Comment on gmd-2023-4

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Adjoint of the GEOS-Chem model has been widely used to constrain the sources of various atmospheric pollutants. Here we provide an updated version (GC-Adjoint-HEMCO) of the adjoint of GEOS-Chem model to support the MERRA-2 meteorological data and Harmonized Emissions Component (HEMCO) emission inventories. State-of-the-art inventories, such as CEDS (Community Emissions Data System), MIX, NEI2011 (National Emissions Inventory), and GFED4 (Global Fire Emission Database), are supported in GC-Adjoint-HEMCO. We find good agreements in carbon monoxide (CO) emissions from various inventories, chemical sources and sinks, and surface and column CO concentrations between GC-Adjoint-HEMCO and GEOS-Chem (v12-8-1) forward simulations. Furthermore, observing system simulation experiments (OSSE) are employed to evaluate the performance of GC-Adjoint-HEMCO in 4D variational (4D-var) assimilations. We find underestimations by approximately 15 % in the a posteriori anthropogenic CO emissions over North America and Europe due to limited coverage of observations by smoothing the pseudo-CO observations with Measurement of Pollution in the Troposphere (MOPITT) averaging kernels. As an example application of GC-Adjoint-HEMCO, we constrain anthropogenic CO emissions in 2015 by assimilating MOPITT CO observations. The a posteriori anthropogenic CO emission estimates derived in this work match well with Jiang et al. (2017) in North America and Africa but are overestimated in Asia, South America and Australia, which could be associated with the different treatment of MOPITT CO observations over ocean grids and the large differences in CO chemical sources and sinks. The updated model developed in this work is a useful extension of the adjoint of GEOS-Chem model.