Anthropogenic imprints on nitrogen and oxygen isotopic composition of precipitation nitrate in a nitrogen-polluted city in southern China

Nitric acid (HNO ₃) or nitrate (NO ₃⁻) is the dominant sink for reactive nitrogen oxides (NO _x = NO + NO ₂) in the atmosphere. In many Chinese cities, HNO ₃ is becoming a significant contributor to acid deposition. In the present study, we measured nitrogen (N) and oxygen (O) isotopic composition of NO ₃⁻ in 113 precipitation samples collected from Guangzhou City in southern China over a two-year period (2008 and 2009). We attempted to better understand the spatial and seasonal variability of atmospheric NO _x sources and the NO ₃⁻ formation pathways in this N-polluted city in the Pearl River Delta region. The δ ¹⁵N values of NO ₃⁻ (versus air N ₂) ranged from −4.9 to +10.1‰, and averaged +3.9‰ in 2008 and +3.3‰ in 2009. Positive δ ¹⁵N values were observed throughout the year, indicating the anthropogenic contribution of NO _x emissions, particularly from coal combustion. Different seasonal patterns of δ ¹⁵N-NO ₃⁻ were observed between 2008 and 2009, which might reflect different human activities associated with the global financial crisis and the intensive preparations for the 16th Asian Games. Nitrate δ ¹⁸O values (versus Vienna Standard Mean Ocean Water) varied from +33.4 to +86.5‰ (average +65.0‰ and +67.0‰ in 2008 and 2009, respectively), a range being lower than those reported for high latitude and polar areas. Sixteen percent of δ ¹⁸O values was observed lower than the expected minimum of +55‰ at our study site. This was likely caused by the reaction of NO with peroxy radicals; peroxy radicals can compete with O ₃ to convert NO to NO ₂, thereby donate O atoms with much lower δ ¹⁸O value than that of O ₃ to atmospheric NO ₃⁻. Our results highlight that the influence of human activities on atmospheric chemistry can be recorded by the N and O isotopic composition of atmospheric NO ₃⁻ in a N-polluted city.