Impact of lightning NO emissions on North American photochemistry as determined using the Global Modeling Initiative (GMI) model

The impact of nitric oxide (NO) emissions by lightning on summertime North American nitrogen oxides (NOx) and ozone is studied using the Global Modeling Initiative (GMI) CTM and an improved lightning NO algorithm. The spatial distributions of modeled and National Lightning Detection Network-based flash rates during the summers of 2004-2006 agree well (R-2 = 0.49, 18% low bias). Despite this reasonable agreement, 9-12 km model NOx during the Intercontinental Chemical Transport Experiment (INTEX-A) campaign is a factor of 2.2-3.6 too low for a simulation that includes a 480 mol per flash midlatitude lightning NO source, the source that provides the best agreement with measurements. Possible causes of this low bias include biases in model convection and/or too rapid NOx chemistry in the upper troposphere. Model tropospheric NO2 columns over the southeastern United States during these summers show a 7% high bias with respect to the OMI DOMINO/GEOS-Chem tropospheric column NO2 product. Observed changes between 2004 and 2006 in upper tropospheric ozone at southeastern U. S. INTEX Ozonesonde Network Study sites are captured by the model and appear to be caused by a stronger upper tropospheric anticyclone in 2006 that led to an increase from 21 to 30 ppbv between 2004 and 2006 in the amount of ozone with a lightning NO source; lightning NO emissions were 15%-20% larger in 2004. The contribution of lightning NO to monthly average summertime 300 hPa NOx over the eastern United States during 2004-2006 varies from 61%-73% (0.09-0.16 ppbv), while the contribution to ozone varies from 19%-31% (15-24 ppbv).