Development of a two-dimensional global tropospheric model: Model chemistry

Abstract

A latitudinally averaged two-dimensional model has been used to study the distributions, budgets, and trends of trace gases in the atmosphere from pole to pole and from the surface to 24 km. The chemical mechanism used contains 56 chemical species, including 12 hydrocarbons and 125 chemical and photochemical reactions, as well as wet removal processes and dry deposition. Apart from the stratospheric sources of ozone and nitrogen oxides the model chemistry is driven completely by the time-dependent photolytic processes and the emission of 17 chemical species distributed according to 10 different source categories. Each source category is parameterized as a function of latitude and time of the year. The model results are generally in good agreement with observations, and the model reproduces the observed temporal and spatial variation in the mixing ratios of methane, carbon monoxide, hydrocarbons, and ozone. The global average concentration of the hydroxyl radical in the troposphere is 8.3×10⁵ molecules cm⁻³, in agreement with recent calculations based on budgets and trends for CH₃CCl₃. Model budgets for NO_x, CO, CH₄, H₂ and O₃ are presented. These show that although the stratospheric source and dry deposition terms for ozone almost balance, the global annual turnover of ozone below 24 km, excluding the O₃/NO/NO₂ null cycle, is four times greater than the stratospheric source strength. These budgets also suggest that the observed increase in the mixing ratio of methane may be due not only to an increasing source strength but also to a downward perturbation in the abundance of the hydroxyl radical.