Nitrogen Stable Isotope Composition (δ¹⁵N) of Vehicle-Emitted NO_<i>x</i>

The nitrogen stable isotope ratio of NO_<i>x</i> (δ¹⁵N-NO_<i>x</i>) has been proposed as a regional indicator for NO_<i>x</i> source partitioning; however, knowledge of δ¹⁵N values from various NO_<i>x</i> emission sources is limited. This study presents a detailed analysis of δ¹⁵N-NO_<i>x</i> emitted from vehicle exhaust, the largest source of anthropogenic NO_<i>x</i>. To accomplish this, NO_<i>x</i> was collected from 26 different vehicles, including gasoline and diesel-powered engines, using a modification of a NO_<i>x</i> collection method used by the United States Environmental Protection Agency, and δ¹⁵N-NO_<i>x</i> was analyzed. The vehicles sampled in this study emitted δ¹⁵N-NO_<i>x</i> values ranging from −19.1 to 9.8‰ that negatively correlated with the emitted NO_<i>x</i> concentrations (8.5 to 286 ppm) and vehicle run time because of kinetic isotope fractionation effects associated with the catalytic reduction of NO_<i>x</i>. A model for determining the mass-weighted δ¹⁵N-NO_<i>x</i> from vehicle exhaust was constructed on the basis of average commute times, and the model estimates an average value of −2.5 ± 1.5‰, with slight regional variations. As technology improvements in catalytic converters reduce cold-start emissions in the future, it is likely to increase current δ¹⁵N-NO_<i>x</i> values emitted from vehicles

Nitrogen Stable Isotope Composition (δ¹⁵N) of Vehicle-Emitted NO_<i>x</i>

The nitrogen stable isotope ratio of NO_<i>x</i> (δ¹⁵N-NO_<i>x</i>) has been proposed as a regional indicator for NO_<i>x</i> source partitioning; however, knowledge of δ¹⁵N values from various NO_<i>x</i> emission sources is limited. This study presents a detailed analysis of δ¹⁵N-NO_<i>x</i> emitted from vehicle exhaust, the largest source of anthropogenic NO_<i>x</i>. To accomplish this, NO_<i>x</i> was collected from 26 different vehicles, including gasoline and diesel-powered engines, using a modification of a NO_<i>x</i> collection method used by the United States Environmental Protection Agency, and δ¹⁵N-NO_<i>x</i> was analyzed. The vehicles sampled in this study emitted δ¹⁵N-NO_<i>x</i> values ranging from −19.1 to 9.8‰ that negatively correlated with the emitted NO_<i>x</i> concentrations (8.5 to 286 ppm) and vehicle run time because of kinetic isotope fractionation effects associated with the catalytic reduction of NO_<i>x</i>. A model for determining the mass-weighted δ¹⁵N-NO_<i>x</i> from vehicle exhaust was constructed on the basis of average commute times, and the model estimates an average value of −2.5 ± 1.5‰, with slight regional variations. As technology improvements in catalytic converters reduce cold-start emissions in the future, it is likely to increase current δ¹⁵N-NO_<i>x</i> values emitted from vehicles

Using ¹⁵N, ¹⁷O, and ¹⁸O To Determine Nitrate Sources in the Yellow River, China

Many previous studies have used δ¹⁵N and δ¹⁸O of nitrate (δ¹⁵N_NO3 and δ¹⁸O_NO3) to determine the nitrate sources in rivers but were subject to substantial uncertainties and limitations, especially associated with evaluating the atmospheric contribution. The Δ¹⁷O of nitrate (Δ¹⁷O_NO3) has been suggested as an unambiguous tracer of atmospheric NO₃^– and may serve as an additional nitrate source constraint. In the present study, triple nitrate isotopes (δ¹⁵N_NO3, Δ¹⁷O_NO3, and δ¹⁸O_NO3) were used for the first time to assess the sources and sinks of nitrate in the Yellow River (YR) basin, which is the second longest river in China. Results showed that the Δ¹⁷O_NO3 of the water from the YR ranged from 0‰ to 1.6‰ during two normal-water seasons. This suggested that unprocessed atmospheric nitrate accounted for 0–7% of the total nitrate in the YR. The corrected δ¹⁵N_NO3 and δ¹⁸O_NO3 values with atmospheric imprints being removed indicated that the main terrestrial sources of nitrate were sewage/manure effluents in the upstream of the YR and manure/sewage effluents and ammonium/urea-containing fertilizer in the middle and lower reaches which made comparable contributions to the nitrate. In addition, there was a significant positive relationship between δ¹⁵N_NO3 and δ¹⁸O_NO3 values of river water (<i>p</i> < 0.01) which may signal the presence of denitrification. This study indicates that the triple nitrate isotope method is useful for assessing the nitrate sources in rivers, especially for the measurements of atmospheric nitrate contribution

Sources and Transport of Nitrogen in Arid Urban Watersheds

Urban watersheds are often sources of nitrogen (N) to downstream systems, contributing to poor water quality. However, it is unknown which components (e.g., land cover and stormwater infrastructure type) of urban watersheds contribute to N export and which may be sites of retention. In this study we investigated which watershed characteristics control N sourcing, biogeochemical processing of nitrate (NO₃^–) during storms, and the amount of rainfall N that is retained within urban watersheds. We used triple isotopes of NO₃^– (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) to identify sources and transformations of NO₃^– during storms from 10 nested arid urban watersheds that varied in stormwater infrastructure type and drainage area. Stormwater infrastructure and land coverretention basins, pipes, and grass coverdictated the sourcing of NO₃^– in runoff. Urban watersheds were strong sinks or sources of N to stormwater depending on runoff, which in turn was inversely related to retention basin density and positively related to imperviousness and precipitation. Our results suggest that watershed characteristics control the sources and transport of inorganic N in urban stormwater but that retention of inorganic N at the time scale of individual runoff events is controlled by hydrologic, rather than biogeochemical, mechanisms

Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from ¹⁵N‑Stable Isotope in Size-Resolved Aerosol Ammonium

The reduction of ammonia (NH₃) emissions is urgently needed due to its role in aerosol nucleation and growth causing haze formation during its conversion into ammonium (NH₄⁺). However, the relative contributions of individual NH₃ sources are unclear, and debate remains over whether agricultural emissions dominate atmospheric NH₃ in urban areas. Based on the chemical and isotopic measurements of size-resolved aerosols in urban Beijing, China, we find that the natural abundance of ¹⁵N (expressed using δ¹⁵N values) of NH₄⁺ in fine particles varies with the development of haze episodes, ranging from −37.1‰ to −21.7‰ during clean/dusty days (relative humidity: ∼ 40%), to −13.1‰ to +5.8‰ during hazy days (relative humidity: 70–90%). After accounting for the isotope exchange between NH₃ gas and aerosol NH₄⁺, the δ¹⁵N value of the initial NH₃ during hazy days is found to be −14.5‰ to −1.6‰, which indicates fossil fuel-based emissions. These emissions contribute 90% of the total NH₃ during hazy days in urban Beijing. This work demonstrates the analysis of δ¹⁵N values of aerosol NH₄⁺ to be a promising new tool for partitioning atmospheric NH₃ sources, providing policy makers with insights into NH₃ emissions and secondary aerosols for regulation in urban environments