Chemical Compositions,Optical Properties And Source Identifications Of Nitrophenol Compounds In PM_2.5 During Winter Haze Period In Xi’an,Northwestern China

Nitrated aromatic compounds（NACs）are one of the important light-absorbing components in brown carbon（Br C）aerosols.Negative consequences of NACs pollution events include increasing toxicity,harm to radiative balances and even change the climate.Current studies on NACs have mainly focused on seasonal variation and their possible formations mechanism.However,the studies on the change of chemical composition,light absorbing properties and source of NACs was unclear.Therefore,in this study,we collected continuously 4h-high-resolution fine particulate matter(PM_2.5)samples from December 24,2019 to January 12,2020 in Xi’an,a typical city in Guanzhong region of China.Eleven NACs constituents were detected by using Ultra-High-Performance Liquid Chromatography interfaced with quadrupole and Time-of-Flight Mass Spectrometry（MS/MS）.In addition,the impacts that the degree of air pollution has on chemical,optical and sources characteristics of NACs were quantitatively discussed.In this study,five periods were selected by air quality classification:haze period 1was December 24,2019;haze period 2 was from January 3 to January 5,2020;haze period 3 was from January 9,to January 11,2020;clean period 1 was from December 25to December 27,2019;clean period 2 was from December 30 to December 31,2019.The results showed that the average concentration of PM_2.5 was 123.7±49.8μg·m^-3 during the whole sampling period,and even increases to 159.7±44.6μg·m^-3 in heavy haze days.The average concentration of∑NACs during the whole sampling period was 26.5±23.5 ng·m^-3(0.9–146.1 ng·m^-3).Among the NACs species,both the 4-nitrophenol（4NP）and 4-nitrocatechol（4NC）were the most abundant,which accounted for 60.2%of the total NACs,and followed by methyl-nitrophenol（MNP）（14.0%）and methyl-nitrocatechol（MNC）（18.0%）.These phenomena indicated that NACs mainly derived from biomass and fossil fuel combustion emissions.Moreover,both the 4NC and MNCs contributed39.2%and 41.6%to the total NACs during haze period 1 and haze period 2,respectively,which was 1.5 times higher than that during haze period 3,stressing that heavy biomass burning emissions mainly occurred during haze period 1 and haze period 2.In contrast,during clear periods,the increased proportions of 4NP,2,6DM4NP can be observed.These compounds can be mainly derived from the secondary formation of phenolic.For diurnal variations,the proportion of NACs in both clean and haze periods was affected by vehicle emissions in daytime（8:00 to 20:00）,whereas the abundances of the burning of coal and biomass emissions occurred from 20:00 to 8:00.The further correlation analysis also indicating the industrial or coal burning emissions were important roles of in haze period 3.The light absorption coefficent of Br C at 365 nm(b_abs,365-Br C)was 23.7±12.0 Mm^-1 during the sampling period,and the variation of b_abs,365-Br C at several periods and 4h-high-resolution was similar to the concentration characteristics of∑NACs.At 365nm,∑NACs contribution to b_abs,365-Br C was 16 times higher than those mass concentrations,indicating that NACs were extremely important light-absorbing substances.Among NACs species,4NC contributed to 37.9%of b_abs,365-NACs,and even increased over 40%during the period of 16:00-20:00,illustrating the influence of biomass burning emission.Meanwhile,the absorption contribution of 4NC and 4-methyl-5-nitrocatechol in haze period was also significant,which stressed the impacts of biomass burning emission.4NP and MNPs,4NC and MNCs were strongly correlated at 4:00–12:00 and 20:00–0:00,indicating that their sources may be similar.Finally,the sources of NACs were analyzed by positive matrix factorization:in the haze period,factory and coal combustion emissions and biomass burning emissions were the main sources of NACs contributed48.8%and 25.1%,respectively,vehicle emission and secondary formation contribution were 16.3%and 9.6%.However,the contribution of secondary formation in clean days increased to 27.1%,with factory and coal combustion emissions（40.1%）becoming the main sources of clean period.In this study,the concentration,light absorption and source characteristics of NACs in winter clean days and heavy haze days in Xi’an were compared in detail,which will greatly enrich the 4h-high-resolution research on fine particle pollutants and provide scientific basis for pollution control measures.