A three-dimensional total odd nitrogen (NO_y) simulation during SONEX using a stretched-grid chemical transport model

The relative importance of various odd nitrogen (NO_y) sources including lightning, aircraft, and surface emissions on upper tropospheric total odd nitrogen is illustrated as a first application of the three-dimensional Stretched-Grid University of Maryland/Goddard Chemical-Transport Model (SG-GCTM). The SG-GCTM has been developed to look at the effect of localized sources and/or small-scale mixing processes on the large-scale or global chemical balance. For this simulation the stretched grid was chosen so that its maximum resolution is located over eastern North America and the North Atlantic; a region that includes most of the Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) flight paths. The SONEX period (October-November 1997) is simulated by driving the SG-GCTM with assimilated data from the Goddard Earth Observing System-Stratospheric Tracers of Atmospheric Transport Data Assimilation System (GEOS-STRAT DAS). A new algorithm is used to estimate the lightning flash rates needed to calculate NO_y emission by lightning. This algorithm parameterizes the flash rate in terms of upper tropospheric convective mass flux. Model-calculated upper tropospheric NO_y and NO_y measurements from the NASA DC-8 aircraft are compared. Spatial variations in NO_y were well captured especially with the stretched-grid run; however, model-calculated peaks due to "stratospheric" NO_y are occasionally too large. The lightning algorithm reproduces the temporally and spatially averaged total flash rate accurately; however, the use of emissions from observed lightning flashes significantly improves the simulation on a few days, especially November 3, 1997, showing that significant uncertainty remains in parameterizing lightning in chemistry and transport models. Aircraft emissions contributed ∼15% of the upper tropospheric NO_y averaged along SONEX flight paths within the North Atlantic Flight Corridor with the contribution exceeding 40% during portions of some flights.