Identifying NOx Sources in Arequipa, Peru Using Nitrogen Isotopes in Particulate Nitrate

We reported on the first time series of δ 15 N in aerosol nitrate from South America. Particulate matter less than 2.5 microns in diameter (PM 2.5 ) was collected at four sites located in Arequipa, a major city in southern Peru. The δ 15 N values for nitrate in PM 2.5 ranged from -1.7–15.9‰ and averaged 5.3 ± 3.0‰, with no significant difference between the four study sites and no discernable seasonal trend. These values are significantly higher than those in aerosol nitrate from southern hemisphere marine environments and those from the northern hemisphere. We explain the elevated values using an isotope mass balance mixing model that estimates a source NO x δ 15 N of -8 ± 3‰, derived mainly from anthropogenic sources (vehicles, industry). An isotope enabled 0-D photochemical box model was used to estimate the isotope enrichment of nitrate relative to NO x due to kinetic, equilibrium, and photolysis isotope effects occurring during NO x oxidation. This “source plus photochemistry” approach resulted in general agreement with the observations. This suggests that if the photochemistry effect can be accounted for, nitrate δ 15 N can be used to assess the relative importance of NO x sources and could be a new tool to validate NO x emission inventories.