Vehicle NO_x emission plume isotopic signatures: Spatial variability across the eastern United States

Abstract

On‐road vehicle nitrogen oxide (NOx) sources currently dominate the U.S. anthropogenic emission budgets, yet vehicle NOx emissions have uncertain contributions to oxidized nitrogen (N) deposition patterns. Isotopic signatures serve as a potentially valuable observational tool to trace source contributions to NOx chemistry and N deposition, yet in situ emission signatures are underconstrained. We characterize the spatiotemporal variability of vehicle NOx emission isotopic signatures (δ15N‐NOx) representative of U.S. vehicle fleet‐integrated emission plumes. A novel combination of on‐road mobile and stationary urban measurements is performed using a field and laboratory‐verified technique for actively capturing NOx in solution to quantify δ15N‐NOx at hourly resolution. On‐road δ15N‐NOx upwind of Providence, RI, ranged from −7 to −3‰. Simultaneous urban background δ15N‐NOx observations showed comparable range and variations with on‐road measurements, suggesting that vehicles dominate NOx emissions in the Providence area. On‐road spatial δ15N‐NOx variations of −9 to −2‰ were observed under various driving conditions in six urban metropolitan areas and rural interstate highways during summer and autumn in the U.S. Northeast and Midwest. Although isotopic signatures were insensitive to on‐road driving mode variations, statistically significant correlations were found between δ15N‐NOx and NOx emission factor extremes associated with heavy diesel emitter contributions. Overall, these results constrain an isotopic signature of fleet‐integrated roadway NOx emission plumes, which have important implications for distinguishing vehicle NOx from other sources and tracking emission contributions to NOx chemistry and N deposition.

Isotopic Composition of In Situ Soil NOx Emissions in Manure‐Fertilized Cropland

“…Soil NO x emissions were measured using a static flux chamber and luminol‐based chemiluminescence NO 2 /NO x analyzer (Figure S1, left; Drummond et al, ) with emission uncertainties of 0.1 ng N‐NO x · m −2 · s −1 ± 8% (details provided in the supporting information). The stable N isotopic composition of soil NO x emissions (δ 15 N‐NO x , where δ 15 N = (( 15 N/ 14 N) sample /( 15 N/ 14 N) air − N2 (reference) − 1) × 1,000‰) was measured via collection of emitted NO x in a KMnO 4 /NaOH solution using an automated, battery‐powered, wirelessly operated field collection system detailed in Wojtal et al () and Miller et al (). Here, a dynamic flux chamber configuration was used with standard flow rates of ~4 L/min at both the inlet and outlet (Figure S1, right).…”

“…The nitrogen isotopic composition of soil NO x emissions (δ 15 N-NO x ) could serve as a potentially valuable observational tool to distinguish soil from other NO x sources (Hastings et al, 2013;Yu & Elliott, 2017) and track source contributions to atmospheric chemistry and N deposition (Felix & Elliott, 2013;Hastings et al, 2013;Zong et al, 2017). Soil microbial processes are expected to produce much lower δ 15 N (À60 to À19‰) than that of on-road vehicles (À9 to À2‰), natural gas combustion (À18 to À15‰), coal combustion (+9 to +20‰), or biomass burning sources (À12 to +7‰) (Felix et al, 2012;Fibiger & Hastings, 2016;Li & Wang, 2008;Miller et al, 2017;Walters et al, 2015;Yu & Elliott, 2017). Furthermore, studies using soil ammonium (NH 4 + ), nitrate (NO 3 À ) and nitrite (NO 2 À ) with 15 N labeling have elucidated soil emission pathways, including distinguishing biotic and abiotic NO production during soil rewetting (Homyak et al, 2016).…”

“…Furthermore, NH 3 exhibits surface affinity inside the aluminum flux chamber. Thus, we expect relatively low NH 3 concentrations reaching the collection solution and negligible NH 3 interference (see Miller et al, 2017). Böhlke et al, 2007) were run as quality controls.…”

Composition, isotopic fingerprint and source attribution of nitrate deposition from rain and fog at a Sub-Arctic Mountain site in Central Sweden (Mt Åreskutan)

“…To our knowledge, there are no studies of d( 15 N) in the oil rig plume NO x . Therefore, considering the isotopic fingerprint of reported mobile and stationary NO x sources, the d( 15 N) of oil rig emissions could be within a wide range between À13 and 20 & (Hastings (2010) and references therein; Felix et al, 2012;Miller et al, 2017). Concomitant to the shift in d( 15 N) values, a decrease in d( 18 O) values from 58 & to 33 & in rain samples was observed between samples R11 and R12 (Figure 8b), which could be due to formation of NO 3 À via oxidation of NO x with hydroxyl radicals (OH) in Table 4.…”

Isotope Fractionation of Nitrate During Volatilization in Snow: A Field Investigation in Antarctica

“…NO 3 − is deposited from the atmosphere in snow and ice, and there has long been interest in using ice core records for reconstructing past atmospheric loadings of NO x from atmospheric (lightning, stratospheric chemistry) and biospheric (soils, biomass burning) sources, as well as levels of major oxidants (i.e., ozone and hydroxyl radical) (Felix & Elliott, 2013;Hastings et al, 2009;Mayewski & Legrand, 1990;Sofen et al, 2014). Atmospheric NO 3 − , however, is not irreversibly deposited in snowpack and can undergo postdepositional processing, significantly among sources, and the ratios are found to be much lower in NO x emitted from soils and vehicles than those from coal combustion or lightning (Hastings et al, 2013;Miller et al, 2017;Yu & Elliott, 2017 (Alexander et al, 2009). In addition, the nitrogen and oxygen isotopic compositions of NO 3 − are sensitive to postdepositional processing.…”