One-dimensional vertical model for ozone and other gases in the atmospheric boundary layer

Abstract

A vertical eddy diffusion coefficient profile for mass transport in the atmospheric boundary layer is established on the basis of a calculation where the equations of motion and the thermodynamic energy equation are solved. A one-dimensional vertical model for the chemical turnover is then developed. It is assumed that the turbulent mass transport is equal to the product of the vertical mean concentration gradient and the vertical eddy diffusion coefficient for mass transport. About 40 chemical species are calculated in the model, and about 90 chemical reactions are involved. A log-linear grid is introduced, with close spacing near the ground and coarser towards the free troposphere. The diurnal cycle of ozone in an atmospheric boundary layer with strong convective mixing during the day and a shallow layer under the influence of mechanical stress at night, is computed and compares well with observations. It is found that in polluted air, depletion of ozone close to the ground through reaction with NO prevents ozone from reaching the ground and be removed by deposition. The concentration of hydroxyl has a marked gradient with height, with maximum in the layers where nitrogen oxides, hydrocarbons and sulphur dioxide are emitted. Nitrogen dioxide, sulphur dioxide and the hydrocarbons (ethane, n- butane , ethylene, propylene and m- xylene ) accumulate throughout the night in the shallow nocturnal layer.