CMAQv5.1 Dry Deposition Updates
Revisions were made to the modules in CMAQ relating to dry deposition. The changes include addition of new depositing species, modifications to the deposition velocity of existing species, and restructuring of the code.
Treatment of organic N species
Consistent with the changes in the chemical mechanisms (Chemical mechanism updates), additional organic N species have been added to the deposition modules. Prior to CMAQv5.1, organic N was modeled as a single species - e.g. NTR (CB05) or RNO3 (SAPRC07). In CMAQv5.1, organic N species are treated more explicitly but are still lumped to reduce model runtime (Pye et al.,2015). Since the organic N species represent a range of species, a representative compound was selected for the calculation of the diffusivity and LeBas molar volume and the specification of the Henry's Law constant for each compound. The full list of available surrogate species can be found in DEPVVARS.F and the corresponding properties for each species can now be found in ASX_DATA_MOD.F. The organic N species are shown in the table below.
Organic N Deposition Velocity Surrogate Species
|New Species||Description||Representative Species||Diffusivity(cm2/s)||LeBas Molar Volume (cm3/mol)||Henry's Law Surrogate|
|NTRPX||Multifunctional hydroperoxide nitrate||2-nitrooxybutylperoxide||0.0646||147.8||HYDROXY_NITRATES|
|NTRM||Isoprene nitrate||Z-1,4-Isoprene nitrate||0.0609||156.1||HYDROXY_NITRATES|
|PPN||Peroxypropionyl nitrate||Peroxypropionyl nitrate||0.0631||118.2||PPN|
|MPAN||Peroxyaceylnitrate from methacrolein||Peroxymethacryloyl nitrate||0.0580||133.0||MPAN|
|MTNO3||Monoterpene nitrate||C10-nitrate-diol (MW=231 g/mol)||0.0453||251.2||MTNO3|
- Enhanced deposition of O3 over oceans was added to account for the influence of iodide
- In conjunction with changes made to the chemistry module to account for halogen species (CMAQv5.1 Halogen chemistry), the deposition velocity of O3 over oceans was updated to include the additional sink due to interaction with iodide in the seawater. Iodide concentrations are estimated based on sea-surface temperature (Sarwar et al., 2015). The effect of this change was to increase deposition and decrease air concentrations of ozone.
- Ozone deposition to vegetation surfaces was revised.
- Wet cuticular resistance was updated to follow Altimir et al. (2006) and dry cuticular resistance was parameterized following Wesely (1989). Cuticular resistance was scaled between dry and wet values to account for physisorbed H2O on the cuticular surfaces (Altimir et al., 2006).
Restructuring of Dry Deposition Modules
Dry deposition and vertical diffusion code was restructured to include a common data module (ASX_DATA_MOD.F). This was used to remove redundant calculations, computations, and reading of input data as well as creating a central location in the code where updates to input data and common environment variables can be made. This update reduces the complexity and redundancy of the code and reduces the effort in maintaining and updating the processes contained in this code.
Significance and Impact
The updates to the organic nitrogen chemistry and dry deposition code reduce a systematic underprediction of organic nitrogen deposition which can make substantial contributions to total nitrogen deposition in many ecosystems.
The updates to ozone deposition to oceans and vegetation include processes that were previously missing in early versions of CMAQ and reduce the model over estimates of background ozone.
Altimir, N., Kolari, P., Tuovinen, J.-P., Vesala, T., Bäck, J., Suni, T., Kulmala, M., and Hari, P., 2006. Foliage surface ozone deposition: a role for surface moisture?, Biogeosciences, 3, 209-228, doi:10.5194/bg-3-209-2006.
Pye, H. O. T., D. J. Luecken, L. Xu, C. M. Boyd, N. L. Ng, K. Baker, B. A. Ayres, J. O. Bash, K. Baumann, W. P. L. Carter, E. Edgerton, J. L. Fry, W. T. Hutzell, D. Schwede, P. B. Shepson, 2015. Modeling the current and future role of particulate organic nitrates in the southeastern United States, Environmental Science & Technology, DOI: 10.1021/acs.est.5b03738.
Sarwar, G., Gantt, B., Schwede, D., Foley, K., Mathur, R., Saiz-Lopez, A., 2015. Impact of Enhanced Ozone Deposition and Halogen Chemistry on Tropospheric Ozone over the Northern Hemisphere. Environmental Science & Technology 49, 9203-9211.
Wesely, M. L., 1989. Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models, Atmospheric Environment, 23, 1293–1304.
Jesse Bash, National Exposure Research Laboratory, U.S. EPA, Donna Schwede, National Exposure Research Laboratory, U.S. EPA