Photochemical model comparisons with lims observations in a stratospheric trajectory coordinate system

Abstract

AbstractA stratospheric photochemical model with 32 species and 77 reactions has been integrated along stratospheric air parcel trajectories for both disturbed and quiescent periods in early 1979. the model has fully time‐dependent photodissociation coefficients, allows for scattering and albedo effects and the solar zenith angle is accurately computed. the trajectories were calculated from satellite data using a radiative scheme to compute the vertical displacements of the air parcels relative to isentropic surfaces θ0. The model results are compared with LIMS observations of NO2, HNO3, H2O and O3 at points of near coincidence along trajectories at five levels θ0 = 500 K (∼20km); 650 K (∼25km); 850 K (∼30km); 1100 K (∼35km); and 1400 K (∼40km). JPL‐85 data for rate coefficients were used as standard.At the upper two levels the amounts of chlorine and methane in the model were found to have an important effect on the results. Using SAMS CH4 and typical but not locally observed mixing ratios of chlorine, agreement between the model and LIMS O3 data could not be obtained using JPL‐85 rate coefficients. This ozone discrepancy could be essentially removed by using the CODATA‐82 rate coefficient data. Large variations in NO2 during the course of a day were successfully modelled at θ0 = 650K, 850K and 1100K. Disagreement between observations and calculations was found for HNO3 at θ0 = 850K and 1100K in the standard case, but agreement could be obtained at θ0 = 850K using CODATA‐82 rates. Sensitivity tests were performed in which molecular oxygen absorption cross‐sections and certain key rate coefficients were varied, among them those for the O + HO2, OH + NO2 + M, NO + O3 and O + O3 reactions. Variations in O3, NO2 and HNO3 during substantial meridional excursions were reproduced by the photochemical model for trajectories where these molecules were expected to be chemically responsive on a time scale of less than 10 days. In other cases, they acted as tracers.