Quantifying uncertainties due to chemistry modelling – evaluation of tropospheric composition simulations in the CAMS model (cycle 43R1)

Abstract. We report on an evaluation of tropospheric ozoneand its precursor gases in three atmospheric chemistryversions as implemented in ECMWF’s Integrated Forecasting System(IFS), referred to as IFS(CB05BASCOE), IFS( MOZART) and IFS(MOCAGE). While the model versions were forced with the same overall meteorology, emissions, transport and deposition schemes, they vary largely in their parameterizations describing atmospheric chemistry, including the organics degradation, heterogeneous chemistryand photolysis, as well as chemical solver. The model results from the three chemistryversions are compared against a range of aircraft field campaigns, ozone sondes and satellite observations, which provides quantification of the overall model uncertainty driven by the chemistryparameterizations. We find that they produce similar patterns and magnitudes for carbon monoxide (CO) and ozone (O 3 ), as well as a range of non-methane hydrocarbons (NMHCs), with averaged differences for O 3 (CO) within 10 % (20 %) throughout the troposphere. Most of the divergence in the magnitude of NMHCs can be explained by differences in OH concentrations, which can reach up to 50 % particularly at high latitudes. Also comparatively large discrepancies between model versions exist for NO 2 , SO 2 and HNO 3 , which are strongly influenced by secondary chemical production and loss. Other, common biases in CO and NMHCs are mainly attributed to uncertainties in their emissions. This configuration of having various chemistryversions within IFS provides a quantification of uncertainties induced by chemistrymodeling in the main CAMS global trace gasproducts beyond those that are constrained by data-assimilation.