The long range transport of airborne material and its removal by deposition and washout—I. General considerations

Abstract

The lifetime and decay distance of airborne pollution downwind from a large area source are examined assuming that the transport takes place between the ground, or sea level, and an inversion and that the only removal mechanisms are deposition and washout by rain. Rainout and chemical reactions are not included in this preliminary account and in Part I the airborne concentrations are taken to be uniform with height. The detailed effects of turbulent diffusion upon concentration profiles are considered in Part II. In the absence of rain the concentration decays exponentially, with a “decay” or “travel” distance equal to the inversion height multiplied by the ratio of mean wind speed to deposition velocity. For sulphur dioxide passing over dry land this ratio is typically several hundred, so that inversion heights of about 1 km give rise to lifetimes of about 1 day and decay distances of several hundred kilometres. It is shown that as conditions vary there is a maximum deposition rate at each distance from the source which is independent of deposition velocity and decreases inversely with distance from the source. When conversion to sulphate aerosol takes place, again under the assumption that rainfall is absent, the corresponding life times and decay distances are increased by an order of magnitude. The extension of the analysis based on the assumptions of Part I to variable inversion heights and deposition velocities is indicated. When moderate rainfall (1 mm h −1) is present washout reduces the lifetime of sulphur dioxide by an order of magnitude. On an annual average basis dry deposition remains a comparable removal process, the deposition rates at several hundred kilometres from large industrial areas being at most a small fraction of 1 g S m −2, per annum. For sulphate aerosol washout is the most effective removal process.