Accounting for non-linear chemistry of ship plumes in the GEOS-Chem global chemistry transport model

Abstract. We present a computationally efficient approach to account for the non-linear chemistry occurring during the dispersion of ship exhaust plumes in a global 3-D model of atmospheric chemistry (GEOS-Chem). We use a plume-in-grid formulation where ship emissions age chemically for 5 h before being released in the global model grid. Besides reducing the original ship NO_x emissions in GEOS-Chem, our approach also releases the secondary compounds ozone and HNO₃, produced during the 5 h after the original emissions, into the model. We applied our improved method and also the widely used "instant dilution" approach to a 1-yr GEOS-Chem simulation of global tropospheric ozone-NO_x-VOC-aerosol chemistry. We also ran simulations with the standard model (emitting 10 molecules O₃ and 1 molecule HNO₃ per ship NO_x molecule), and a model without any ship emissions at all. The model without any ship emissions simulates up to 0.1 ppbv (or 50%) lower NO_x concentrations over the North Atlantic in July than our improved GEOS-Chem model. "Instant dilution" overestimates NO_x concentrations by 0.1 ppbv (50%) and ozone by 3–5 ppbv (10–25%), compared to our improved model over this region. These conclusions are supported by comparing simulated and observed NO_x and ozone concentrations in the lower troposphere over the Pacific Ocean. The comparisons show that the improved GEOS-Chem model simulates NO_x concentrations in between the instant dilution model and the model without ship emissions, which results in lower O₃ concentrations than the instant dilution model. The relative differences in simulated NO_x and ozone between our improved approach and instant dilution are smallest over strongly polluted seas (e.g. North Sea), suggesting that accounting for in-plume chemistry is most relevant for pristine marine areas.