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The Nitrogen Stable Isotope Composition (#15N) of Vehicle Emitted NOx Wendell William Walters, Stanford R Goodwin, and Greg Michalski Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505580v • Publication Date (Web): 26 Jan 2015 Downloaded from http://pubs.acs.org on February 3, 2015
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The Nitrogen Stable Isotope Composition (δ15N) of
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Vehicle Emitted NOx
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Wendell W. Walters*†, Stanford R. Goodwin‡, and Greg Michalski†‡
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†
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Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, United States
Department of Earth, Atmospheric, and Planetary Sciences and ‡Department of Chemistry,
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Abstract
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The nitrogen stable isotope ratio of NOx (δ15N-NOx) has been proposed as a regional indicator
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for NOx source partitioning; however, knowledge of δ15N values from various NOx emission
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sources is limited. This study presents a detailed analysis of δ15N-NOx emitted from vehicle
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exhaust, the largest source of anthropogenic NOx. To accomplish this, NOx was collected from
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26 different vehicles including gasoline and diesel-powered engines using a modification of a
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U.S. EPA NOx collection method, and δ15N-NOx was analyzed. The vehicles sampled in this
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study emitted δ15N-NOx values ranging from (-19.1 to 9.8‰) that negatively correlated with the
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emitted NOx concentrations (8.5 to 286 ppm) and vehicle run time due to kinetic isotope
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fractionation effects associated with the catalytic reduction of NOx. A model for determining the
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mass-weighted δ15N-NOx from vehicle exhaust was constructing based on average commute
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times and estimates an average value of -2.5 ± 1.5‰ with slight regional variations.
As
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technology improvements in catalytic converters reduce cold-start emissions in the future, it is
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likely to increase current δ15N-NOx values emitted from vehicles.
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Introduction
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Nitrogen oxides (NOx = NO + NO2) are important trace gases that affect atmospheric
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chemistry, air quality, and climate.1 NOx plays a key role in the troposphere by acting to control
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the concentrations of ozone (O3) and the hydroxyl radical (OH) as well as forming nitrate (NO3-)
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aerosols.2 The reactions between NOx and hydrocarbons (HC) are major sources of tropospheric
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O3, which is a greenhouse gas, an oxidizing pollutant, and influences lifetimes of other
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greenhouse gases.3-5 Ultimately, NOx is oxidized to nitric acid (HNO3) and deposited as acid
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rain leading to degradation of drinking water, soil acidification, lacustrine and estuarine
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eutrophication, and biodiversity changes in terrestrial ecosystems.6
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Sources of NOx are both natural and anthropogenic, but there are uncertainties in the
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relative importance of these sources.6-8
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nitrification/denitrification, and wildfires.6,8 Anthropogenic sources of NOx include fossil fuel
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and biofuel combustion, mainly emitted from power plants, transport (vehicles, ships, and
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aircrafts) and industry.7-8 Since the industrial revolution, anthropogenic emissions of NOx have
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surpassed natural NOx emissions.6-7 While significant improvements have been made to reduce
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NOx emissions from stationary and mobile sources, further progress is needed to reduce the
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health and ecosystems impacts associated with NOx emissions;9 however, the uncertainty of total
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NOx emissions is relatively high, with an estimated uncertainty on the order of 30% to 50%.6 In
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order to estimate the relative importance of various NOx sources, and to assess the effectiveness
Natural sources of NOx include lightning, soil
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of NOx reduction technologies, a way of partitioning NOx sources based on nitrogen (N)
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deposition studies is required.
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Once emitted in the atmosphere, NOx is primarily oxidized to HNO3 and subsequently
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removed from the atmosphere via wet or dry deposition. Therefore, analysis of the N stable
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isotopes of atmospheric derived nitrate (δ15N-NO3-) could be used as a regional indicator for
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partitioning NOx sources, which would help identify the contribution of various NOx sources to
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local/regional N deposition, as well as help evaluate the effectiveness of NOx emission
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reductions.
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deposition that correlated with the δ15N-NOx of the surrounding areas stationary NOx emission
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sources (one-tailed, t-test, p
