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

Abstract. Nitric acid (HNO₃) or nitrate (NO₃⁻) is the dominant sink for reactive nitrogen oxides (NO_x = NO + NO₂) in the atmosphere. In many Chinese cities, HNO₃ is becoming a significant contributor to acid deposition. In the present study, we measured nitrogen (N) and oxygen (O) isotopic composition of NO₃⁻ 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 NO₃⁻ formation pathways in this N-polluted city in the Pearl River Delta region. The δ¹⁵N values of NO₃⁻ (versus air N₂) ranged from −4.9 to +10.1‰, and averaged +3.9‰ in 2008 and +3.3‰ in 2009. Positive δ¹⁵N values were observed throughout the year, indicating the anthropogenic contribution of NO_x emissions, particularly from coal combustion. Different seasonal patterns of δ¹⁵N-NO₃⁻ 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 δ¹⁸O 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 δ¹⁸O 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 O₃ to convert NO to NO₂, thereby donate O atoms with much lower δ¹⁸O value than that of O₃ to atmospheric NO₃⁻. Our results highlight that the influence of human activities on atmospheric chemistry can be recorded by the N and O isotopic composition of atmospheric NO₃⁻ in a N-polluted city.