Submitted as: model evaluation paper 01 Oct 2021

Submitted as: model evaluation paper | 01 Oct 2021

Review status: this preprint is currently under review for the journal GMD.

Regional evaluation of the performance of the global CAMS chemical modeling system over the United States (IFS cycle 47r1)

Jason Edward Williams1, Vincent Huijnen1, Idir Bouarar2, Mehdi Meziane3, Timo Schreurs4, Sophie Pelletier3, Virginie Marécal3, Beatrice Josse3, and Johannes Flemming4 Jason Edward Williams et al.
  • 1R&D Weather and Climate Modeling, Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
  • 2Max Planck Institute for Meteorology, Hamburg, Germany
  • 3Centre National de Recherches Météorologiques, Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • 4ECMWF, Shinfield Park, Reading, UK

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.

Jason Edward Williams et al.

Status: final response (author comments only)

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Jason Edward Williams et al.

Jason Edward Williams et al.


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Short summary
The global CAMS air quality model is used for providing such information to end users, particularly on tropospheric ozone. This paper updates the chemical mechanism employed (CBA) and compares against two other mechanisms (MOCAGE, MOZART) and a multi-decadel dataset based on a previous version of CBA.