Formation Pathways and Source Apportionments of Inorganic Nitrogen-Containing Aerosols in Urban Environment: Insights from Nitrogen and Oxygen Isotopic Compositions in Guangzhou, China

Air quality issues caused by PM2.5-induced persistent extreme haze episodes have become increasingly serious in urban environments in recent years. Secondary water-soluble inorganic sulfate (SO42-), nitrate (NO3-) and ammonium (NH4+) ions are the major components of PM2.5. However, the contributions of inorganic nitrogen species, especially nitrate, to PM2.5 have greatly increased during haze episodes in many cities in China. Therefore, better understanding of their emission sources and formation pathways holds the key to controlling urban PM2.5 pollution more efficiently and effectively. In this study, water-soluble ionic characteristics and isotopic compositions and sources of NH4+ and NO3-, as well as NO3- formation pathways were determined in PM2.5 aerosol samples collected in Guangzhou, China, during 2015-2018. The PM2.5 concentrations varied from 30.5 to 189.8 mu g.m(-3) and their mean values were highest in spring (111.1 mu g.m(-3)) and lowest in summer (63.5 mu g.m(-3)). The delta N-15-NH4+ and delta N-15-NO3- values ranged from +4.5 to +20.2 parts per thousand and from +4.8 to +14.8 parts per thousand, respectively, with their mean values being highest in winter (14.0 parts per thousand and 9.5 parts per thousand, respectively) and lowest in summer (10.4 parts per thousand and 7.1 parts per thousand, respectively). The seasonal delta N-15 variability was mainly attributed to isotopic equilibrium fractionation, and partly due to the changes in NH3 and NOx sources. The average delta O-18-NO3- value of 63.0%, ranging seasonally from +58.6% in summer to +68.0% in spring, suggests that NO2 +.OH pathway played a vital role (70.3-85.9 parts per thousand) in NO3- formation. The Bayesian isotope mixing model results revealed fossil fuel combustion sources as dominant sources of atmospheric NH3 and NOx. This study suggests that more effort should be devoted to reduce NH3 and NOx from combustion-related processes and highlights the importance of delta O-18-NO3- analysis for exploring variations of nitrate formation pathways in urban atmospheres.