MOZART Chemistry Mechanisms Development

The family of MOZART (Model of Ozone and Related chemical Tracers) chemical mechanisms are used in the Community Earth System Model (CESM), including CAM-chem and WACCM, as well as in WRF-Chem. 

The first version of MOZART had 46 transported chemical species to represent tropospheric ozone production based on nitrogen oxides (NOx), methane (CH4), carbon monoxide (CO) and nonmethane hydrocarbons (NMHCs), including specific C2 and C3 alkanes and alkenes (ethane, ethene, propane, propane) and isoprene (C5H8) as well as lumped larger hydrocarbons (treated as butane) and a lumped monoterpene (Brasseur et al., 1998).  The MOZART-2 chemistry had 63 chemical species with an updated isoprene oxidation scheme over MOZART-1, which included the representation of a few lumped hydroperoxides and isoprene nitrates (Horowitz et al., 2003).  Heterogeneous uptake of N2O5 and NO3 on sulfate aerosols (based on a prescribed distribution) were also added in this version. The oxidation of the lumped hydrocarbons and monoterpenes remained the same as MOZART-1, and was represented with the oxidation products of smaller compounds (propane and isoprene, respectively).  Versions 3 and 4 of MOZART were developed in parallel, with MOZART-3 including a detailed stratospheric chemistry mechanism added to MOZART-2 (Kinnison et al., 2007), while MOZART-4 included a further expansion of the tropospheric chemistry mechanism to 85 gas-phase species, including more specific representation of larger hydrocarbons with a lumped alkane, a lumped alkene and a lumped aromatic species with corresponding oxidation products (Emmons et al., 2010).  The MOZART-3 stratospheric chemistry and MOZART-4 mechanism were combined and coupled to the Community Atmosphere Model (CAM) to create CAM-chem (Lamarque et al., 2012; Tilmes et al., 2015), a component of the Community Earth System Model (CESM). Over the past few years the tropospheric and stratospheric (abbreviated “TS”) mechanism has been further updated to have a total of 151 gas-phase species in the MOZART-TS1 chemical mechanism. This mechanism includes an expansion of the isoprene oxidation scheme, splitting lumped aromatics and terpenes into individual species and improving their oxidation scheme, oxidation of the biogenic compound MBO, and a more detailed representation of organic nitrates (Emmons et al., 2019).

More complex and updated isoprene and terpene chemistry has recently been added into MOZART-TS1 to create the MOZART-TS2 chemical mechanism (Schwantes et al., 2019). Isoprene and terpenes are biogenic hydrocarbons that are emitted from vegetation. During the summer in the southeastern U.S. where there are high biogenic emissions, many regional and global models are biased high for surface ozone compared to observations. The MOZART-TS2 chemical mechanism due to the improved isoprene and terpene chemistry greatly reduces the simulated surface ozone bias in the eastern U.S. compared to U.S. EPA CASTNET monitoring station data as shown in Figure 1. The MOZART-TS2 mechanism is of similar complexity to other recently updated mechanisms for isoprene and significantly more complex than other reduced mechanisms for terpenes. Terpene oxidation in particular has been ignored or heavily reduced in chemical schemes used in many regional and global models. Schwantes et al. (2019) clearly demonstrates that comprehensive isoprene and terpene chemistry is needed to reduce surface ozone model biases. Accurately simulating ozone for the right reasons in models is important for air quality forecasting and source apportionment studies. The MOZART-TS2 chemical mechanism has been implemented into CESM/CAM-chem (Community Earth System Model/Community Atmosphere Model with chemistry) and MOZART-TS2 can be adapted for use in other models as well. 

Surface Ozone Daily Max 8-hr Average
Figure 1. Surface Ozone Daily Max 8-hr Average (MDA8) CESM/CAM-chem results over CONUS using the default MOZART-TS1 mechanism (panel a) and the updated MOZART-TS2 mechanism (panel b) compared to U.S. EPA CASTNET data (filled circles) averaged over August 2013. The same data are presented in a scatter plot for MOZART-TS1 (panel c) and MOZART-TS2 (panel d) with Eastern U.S. (longitude > -96°) in blue and Western U.S. (longitude < -96°) in red from Schwantes et al. (2019). Click for larger image.


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Schwantes, R. H., Emmons, L. K., Orlando, J. J., Barth, M. C., Tyndall, G. S., Hall, S. R., Ullmann, K., St. Clair, J. M., Blake, D. R., Wisthaler, A., and Bui, T. V.: Comprehensive isoprene and terpene chemistry improves simulated surface ozone in the southeastern U.S., Atmos. Chem. Phys. Discuss.,, in review, 2019.

Tilmes, S., Lamarque, J. F., Emmons, L. K., Kinnison, D. E., Ma, P. L., Liu, X., . . . Martin, M. V. (2015). Description and evaluation of tropospheric chemistry and aerosols in the Community Earth System Model (CESM1.2). Geoscientific Model Development, 8(5), 1395-1426. doi:10.5194/gmd-8-1395-2015

Tilmes, S., A. Hodzic, L. K. Emmons, M. J. Mills, A. Gettelman, D. E. Kinnison, M. Park, J.-F. Lamarque, F. Vitt, M. Shrivastava, P. Campuzano Jost, J. Jimenez, X. Liu. Climate forcing and trends of organic aerosols in the Community Earth System Model (CESM2), submitted to J. Advances in Modeling Earth Systems.  Available at: