Global and seasonal variations of O3 and NO2 photodissociation rate coefficients -: art. no. 4216

[1] The global and seasonal variations in the photodissociation rate coefficients (J values) of the reactions of ozone forming O(D-1) (J((OD)-D-1)) and nitrogen dioxide forming O(P-3) (J(NO2)) were investigated systematically with a focus on the troposphere. The corresponding mechanisms were studied through sensitivity modeling experiments of temperature, surface albedo, aerosols, ozone column, and clouds. The one-dimensional radiative transfer model PHODIS was applied with Total Ozone Mapping Spectrometer (TOMS) aerosol optical depth and surface reflectivity, observation-based three-dimensional ozone distributions, and other numerically simulated climate variables. Our results showed that under clear-sky conditions, the zonal averaged J((OD)-D-1) and J(NO2) peaked in magnitude at 3 x 10(-5) and 8 x 10(-3) s(-1), respectively, in the region of 30degreesS to 30degreesN with small latitudinal and vertical variations, and decreased precipitously at 30degrees/60degrees in the winter/summer hemisphere. J((OD)-D-1) exhibited a large meridional and vertical gradient as a result of high sensitivity to temperature and ozone. The predominant effect of surface albedo led to significant longitudinal variation in J(NO2) and a poleward increase beyond 60degrees of the summer hemisphere. Surface J((OD)-D-1) changes were regressed as a second-degree polynomial function of changes in temperature (+/-20degreesK). The impact of low and middle clouds on J values was stronger than high clouds by a factor of 2 to 3 because of thick optical depth. In addition, the cloud-induced changes in J values calculated by PHODIS differed by up to 20% from those parameterized in the Regional Acid Deposition Model (RADM) and Intermediate Model for the Global and Annual Evolution of Species ( IMAGES). Regional photochemical model simulations showed that an arbitrary increase/decrease of 20% above/below the cloud base (750 m) in J((OD)-D-1) resulted in decreases up to 6% in the ozone concentration below 200 m at noon and late afternoon and increases ubiquitously elsewhere.