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 (NOx Combining double low line NOx + 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 NOx SOURCES and the NO 3 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 NOx 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 O 3 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. © 2011 Author(s).