CMAQv5.0.2 Aerosol updates
Under cold conditions (approximately 210 K) the NO3 aerosol can become slightly negative in the i and j modes. In CMAQ 5.0.1 and earlier, this was initially corrected with a mass balance adjustment but because this was only done under the j mode loop and added the mass from the i mode into the j mode. This can continually add mass to the j mode and in the right conditions can crash the model with an ISOROPIA-II error or a photolysis error. aero_subs.F was corrected to apply the mass balance correction to both the i and j modes.
Significance and Impact
This correction resulted in a reduction of NO3 and HNO3 in the layers near the tropopause of the model simulation and, due to vertical transport from these layers to the surface, this results in approximately a 3% reduction in NO3 and HNO3 at the surface level and a similar reduction in wet deposition. These changes are larger over higher terrain where vertical mixing is more vigorous. Overall, this may increase the bias by approximately 3%-5% in ambient NO3 and HNO3 and NO3 wet deposition observations but also generally decreases the model error in these model variables.
Jesse Bash, Atmospheric Modeling and Analysis Division, U.S. EPA