Abstract
Anthropogenic enrichment of reactive nitrogen (Nr) deposition is an ecological concern. We use the adjoint of a global 3-D chemical transport model (GEOS-Chem) to identify the sources and processes that control Nr deposition to an ensemble of biodiversity hotspots worldwide and two U.S. national parks (Cuyahoga and Rocky Mountain). We find that anthropogenic sources dominate deposition at all continental sites and are mainly regional (less than 1000 km) in origin. In Hawaii, Nr supply is controlled by oceanic emissions of ammonia (50%) and anthropogenic sources (50%), with important contributions from Asia and North America. Nr deposition is also sensitive in complicated ways to emissions of SO2, which affect Nr gas-aerosol partitioning, and of volatile organic compounds (VOCs), which affect oxidant concentrations and produce organic nitrate reservoirs. For example, VOC emissions generally inhibit deposition of locally emitted NOx but significantly increase Nr deposition downwind. However, in polluted boreal regions, anthropogenic VOC emissions can promote Nr deposition in winter. Uncertainties in chemical rate constants for OH + NO2 and NO2 hydrolysis also complicate the determination of source-receptor relationships for polluted sites in winter. Application of our adjoint sensitivities to the representative concentration pathways (RCPs) scenarios for 2010-2050 indicates that future decreases in Nr deposition due to NOx emission controls will be offset by concurrent increases in ammonia emissions from agriculture.
Highlights
Reactive nitrogen (Nr) deposition has more than tripled since preindustrial times primarily due to Nr production and release from the agriculture and energy sectors.[1]
Anthropogenic Nr is released to the atmosphere either as nitrogen oxides (NOx ≡ NO + NO2), mainly from combustion, or as ammonia (NH3), mainly from agriculture
Estimates of Nr deposition generally rely on chemical transport models (CTMs), which relate sources to deposition through atmospheric transport and chemistry and can be evaluated with available observations.[11,12]
Summary
Reactive nitrogen (Nr) deposition has more than tripled since preindustrial times primarily due to Nr production and release from the agriculture and energy sectors.[1]. Estimates of Nr deposition generally rely on chemical transport models (CTMs), which relate sources to deposition through atmospheric transport and chemistry and can be evaluated with available observations.[11,12] The processes controlling Nr deposition at a particular receptor site can in principle be fully characterized by perturbing successive source regions, source types, and other processes in the CTM.[12,13]. This approach is in practice severely limited by the computational expense of conducting a large number of CTM simulations. Cuyahoga is located in the vicinity of large anthropogenic sources of Nr, while Rocky Mountain is more remote but highly sensitive to Nr deposition enrichments.[27,28]
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