Application and evaluation of a new in-line photolysis module for the Community Multiscale Air Quality (CMAQ) modeling system

Photolysis of ozone and other atmospheric constituents drives many of the chemical interactions that lead to the production of photochemical smog and secondary aerosol formation. Typically, air quality models, such as CMAQ, pre-calculate the photolytic rates of the constituents using meteorological data and solar radiation, based on assumed “standard” profiles of aerosols and ozone. Photolysis rates in CMAQ are taken from a table and are interpolated to local conditions of solar zenith angle. This approach cannot account for the feedbacks resulting from model predicted concentrations of particles and gases on the solar radiation field. A new module has been developed that calculates photolysis rates interactively within CMAQ. Although more time consuming than using tables, a significant advantage of this approach is that the effects (scattering and absorption) of the aerosol particles produced during model runs are then taken into account.

Photolysis rates within CMAQ are calculated using a modification of the TUV algorithm (Madronich 1987) that calculates contributions from several broad wavelength bins. Comparisons are made using different bins. Results from a case study comparing the new in-line approach with that using the pre-calculated approach will be presented. Predicted aerosol optical depths, using the two approaches, will also be compared with observed values where available