion of TC based on 14 C results was less than 9%,indicating the good performance of the developed method.The research results can provide scientific basis for the formulation of regional pollution prevention and controlling policies and measures.