On-road emissions of ammonia: An underappreciated source of atmospheric nitrogen deposition
Graphical abstract
Introduction
Increasingly, both agricultural and transport sector emissions of NH3 have been recognized as important sources of reduced N (Xing et al., 2013), although in some large-scale analyses only agricultural sources are considered (Battye et al., 2017, Moldanová et al., 2011, Pye et al., 2009). Estimated emissions of NH3 in the United States increased by 91, 70 and 7% in the on-road, power plant and agricultural sectors, respectively, between 1990 and 2010 (Xing et al., 2013). Emissions of NH3 from motor vehicles (Kean et al., 2009, Leip et al., 2011) and industrial stacks (Hertel et al., 2011, Li et al., 2017, Vallero, 2014) are largely a byproduct of stringent NOx control technologies. In many regions of the developed world, NOx emissions have steadily declined since around 1990 (Leip et al., 2011) while emissions of NH3 have remained constant or increased slightly, such as in the United States where NH3 emissions have increased by 11% (Bytnerowicz et al., 2016, Hertel et al., 2011, Xing et al., 2013). As a result, the proportion of N deposition in reduced forms in many areas of the developed world, has increased in recent years (Du et al., 2014, Hůnová et al., 2017, Li et al., 2016).
Light and medium duty motor vehicles equipped with three-way catalytic converters dominate our roadways and produce NH3 (Bishop and Stedman, 2015). As a result of the introduction of more stringent standards for emissions of NOx and particulate matter in California, nationwide by the U.S. Environmental Protection Agency (USEPA), and in Europe (Hertel et al., 2011, Leip et al., 2011) heavy-duty vehicles now include a mix of natural gas engines with three-way catalytic converters and diesel engines equipped with selective catalytic reduction (SCR). In SCR, which is also used in light-duty diesel powered cars and fleet vehicles, aqueous urea is injected as a reductant for NOx control, resulting in production of NH3 (Bishop and Stedman, 2015, Thiruvengadam et al., 2016). Heavy duty vehicles, including city buses, refuse trucks, cargo vehicles, and clean diesel trains equipped with current emissions control technology are becoming an increasingly important source of NH3 emissions in urban areas and along transport corridors.
Air-fuel ratios that are slightly rich result in maximal NOx reductions, yet are the most conducive to NH3 production from three-way catalytic converters (Thiruvengadam et al., 2016). Therefore a trade-off exists whereby conditions favoring NOx reduction generally favor NH3 production by three-way catalytic converters (Heeb et al., 2006). As a result of NH3 production from NOx control mechanisms, urban areas may represent more important NH3 emissions source areas for downwind ecosystems than commonly recognized, particularly as urbanization continues to expand its footprint. Livingston et al. (2009) argue that broad scale emissions inventories of NH3 “fail to take into account the spatial distribution of ammonia emissions and the potential for relatively high emissions from mobile sources in dense, highly urbanized airsheds”.
This study builds on the work of Du et al. (2014) and Li et al. (2016) who demonstrated the increasing importance of reduced forms of N deposition in the U.S., and the work of Sun et al. (2017) who describe an improved method for estimating on-road emissions of NH3. The overarching objective of this analysis is to evaluate the increasing importance of reduced forms of reactive N in areas influenced by urban and on-road emissions. More specifically, the supporting objectives of this study are three-fold: (1) to provide national scale on-road NH3 emissions data and spatial patterns; (2) to compare temporal trends in NO3− and NH4+ wet deposition in sites primarily affected by urban versus agricultural emissions; and (3) to present case studies of regional-scale empirical deposition and atmospheric exposure data showing the contribution of on-road NH3 emissions towards elevated atmospheric N exposure and deposition in urban and urban-influenced ecosystems.
Section snippets
Ammonia emissions data
Agricultural and on-road emissions of NH3 are from the 2014 USEPA National Emissions Inventory (NEI; USEPA, 2017). Downloaded NH3 data was aggregated by the sector: “Agriculture” (Crops and Livestock Dust, Fertilizer Application, and Livestock Waste) and the Tier 1 category “Highway Vehicles”. Emissions of NH3 were also estimated based on vehicular CO2 emissions and well-documented on-road NH3:CO2 emissions ratios (summarized in Sun et al., 2017). This ratio approach is considered to be a
Ammonia emissions sources in CONUS
Comparisons of NH3 emissions from on-road sources with those from agriculture across the U.S. are shown in Fig. 2. As expected, elevated NH3 emissions from agriculture are more widespread and are dominant in the Midwest region and in a few isolated hotspots; Agricultural emissions reach higher levels in the most affected counties compared to on-road emissions. Extensive portions of the Southeast and Northeast regions of the U.S. receive high proportions of the total NH3 emissions from urban
Discussion
The improved national scale estimates of on-road NH3 emissions reported herein show that areas with elevated on-road NH3 emissions are widespread in the eastern half of the U.S., along the west coast, and across urbanized regions of the interior West. This spatial extensiveness of elevated on-road NH3 emissions is also evident by the fact that 440 counties across the U.S. receive at least one-third of their NH3 emissions from on-road sources. The universal and increasing importance of reduced
Conclusions
Emissions inventories and trends in atmospheric NH3 concentrations are uncertain, largely because NH3 is not a USEPA criteria pollutant, and thus data are lacking. Our estimates of on-road emissions of NH3 within CONUS (0.26 Tg yr− 1) are 2.9 times greater than those of the USEPA's NEI (0.09 Tg yr− 1). Empirical monitoring data indicate the increasing importance of reduced forms of atmospheric N in the environment. Ammonium constitutes 58.7% of wet inorganic N deposition at sites within CONUS. Only
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