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 twoyear 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 Npolluted city in the Pearl River Delta region. The delta(15)N values of NO(3)(-) (versus air N(2)) ranged from -4.9 to +10.1 parts per thousand, and averaged +3.9 parts per thousand in 2008 and +3.3 parts per thousand in 2009. Positive delta(15)N values were observed throughout the year, indicating the anthropogenic contribution of NO(x) emissions, particularly from coal combustion. Different seasonal patterns of delta(15)N-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 delta(18)O values (versus Vienna Standard Mean Ocean Water) varied from +33.4 to +86.5 parts per thousand (average +65.0 parts per thousand and +67.0 parts per thousand in 2008 and 2009, respectively), a range being lower than those reported for high latitude and polar areas. Sixteen percent of delta(18)O values was observed lower than the expected minimum of +55 parts per thousand 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 delta(18)O 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.