Why do Different Anthropogenic Sources of Atmospheric Nitrate have Distinctive Isotopic Signatures?
Abstract
Do different sources of atmospheric nitrate (power plants, vehicles, agricultural emissions) have distinctive isotopic signatures? To answer this question, we conducted a national survey of nitrate isotopes in wet deposition samples collected throughout the USA from 156 NADP sites. Archived samples from the year 2000 were pooled into bimonthly composites and analyzed for d15N and d18O, with a subset analyzed for D17O. In this presentation, we present our current thoughts about why the different sources have distinctive isotopic signatures, focusing mainly on oxygen isotopes of nitrate. Our original conceptual model was based largely on (1) Heaton's 1990 paper that showed that NOx emissions from power plants in South Africa had d15N values significantly higher than exhaust from vehicles, and (2) anecdotal data from several studies showing higher d15N and/or d18O values of nitrate in precipitation downwind of areas dominated by power plants. Our model proposed that atmospheric nitrate derived from near- surface sources (e.g., vehicle and biogenic emissions) would obtain d18O and D17O signatures predominantly from near-surface O2 produced by photosynthesis (averaging +23 permil), whereas NOx exiting power plant stacks would circulate higher into the atmosphere and obtain nitrate d18O and D17O signatures predominantly from tropospheric O3 (+95 and +35 permil, respectively). We speculated that the source discrimination seen in NOx emissions would be maintained in the atmospheric nitrate ultimately derived from it, despite potential isotopic fractionation during conversion to nitrate. Furthermore, because of the likelihood of temporal and spatial variation in the compositions of NOx exiting individual combustion engines due to changes in temperature, fuel types, and other operating conditions, we suspected that source signatures would best be determined after the NOx had been converted to nitrate. Hence, we proposed to determine source signatures primarily from statistical analysis of a large set of wet deposition samples, instead of using emission samples. The most striking aspect of our data are the distinct but different spatial patterns shown by the d15N, d18O, and D17O values. We observe strong correlations between d15N and power plant NOx, suggesting this source is well mixed relative to vehicle emissions, which appear to be largely deposited near roadways (Elliott et al., in press). Several recent papers have developed models to explain seasonal nitrate isotopic variations in wet and dry deposition in localized areas. In general, these models explain the variations in terms of seasonality in oxidative reactions in the atmosphere, not source signatures. We will evaluate these models and our original conceptual model with our various large data sets, and present the current state of our understanding of the isotopic signatures of different sources and how they are established during combustion, transport, mixing, and atmospheric reactions during the conversion of NOx emissions to nitrate in deposition.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.B31A0059K
- Keywords:
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- 0345 Pollution: urban and regional (0305;
- 0478;
- 4251);
- 0426 Biosphere/atmosphere interactions (0315);
- 0469 Nitrogen cycling;
- 1041 Stable isotope geochemistry (0454;
- 4870);
- 1854 Precipitation (3354)