Nitrous Oxide (N2O) Emissions from California based on 2010 CalNex Airborne Measurements

Nitrous oxide (N₂O) has become the third most important long-live greenhouse gas in the troposphere and the top ozone-depleting substance in the stratosphere. Its global atmospheric concentration dramatically increased from a preindustrial level of 275 ppb to the current global average of 325 ppb, this makes accurate emissions constraints and effective mitigation strategies all the more important. Agricultural management is known to be the largest anthropogenic source, but estimates of emissions remain highly uncertain, especially at regional scales. This work is among a small number of top-down studies which use in-situ atmospheric observations to estimate regional-scale N₂O emissions. Airborne data were collected during the 2010 growing season in the heavily agricultural Central Valley of California. We coupled WRF (Weather Research and Forecasting) meteorology and the STILT (Stochastic Time-Inverted Lagrangian Transport) transport model to link the in-situ observation with surface emissions, and we used an array of statistical methods to optimize emission sources. Results indicate that both fertilizer application and fossil fuel burning are the largest Central Valley N₂O sources. High resolution (0.1 degree by 0.1 degree) optimized surface emission maps show similar patterns to the EDGAR inventory (version 4.0) but are approximately three times higher in magnitude during the study period. Based on this work, global N₂O inventories should be updated with improved spatial patterns and should include seasonal variations.