Implications of variations in photodissociation rates for global tropospheric chemistry

Variations in global photodissociation rates in the troposphere are calculated and the consequences of these variations for tropospheric chemistry are examined. Most of the variations are due to the effects of clouds, surface reflection and stratospheric ozone. Parameterizations are developed and they permit the photodissociation rates to be calculated rapidly as a function of these three factors. The errors introduced by using the parameterization are considered to be acceptable in view of the uncertainty in the data available. The daily-integrated photodissociation rates (DIPRs) of ozone and nitrogen dioxide are presented in detail for the month of January. The DIPR of nitrogen dioxide peaks in the upper Antarctic troposphere due to the combined effects of long daylength and high surface reflection. The DIPR of ozone, on the other hand, is found to attain its global maximum at about 300 hPa in the tropical troposphere. The sensitivity of the DIPRs to the global input data is studied. Perturbed DIPRs are fed into a global chemistry transport model to assess the effects on tropospheric chemistry. The globally averaged lifetime of methane in an atmosphere with clouds is calculated to be 8.8 yr. The removal of all clouds reduces its lifetime with only 2.4%. This fairly small reduction in lifetime, however, is due to the fact that the effects above and below clouds cancel each other out. The DIPR of ozone and therefore the OH production is sensitive to the amount of stratospheric ozone. Whereas a reduction in stratospheric ozone affects predominantly the ozone photodissociation rate, clouds and surface reflection affect all photodissociation rates concurrently.