Ammonia (NH3) in the UTLS: GLORIA airborne measurements for CAMS model evaluation in the Asian Monsoon and in biomass burning plumes above the South Atlantic
Abstract. Ammonia (NH3) is the major alkaline species in the atmosphere and plays an important role in aerosol formation, which affects local air quality and the radiation budget. NH3 in the Upper Troposphere and Lower Stratosphere (UTLS) is difficult to detect and only limited observations are available. We present two dimensional trace gas measurements of NH3 obtained by the airborne infrared imaging limb sounder GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) that has been operated onboard the research aircraft Geophysica within the Asian Monsoon during the StratoClim campaign (July 2017) and onboard HALO (High Altitude and Long Range Research Aircraft) above the South Atlantic during the SouthTRAC campaign (September–November 2019). We compare these GLORIA measurements in the UTLS with results of the CAMS (Copernicus Atmosphere Monitoring Service) reanalysis and forecast model. The GLORIA observations reveal large enhancements of NH3 of more than 1 ppbv in the Asian Monsoon upper troposphere, but no clear indication of NH3 in biomass burning plumes in the upper troposphere above the South Atlantic above the instrument's detection limit of around 20 pptv. In contrast, CAMS reanalysis and forecast simulation results indicate strong enhancements of NH3 in both measured scenarios. Comparisons of other retrieved pollution gases, such as peroxyacetyl nitrate (PAN) show the ability of CAMS models to reproduce the biomass burning plumes above the South Atlantic in general. However, NH3 concentrations are largely overestimated by the CAMS models within these plumes. We suggest that emission strengths used by CAMS models are of different accuracy for biomass burning and agricultural sources in the Asian Monsoon. Further, we suggest that loss processes of NH3 during transport to the upper troposphere may be underestimated for the biomass burning cases above the South Atlantic. Since NH3 is strongly undersampled, in particular at higher altitudes, we hope for regular vertically resolved measurements of NH3 from the proposed CAIRT mission to strengthen our understanding of this important trace gas in the atmosphere.
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