Governing processes for reactive nitrogen compounds in the European atmosphere

Abstract. Reactive nitrogen (N_r) compounds have different fates in the atmosphere due to differences in the governing processes of physical transport, deposition and chemical transformation. N_r compounds addressed here include reduced nitrogen (NH_x: ammonia (NH₃) and its reaction product ammonium (NH₄⁺)), oxidized nitrogen (NO_y: 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. NO_x (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 NO_x concentrations. NO_x emissions generally have little impact on nearby ecosystems because of the small dry deposition rates of NO_x. 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 NO_x 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 PM_2.5 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.