The long-range transport of air pollutants—some thoughts on the state of modelling

Abstract

Various aspects of long-range transport modelling are illustrated using a very simple model. It is shown that annual average concentrations and depositions from distant sources are dependent on a few main factors, such as emission strength and removal rates, and other factors, such as source height, are of lesser importance. Most dilution arises from the variation in direction of air mass trajectories, and this is illustrated by maps of deposition over Europe in which the deposition rate is shown to be only a small fraction of the emission rate, except in the grid square containing the source. For sulphur oxides simple parameterizations of the transformation process are discussed, based on typical atmospheric conditions, and it is argued that these can be used, provided the resulting long-term average predictions have been tested against measurements. Ideas as to how similar approaches may be applied to the other species, such as nitrogen oxides ( NO x ) and ammonia, affecting acid rain, are suggested and some preliminary results presented. Attention is drawn to the large areas of uncertainty still remaining, particularly in the area of wet deposition, and it is concluded that precise predictions of the effect of large changes in emission cannot be made at present.