Formation pathways and source apportionments of inorganic nitrogen-containing aerosols in urban environment: Insights from nitrogen and oxygen isotopic compositions in Guangzhou, China

Atmospheric Environment

10.1016/j.atmosenv.2023.119888

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 (SO4 2− ), 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 μg⋅m− 3 and their mean values were highest in spring (111.1 μg⋅m− 3 ) and lowest in summer (63.5  μg⋅m− 3 ). The δ15N–NH4 + and δ15N–NO3 - values ranged from +4.5 to +20.2‰ and from +4.8 to +14.8‰,  respectively, with their mean values being highest in winter (14.0‰ and 9.5‰, respectively) and lowest in  summer (10.4‰ and 7.1‰, respectively). The seasonal δ15N variability was mainly attributed to isotopic equilibrium fractionation, and partly due to the changes in NH3 and NOx sources. The average δ18O–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%) 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  δ18O–NO3 - analysis for exploring variations of nitrate formation pathways in urban atmospheres.