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. Reactive nitrogen (N) emissions have increased over the last 150 years as aresult of greater fossil fuel combustion and food production. The resultingincrease in N deposition can alter the function of ecosystems, butcharacterizing its ecological impacts remains challenging, in part because ofuncertainties in model-based estimates of N dry deposition. Here, we use theGeophysical Fluid Dynamics Laboratory (GFDL) atmospheric chemistry–climatemodel (AM3) coupled with the GFDL land model (LM3) to estimate dry depositionvelocities. We leverage the tiled structure of LM3 to represent the impact ofphysical, hydrological, and ecological heterogeneities on the surface removalof chemical tracers. We show that this framework can be used to estimate Ndeposition at more ecologically relevant scales (e.g., natural vegetation,water bodies) than from the coarse-resolution global model AM3. Focusing onNorth America, we show that the faster removal of N over forested ecosystemsrelative to cropland and pasture implies that coarse-resolution estimates ofN deposition from global models systematically underestimate N deposition tonatural vegetation by 10 % to 30 % in the central and eastern US.Neglecting the sub-grid scale heterogeneity of dry deposition velocities alsoresults in an underestimate (overestimate) of the amount of reduced(oxidized) nitrogen deposited to water bodies. Overall, changes in land coverassociated with human activities are found to slow down the removal of N fromthe atmosphere, causing a reduction in the dry oxidized, dry reduced, andtotal (wet + dry) N deposition over the contiguous US of 8 %, 26 %, and6 %, respectively. We also find that the reduction in the overall rate ofremoval of N associated with land-use change tends to increase N depositionon the remaining natural vegetation and facilitate N export to Canada. Weshow that sub-grid scale differences in the surface removal of oxidized andreduced nitrogen imply that projected near-term (2010–2050) changes inoxidized ( −47  %) and reduced ( +40  %) US N emissions will causeopposite changes in N deposition to water bodies (increase) and naturalvegetation (decrease) in the eastern US, with potential implications foracidification and ecosystems.

atmospheric chemistry and physics

Representing sub-grid scale variations in nitrogen deposition associated with land use in a global Earth system model: implications for present and future nitrogen deposition fluxes over North America