Anthropogenic activities have nearly doubled the global creation of reactive nitrogen (Nr), deliberately for food production (Haber-Bosch process) and unintentionally via fossil fuel combustion. The associated atmospheric emissions of nitrogen oxides (NOx), ammonia (NH3), and nitrous oxide (N2O) result in greater production of ozone and particulate matter, degraded surface air quality, increased nitrogen deposition to vulnerable areas, and impacts on regional and global climate. Atmospheric Nr deposition and agricultural Nr runoff into rivers affect water quality and coastal ecosystem functioning. The GFDL climate and Earth System Models (ESMs) used for CMIP5 included representations of Nr cycling within the atmosphere and oceans. A new version of the GFDL land model, LM3-TAN, represents processes of ecological and biogeochemical transformation of Nr in soils, vegetation, and rivers. The ocean, land, and atmospheric Nr dynamics are currently uncoupled, relying on prescribed exchanges between these pools. We propose to advance the representation of atmosphere-land interactions of Nr in GFDL ESMs on timescales from sub-hourly to centennial. These new representations of Nr processing will be evaluated and constrained through detailed evaluation with observational datasets of atmospheric and terrestrial Nr. The overarching goal of this project is to advance understanding of the intended and unintended consequences of increased anthropogenic Nr creation for terrestrial and aquatic ecosystems, The project will improve the representation of the formation, radiative properties, internal mixing, and heterogeneous sources and sinks of nitrate aerosol, to be incorporated into a new version of the GFDL atmospheric model, AM3N. In the land model, we plan to represent Nr loss through denitrification in soils under anoxic conditions, including an explicit treatment of microbial processes. Runoff of Nr and cycling in rivers will be represented globally. The model will use improved scenarios of historical and future land management practices. The project will improve the characterization of the Nr emissions and deposition through online coupling between the atmosphere and land components of the GFDL ESM. Speciated Nr emissions (NH3, NO, N2O) from unmanaged ecosystems, managed agricultural lands, and biomass burning will be represented. Wet and dry deposition fluxes of Nr species from the atmosphere will be The project will leverage and extend ongoing observations of Nr exchange between soil and atmosphere in managed ecosystems (feedlots, croplands) and to perform new observations in an unmanaged environment (forest or grassland). These detailed observations will provide important constraints for process-level evaluation of the coupled atmosphere-land Nr system in the ESM. Regional evaluation of AM3N will include comparison against ship-based, ground-based, mobile laboratory, airborne, and satellite measurements of Nr gases and aerosols.