Global and seasonal variations of O3 and NO2 photodissociation rate coefficients

Abstract

The global and seasonal variations in the photodissociation rate coefficients (J values) of the reactions of ozone forming O(1D) (J(O1D)) and nitrogen dioxide forming O(3P) (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(O1D) and J(NO2) peaked in magnitude at 3 × 10-5 and 8 × 10-3 s-1, respectively, in the region of 30°S to 30°N with small latitudinal and vertical variations, and decreased precipitously at 30°/60° in the winter/summer hemisphere. J(O1D) 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 60° of the summer hemisphere. Surface J(O1D) changes were regressed as a second-degree polynomial function of changes in temperature (±20°K). 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(O1D) resulted in decreases up to 6% in the ozone concentration below 200 m at noon and late afternoon and increases ubiquitously elsewhere.