The application of the Modified Band Approach for the calculation of on-line photodissociation rate constants in TM5: Implications for oxidative capacity

Abstract

A flexible and explicit on-line parameterization for the calculation oftropospheric photodissociation rate constants (J-values) has been integratedinto the global Chemistry Transport Model TM5. Here we provide acomprehensive description of this Modified Band Approach (MBA) includingdetails of the optimization procedure employed, the methodology applied forcalculating actinic fluxes, the photochemical reaction data used for eachchemical species, the aerosol climatology which is adopted and theparameterizations adopted for improving the description of scattering andabsorption by clouds. The resulting J-values change markedly throughout thetroposphere when compared to the offline approach used to date, withsignificant increases in the boundary layer and upper troposphere.Conversely, for the middle troposphere a reduction in the actinic fluxresults in a decrease in J-values. Integrating effects shows that applicationof the MBA introduces seasonal dependent differences in important trace gasoxidants. Tropospheric ozone (O3) changes by ±10% in theseasonal mean mixing ratios throughout the troposphere, especially overland. These changes and the perturbations in the photolysis rate of O3induce changes of ±15% in tropospheric OH. In part this is due to anincrease in the re-cycling efficiency of nitrogen oxides. The overallincrease in northern hemispheric tropospheric ozone strengthens theoxidizing capacity of the troposphere significantly and reduces the lifetimeof CO and CH4 by ∼5 % and ∼4%, respectively. Changesin the tropospheric CO burden, however, are limited to a few percent due tocompeting effects. Comparing the distribution of tropospheric ozone in theboundary layer and middle troposphere against observations in Europe showsthere are improvements in the model performance during boreal winter in theNorthern Hemisphere near regions affected by high nitrogen oxide emissions.Monthly mean total columns of nitrogen dioxide and formaldehyde also comparemore favorably against OMI and SCIAMACHY total column observations. © Author(s) 2012. CC Attribution 3.0 License.