Export of NOy from the North American boundary layer: Reconciling aircraft observations and global model budgets

Fossil fuel combustion accounts for >50% of the global atmospheric emission of NO x , but this source is concentrated in the polluted continental boundary layer (CBL) and only a small fraction is exported as NO y (NO x and its oxidation products) to the global troposphere. Better quantification of this export efficiency is needed because of its implications for global tropospheric ozone. A recent Lagrangian analysis of the NO y ‐CO correlations observed from the North Atlantic Regional Experiment in September 1997 (NARE'97) aircraft campaign downwind of eastern North America (September 1997) indicated a NO y export efficiency of <10%, with <10% of the exported NO y present as NO x . In contrast, previous three‐dimensional (3‐D) model Eulerian budget analyses for the North American boundary layer indicated NO y export efficiencies of 25–30%, with 30–35% of the exported NO y present as NO x . We investigated this apparent discrepancy by simulating the NARE'97 aircraft observations with a global 3‐D model of tropospheric chemistry (GEOS‐CHEM) and using the model to calculate the NO y export efficiency both through a Lagrangian analysis of the NO y ‐CO correlations along the aircraft flight tracks and through an Eulerian budget analysis for the North American boundary layer. The model reproduces the variability and NO y ‐CO correlations observed in the aircraft data and also at the Harvard Forest surface site in the northeastern United States. We show that the previous Lagrangian analyses of the NO y export efficiency during NARE'97 were probably biased low because of underestimation of the CO background. Correcting for this bias, we find a NO y export efficiency of 17 ± 7% in the model and 15 ± 11% in the observations. A similar NO y export efficiency (20%) in the model is obtained from the Eulerian budget analysis, demonstrating that the Lagrangian and Eulerian approaches are in fact consistent. Export efficiencies of NO y in previous 3‐D model Eulerian budget analyses were probably too high because of insufficient scavenging out of the CBL. Model results indicate that only 6% of the exported NO y is present as NO x along the aircraft flight tracks, in agreement with the observations, but that 40% of the NO y export flux is present as NO x , in agreement with the previous 3‐D model analyses. This result reflects the fast oxidation of NO x between the point of exit from the CBL and the point of sampling by the aircraft. The eventual ozone production in the global troposphere due to exported NO x and peroxyacetylnitrate (PAN), with equal contributions from each, is comparable in magnitude to the direct export of ozone pollution from the North American boundary layer.