Quantifying the major sources of nitrate in PM2.5 in a port city in southeastern China: Perspectives from nitrate dual isotopes and NOx emission inventory

Nitrate is a major component of PM_2.5 and has important implications for air quality, ecosystems and climate change. Understanding the formation mechanisms and sources of nitrate is crucial for formulating effective NO_x reduction policies. In this study, PM_2.5 samples were collected simultaneously at two sites in Xiamen in 2017. Water-soluble inorganic ions and stable N and O isotopes of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were determined. The results showed that NO₃⁻ was less important than SO₄²⁻ (NO₃⁻/SO₄²⁻ = 0.8), but had a more pronounced seasonal variability. The means of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- at the urban site (+3.9 ± 2.8 ‰ and +63.0 ± 8.2 ‰) were not significantly different from those at the suburban site (+4.5 ± 2.7 ‰ and +63.4 ± 8.8 ‰). Different from δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^- showed a clear seasonal variation and was negatively correlated with temperature and positively correlated with NO₃⁻ concentration. Using a Bayesian mixing model, NO₂ + •OH was estimated to be the dominant formation pathway for nitrate (51.3−52.4 %), followed by the N₂O₅ hydrolysis (25.7−25.8 %), the N₂O₅ + Cl (10.9−11.3 %) and the NO₃ + HCs (10.9−11.6 %). On average, coal combustion, vehicle exhaust, ship emissions and soil emissions contributed 30.3−36.6 %, 27.8−30.4 %, 19.8−22.2 % and 15.8−17.2 % respectively to NO₃⁻ in PM_2.5. However, NO_x emission inventory showed that soil emissions were negligible (0.09−1.19 %) compared to that of coal combustion (35.6 %), vehicle exhaust (37.2 %) and ship emissions (20.1 %) in Xiamen. The results highlighted that more attention should be paid to ship emissions in coastal areas and that NO_x emissions from soil microbes need further investigation.