Abstract
The Copernicus Atmosphere Monitoring Service (CAMS) provides routine analyses and forecasts of trace gases and aerosols on a global scale. The core is ECMWF’s Integrated Forecast System (IFS), where modules for atmospheric chemistry and aerosols have been introduced, and which allows data-assimilation of satellite retrievals of composition. We have updated both the homogeneous and heterogeneous NOx chemistry applied in the three independent tropospheric-stratospheric chemistry modules maintained within CAMS, referred to as IFS(CB05BASCOE), IFS(MOCAGE) and IFS(MOZART). Here we focus on the evaluation of main trace gas products from these modules that are of interest as markers of air quality, namely lower tropospheric O3, NO2 and CO, with a regional focus over the contiguous United States without data assimilation. Evaluation against lower tropospheric composition reveals overall good performance, with chemically induced biases within 10 ppb across species across regions within the US with respect to a range of observations. The versions show overall equal or better performance than the CAMS Reanalysis. Evaluation of surface air quality aspects shows that annual cycles are captured well, albeit with variable seasonal biases. During wintertime conditions there is a large model spread between chemistry schemes in lower-tropospheric O3 (~10–35 %) and, in turn, oxidative capacity related to NOx lifetime differences. Analysis of differences in the HNO3 and PAN formation, which act as reservoirs for reactive nitrogen, revealed a general underestimate in PAN formation over polluted regions likely due to too low organic precursors. Particularly during wintertime, the fraction of NO2 sequestered into PAN has a variability of 100 % across chemistry modules indicating the need for further constraints. Notably a considerable uncertainty in HNO3 formation associated with wintertime N2O5 conversion on wet particle surfaces remains. In summary this study has indicated that the chemically induced differences in the quality of CAMS forecast products over the United States depends on season, trace gas, altitude and region. Whilst analysis of the three chemistry modules in CAMS provide a strong handle on uncertainties associated with chemistry modeling, the further improvement of operational products additionally requires coordinated development involving emissions handling, chemistry and aerosol modeling, complemented with data-assimilation efforts.
Highlights
Poor air quality has a significant impact on visibility, human health and lifespan, crop production and ecosystems, while this impact is expected to be accentuated due to climatic change (Silva et al, 2017; Reddington et al, 2019; 45 Schneidemesser et al, 2020)
In this study we present an evaluation of key trace gas products simulated by the 120 chemistry modules implemented in the Copernicus Atmosphere Monitoring Service (CAMS) system for the contiguous United States
This indicates the absence of significant local NOx emissions, being influenced by land-sea air movements allowing the sampling of clean pacific air (Oltmans et al, 2008)
Summary
Poor air quality has a significant impact on visibility, human health and lifespan, crop production and ecosystems, while this impact is expected to be accentuated due to climatic change (Silva et al, 2017; Reddington et al, 2019; 45 Schneidemesser et al, 2020). For IFS(CBA) there have been extensive modifications to four main components of the tropospheric chemistry module, namely: (i) the inclusion of HONO and CH3O2NO2 into the NOx reaction cycle, (ii) the replacement of the isoprene (C5H8) oxidation scheme with a hybrid from the literature, (iii) the coupling of the formation of 170 Secondary Organic Aerosol (SOA) to oxidation products of aromatics and (iv) the inclusion of hydrogen cyanide (HCN) and acetonitrile (CH3CN) from Biomass Burning (BB) sources. This tropospheric chemistry version is referenced as ‘tc06f’ and is further described below. Validation of these new tracers is not relevant to this study not 220 presented here
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