Relative Contributions of Anthropogenic and Lightning Nitrogen Sources in the Upper Troposphere during the Asian Summer Monsoon

We examine the role of the Asian Summer Monsoon (ASM) in influencing the chemical composition of the upper troposphere and lower stratosphere (UTLS) using the Whole Atmosphere Community Climate Model, version 6 (WACCM6). This version of WACCM6 uses a Finite Volume dynamical core, with a horizontal resolution of ~1.0º and a vertical resolution of ~500m in the UTLS. For this study, the specified dynamics option is applied where the temperature, zonal and meridional winds are nudged towards MERRA-2 reanalysis fields from the NASA Goddard Earth Observing System version 5 (GEOS5). This model study examines the relative contribution of anthropogenic and lightning nitrogen oxide (NOx) sources in the UTLS during the Asian Summer Monsoon (ASM) using a tagged NOx mechanism (Emmons et al., Geosci. Model Dev., doi:10.5194/gmd-5-1531-2012). In this tagging mechanism, the NOx source regions in South and East Asia are examined separately. NOx sources from outside South and East Asia and the amount transported from the stratosphere are also derived. The model simulated NOx for the year 2022 is evaluated by comparing it to in situ measurements from the Asian summer monsoon Chemical and Climate Impact Project (ACCLIP). The model results suggest that the major contribution of NOx concentration within the ASM anticyclone is from South Asia lightning and South Asia anthropogenic sources, which contribute more than 55% to the upper troposphere NOx for the year 2022. In the shedding region, both South and East Asia anthropogenic emissions play an important role in the NOx budget. In addition, we also explore the hydroxyl radical (OH) and peroxyacetylnitrate (PAN) formation from different NOx sources, which are of importance to atmospheric compositions such as ozone and aerosols in the free troposphere.