CMAQv5.1 ClNO2 chemistry
Heterogeneous production of ClNO2 is implemented. Though its production is small, gas-phase ClNO2 chemistry is also included for completeness. CMAQv5.0.2 includes the uptake of N2O5 on fine-mode aerosols to produce HNO3 as the only reaction product using the Davis et al.  parameterization. The ClNO2 chemistry includes the uptake of N2O5 on fine- and coarse-mode aerosols to produce HNO3 and ClNO2 in the presence of particulate chloride. When particulate chloride is not present, it produces only HNO3. Similar to CMAQv5.0.2, it also uses the Davis et al.  parameterization for fine-mode aerosols. However, it uses the Bertram et al.  parameterization for coarse-mode aerosols. The yield of ClNO2 depends on particulate chloride concentration and particle liquid water content and has been parameterized following Bertram and Thornton  and Roberts et al. .
CMAQv5.0.2 includes several options for calculating the heterogeneous uptake of N2O5 on fine-mode aerosols. Another option has been added in CMAQv5.1 for calculating the heterogeneous uptake of N2O5 on fine-mode aerosols using the Bertram et al.  parameterization. It continues to use the Davis et al.  parameterization as the default option.
Significance and Impact
Ten-day unit tests were performed. The heterogeneous chemistry produces ClNO2 both in winter and summer. However, the production is more pronounced in winter than in summer. In winter, it reduces total nitrate by up to 0.23 ug/m3, increases ozone by up to 0.9 ppbv, and sulfate by up to 0.06 ug/m3 (10-day average). Its impact in summer is smaller.
More detailed results can be found in Sarwar et al. (2012) and Sarwar et al. (2014)
No significant impact on model run time is expected.
Note that the asterisk symbol denotes a wildcard string and that the saprc07t based mechanisms already had CLNO2 as a species so this update does not alter their GC namelists.
Bertram, T. H., and J. A. Thornton (2009) Toward a general parameterization of N2O5 reactivity on aqueous particles: The competing effects of particle liquid water, nitrate and chloride, Atmospheric Chemistry & Physics, 9, 8351–8363.
Davis, J. M., P. V. Bhave, and K. M. Foley (2008) Parameterization of N2O5 reaction probabilities on the surface of particles containing ammonium, sulfate, and nitrate, Atmospheric Chemistry & Physics, 8, 5295–5311.
Roberts, J. M., H. D. Osthoff, S. S. Brown, A. R. Ravishankara, D. Coffman, P. Quinn, and T. Bates (2009), Laboratory studies of products of N2O5 uptake on Cl- containing substrates, Geophysical Research Letter, 36, L20808.
Sarwar, G., H. Simon, P. Bhave, G. Yarwood (2012) Examining the impact of heterogeneous nitryl chloride production on air quality across the United States, Atmospheric Chemistry & Physics, 12, 1-19.
Sarwar, G., H. Simon, J. Xing, R. Mathur (2014) Importance of tropospheric ClNO2 chemistry across the Northern Hemisphere, Geophysical Research Letters, 41, 4050-4058.
Golam Sarwar, National Exposure Research Laboratory, U.S. EPA