Quantifying the source and formation of nitrate in PM2.5 using dual isotopes combined with Bayesian mixing model: A case study in an inland city of southeast China

Atmospheric nitrate has been attracting increasing attention because it is one of the important components of PM_2.5. Understanding the sources and formation mechanism of nitrate in PM_2.5 is essential to take effective measures to prevent and control the emission of nitrogen oxides in the atmosphere and reduce the formation of haze. In this study, PM_2.5 samples were collected from Ganzhou, an inland city in southeast China, during summer and winter. The concentrations of PM_2.5 and NO₃^- were determined, and the isotopic compositions of NO₃^- (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were analyzed in order to quantify the relative contributions of different emission sources and formation pathways of nitrate in PM_2.5. The results showed that PM_2.5 and NO₃^- concentrations were lower in summer (39.80 ± 11.10 μg·m^-3 and 1.79 ± 0.70 μg·m^-3) while higher in winter (69.85 ± 29.58 μg·m^-3 and 10.83 ± 9.89 μg·m^-3). The values of δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- ranged from 42.84‰ to 56.80‰ and from -11.17‰ to -2.08‰ in summer, while from 55.86‰ to 78.66‰ and from -10.63‰ to -1.88‰ in winter, respectively. The results of δ¹⁵N-NO₃^- combined with Bayesian isotope mixing model showed that the relative contributions of vehicle exhaust, soil microbial activity, biomass combustion and coal fired power plants were 59.3%, 28.5%, 8.7% and 3.4% in summer, while 65.1%, 20.1%, 10.6% and 4.1% in winter, respectively. The results of δ¹⁸O-NO₃^- combined with Bayesian isotope mixing model showed that the possible relative contributions of pathway 1 (P1) (NO₂ + ·OH), pathway 2 (P2) (NO₃ + HC) and pathway 3 (P3) (N₂O₅ + H₂O) were 73.5%, 12.4% and 14.1% in summer, while 41.6%, 28.9% and 29.5% in winter, respectively. Moreover, P2 and P3 contributed more when NO₃^- concentrations were higher, suggesting that P2 and P3 were of significance to the formation of nitrate in PM_2.5, especially during winter.

Oxygen anomaly in near surface carbon dioxide reveals deep stratospheric intrusion

Stratosphere-troposphere exchange could be enhanced by tropopause folding, linked to variability in the subtropical jet stream. Relevant to tropospheric biogeochemistry is irreversible transport from the stratosphere, associated with deep intrusions. Here, oxygen anomalies in near surface air CO₂ are used to study the irreversible transport from the stratosphere, where the triple oxygen isotopes of CO₂ are distinct from those originating from the Earth’s surface. We show that the oxygen anomaly in CO₂ is observable at sea level and the magnitude of the signal increases during the course of our sampling period (September 2013-February 2014), concordant with the strengthening of the subtropical jet system and the East Asia winter monsoon. The trend of the anomaly is found to be 0.1‰/month (R² = 0.6) during the jet development period in October. Implications for utilizing the oxygen anomaly in CO₂ for CO₂ biogeochemical cycle study and stratospheric intrusion flux at the surface are discussed.