Cirrus clouds, observed to occur in the upper troposphere mainly in the tropics, attenuate solar radiation with subsequent effects on photodissociation processes concerning ozone. In this study, the effects of cirrus clouds on photodissociation rate coefficients J(O1D) and J(NO2) were studied by using a radiative transfer model with cloud optical properties derived from observations. The sensitivity of these two J values to cloud optical depth, solar zenith angle and surface albedo was examined.
It is found that above and below cirrus clouds, a typical cirrus cloud optical depth (tau =2) can increase J(O1D) and J(NO2) by nearly 20% compared to clear sky condition under overhead sun. As solar zenith angle exceeds a threshold, the below-cloud J values are reduced. At tau=8, the threshold is 40°. In the contrast of affecting J(NO2) throughout the whole atmosphere, changes in surface reflectivity only affect J(O1D) between the cirrus cloud layer and surface, and have no effects on above-cloud J(O1D). The presence of low level cloud can enhance the increase of J values solely due to one cirrus cloud layer, especially around the cirrus cloud base.
The implications of J value changes are also discussed.
Symposium on Interdisciplinary Issues in Atmospheric Chemistry