Impact of lightning-NO emissions on eastern United States photochemistry determined using the CMAQ model

Production of NO by lightning (LNOx) is an important part of the summertime tropospheric NOx budget over the United States, but it is also its most uncertain component. Global model simulations indicate that LNOx increases summertime upper tropospheric NOx concentrations over the eastern United States by 60-75%, upper tropospheric ozone amounts by 15-25% and surface ozone concentrations by several ppbv. A simulation with the Community Multiscale Air Quality Model (CMAQ) that did not include lightning-NO emissions greatly underestimated upper tropospheric NOx concentrations measured during NASAs INTEX field campaign. These large biases made it difficult to constrain ground-level NOx emissions using SCIAMACHY retrievals and CMAQ output. A lightning-NO parameterization has been developed that can be used with the CMAQ model. Flash rates in this scheme are assumed to be proportional to convective precipitation rates from the driving meteorological model (MM5 or WRF) and scaled so that monthly average flash rates in each grid box match National Lightning Detection Network (NLDN) observed flash rates after adjusting for climatological intracloud to cloud-to-ground (IC/CG) ratios. Production of 500 moles of NOx is assumed per flash, which results from cloud-resolved simulations of individual midlatitude and subtropical thunderstorm constained by aircraft observations. The LNOx emissions are distributed in the vertical according to the climatological histogram of VHF lightning sources from Lightning Mapping Arrays. The contribution of lightning-NO emissions to eastern United States NOx and ozone distributions during the summers of 2004 and 2006 will be evaluated by comparing results of CMAQ simulations with and without lightning-NO emissions to measurements from aircraft field campaigns, to ozone profiles from the IONS ozonesonde campaigns, and to satellite NO2 retrievals from the SCIAMACHY and OMI satellite instruments.