Using the Nitrogen Isotopic Composition of NO2 (?15N-NO2) for Source Apportionment in an Urban Area

摘要

Introduction: Nitrogen oxides (NOx), produced from combustion sources and natural processes, are ozone precursors and respiratory irritants. Recent studies document distinct source-specific isotopic compositions in NO2 (615N-NO2). Despite source signatures which isotopic composition may provide, few studies have used isotopes to distinguish multiple pollution source contributions within an urban area. Methods: Ogawa passive samplers collected NO2 for 2 week periods at 73 locations throughout Pittsburgh, PA, January-March 2013. Sites were selected using stratified random sampling, capturing variability in hypothesized key NO2 sources in our region (e.g., traffic density, proximity to industry, elevation, bus route frequency density). For isotopic analysis, denitrifying bacteria, Pseudomonas aureofaciens, is used to convert sample NO2- into gaseous N2O. The N2O is analyzed using a trace gas and autosampler coupled with a continuous flow isotope ratio mass spectrometer. Spatial patterns in δ15N-NO2, and relationships with source density, will be explored using GIS based covariates. Land use regression modeling methods will be explored, to explain spatial variability in isotope signatures. Results: A pilot study revealed wide variability in δ15N-NO2 across the Pittsburgh region [μ = -0.66 0/00 (range= -7.75 to +7.37 0/00)], supporting the utility of isotopic signatures in quantifying urban source contributions, and possible improvement of source apportionments. We are currently extending this dataset to include sampling sites across 500 km2, including 36 sites in downtown Pittsburgh, and will explore associations with local air pollution sources. Conclusions: Few studies have used isotopic signatures for source apportionment in urban areas. Significant variation in δ15N values may support quantification of urban source contributions, thus improving upon existing methods for intra-urban source apportionment.