Modeling lightning-NO_x chemistry on a sub-grid scale in a global chemical transport model

Abstract. For the first time, a plume-in-grid approach is implemented in a chemical transport model (CTM) to parameterize the effects of the nonlinear reactions occurring within high concentrated NO_x plumes from lightning NO_x emissions (LNO_x) in the upper troposphere. It is characterized by a set of parameters including the plume lifetime, the effective reaction rate constant related to NO_x–O₃ chemical interactions, and the fractions of NO_x conversion into HNO₃ within the plume. Parameter estimates were made using the Dynamical Simple Model of Atmospheric Chemical Complexity (DSMACC) box model, simple plume dispersion simulations, and the 3-D Meso-NH (non-hydrostatic mesoscale atmospheric model). In order to assess the impact of the LNO_x plume approach on the NO_x and O₃ distributions on a large scale, simulations for the year 2006 were performed using the GEOS-Chem global model with a horizontal resolution of 2° × 2.5°. The implementation of the LNO_x parameterization implies an NO_x and O₃ decrease on a large scale over the region characterized by a strong lightning activity (up to 25 and 8 %, respectively, over central Africa in July) and a relative increase downwind of LNO_x emissions (up to 18 and 2 % for NO_x and O₃, respectively, in July). The calculated variability in NO_x and O₃ mixing ratios around the mean value according to the known uncertainties in the parameter estimates is at a maximum over continental tropical regions with ΔNO_x [−33.1, +29.7] ppt and ΔO₃ [−1.56, +2.16] ppb, in January, and ΔNO_x [−14.3, +21] ppt and ΔO₃ [−1.18, +1.93] ppb, in July, mainly depending on the determination of the diffusion properties of the atmosphere and the initial NO mixing ratio injected by lightning. This approach allows us (i) to reproduce a more realistic lightning NO_x chemistry leading to better NO_x and O₃ distributions on the large scale and (ii) to focus on other improvements to reduce remaining uncertainties from processes related to NO_x chemistry in CTM.