The relative importance of various odd nitrogen (NOy) sources including lightning, aircraft, and surface emissions on upper tropospheric total odd nitrogen was estimated using the three-dimensional Stretched-Grid University of Maryland/Goddard Chemical-Transport Model (SG-GCTM). For this simulation, the stretched-grid was chosen so that its maximum resolution was located over eastern North America and the North Atlantic; a region that included most of the SONEX (the SASS (Subsonic Assessment) Ozone and Nitrogen Oxides Experiment) flight paths. The SONEX period (October-November 1997) was simulated by driving the SG-GCTM with assimilated data from the GEOS-STRAT DAS (Goddard Earth Observing System-STRAT Data Assimilation System). A new algorithm was used to estimate the lightning flash rates needed to calculate NOy emission by lightning. This algorithm parameterized the flash rate in terms of upper tropospheric convective mass flux.
Model-calculated upper tropospheric NOy and NOy measurements from the NASA DC-8 aircraft were compared. Spatial variations in NOy were well captured especially with the stretched-grid run. This experiment was especially useful for identifying the cause(s) of several observed NOy peaks. Peaks due to lightning and stratospheric NOy were captured on numerous occasions; although comparison with measurements indicates that the magnitude of stratospheric peaks was often overestimated. The lightning algorithm reproduces the temporally and spatially averaged total flash rate accurately. However, the location and strength of individual convective (lightning) events is occasionally missed and the use of observed lightning emissions significantly improves the simulation on a few occasions, especially November 3, 1997 and possibly October 29, 1997. Aircraft (lightning) emissions contributed 15% ( 22%) of the upper tropospheric NOy averaged along SONEX flight paths within the North Atlantic Flight Corridor (NAFC) with the contribution by aircraft (lightning) exceeding 40% (75%) during portions of some flights. The contribution of both sources to upper tropospheric NOx is likely to be larger because upper tropospheric NOy over the NAFC with an aircraft or lightning source is likely to be fresher (have a larger NOx / NOy ratio) than NOy from other sources.
The uncertainty associated with the estimated contributions is large because uncertainties in the simulation and/or parameterization of deep convective mixing, wet scavenging, stratosphere-troposphere exchange, aircraft NOy emissions and lightning NOy emissions remain large. In addition, comparison of model output and measurements suggests that the relative importance of the fossil fuel/soil and stratospheric sources may be overestimated by the model. Therefore, the estimated contributions by lightning and aircraft emissions are more likely to be an underestimate than an overestimate.
These simulations have improved our understanding of lightning NOx emissions and the NOy budget over the NAFC; however, the dynamical fields used to drive this model were obtained from a coarser uniform grid model. Future calculations of the SONEX period will use driving fields from a stretched-grid DAS.