Abstract
Abstract. Reactive nitrogen (Nr) compounds have different fates in the atmosphere due to differences in the governing processes of physical transport, deposition and chemical transformation. Nr compounds addressed here include reduced nitrogen (NHx: ammonia (NH3) and its reaction product ammonium (NH4+)), oxidized nitrogen (NOy: nitrogen monoxide (NO) + nitrogen dioxide (NO2) and their reaction products) as well as organic nitrogen compounds (organic N). Pollution abatement strategies need to take into account the differences in the governing processes of these compounds when assessing their impact on ecosystem services, biodiversity, human health and climate. NOx (NO + NO2) emitted from traffic affects human health in urban areas where the presence of buildings increases the residence time in streets. In urban areas this leads to enhanced exposure of the population to NOx concentrations. NOx emissions generally have little impact on nearby ecosystems because of the small dry deposition rates of NOx. These compounds need to be converted into nitric acid (HNO3) before removal through deposition is efficient. HNO3 sticks quickly to any surface and is thereby either dry deposited or incorporated into aerosols as nitrate (NO3−). In contrast to NOx compounds, NH3 has potentially high impacts on ecosystems near the main agricultural sources of NH3 because of its large ground-level concentrations along with large dry deposition rates. Aerosol phase NH4+ and NO3− contribute significantly to background PM2.5 and PM10 (mass of aerosols with an aerodynamic diameter of less than 2.5 and 10 μm, respectively) with an impact on radiation balance as well as potentially on human health. Little is known quantitatively and qualitatively about organic N in the atmosphere, other than that it contributes a significant fraction of wet-deposited N, and is present in both gaseous and particulate forms. Further studies are needed to characterise the sources, air chemistry and removal rates of organic N emissions.
Highlights
Reactive nitrogen compounds (Nr) affect ecosystem health (Sutton et al, 2011), human health (Aneja et al, 2009) and contribute to climate change (Butterbach-Bahl et al, 2011b; Erisman et al, 2011; Xu and Penner, 2012)
Another US study showed that HNO3 and peroxyacetyl nitrate (PAN) concentrations in the urban areas of Houston, Texas were of similar magnitude and could be up to 1.5 ppbv during daytime (Luke et al, 2010)
PAN deposition has been modelled in many studies (Wu et al, 2012); the dry deposition is much faster than expected on the basis of its solubility, with dry deposition velocities in daytime that may exceed 15 mm s−1 and nighttime values of half this magnitude (Doskey et al, 2004; Schrimpf et al, 1996), so much is still unknown about the mechanisms of its atmospheric removal (Turnipseed et al, 2006)
Summary
Reactive nitrogen compounds (Nr) affect ecosystem health (Sutton et al, 2011), human health (Aneja et al, 2009) and contribute to climate change (Butterbach-Bahl et al, 2011b; Erisman et al, 2011; Xu and Penner, 2012). Strong NH3 sources contribute to high N loads to nearby ecosystems through a fast dry deposition rate (dry deposition velocities are typically 5–20 mm s−1, Flechard et al, 2011, but may approach 40 mm s−1, Phillips et al, 2004) This process competes with reactions that lead to the formation of aerosol phase ammonium (NH+4 ). The presence of buildings, especially in streets with close building facades on both sides of the street, increases the local residence time of traffic pollutants emitted in the street sufficiently to significantly increase the exposure of the population (Hertel and Goodsite, 2009) These processes – the flow conditions and the chemical conversion – affect the ratio between NO which has little direct impact on human health and the airway irritant NO2 that, for example, may enhance the impact of airborne allergens (Tunnicliffe et al, 1994). It should be noted though that it does play a role in the chemistry of the stratosphere and as a climate forcer (Pinder et al, 2012), as well as in leading to depletion of stratospheric ozone (Brink et al, 2011; Butterbach-Bahl et al, 2011b)
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