The influence of atmospheric stability on pollutant transport by slope winds

The results of a field investigation into pollutant transport by slope flows are presented. During the study, ozone levels were monitored for eight days at three locations along a mountain slope extending above the Los Angeles Air Basin. In addition, tracer tests were conducted during two afternoons during the study period. The tests were conducted on days with widely varying atmospheric stability and indicate the effect of a strongly stable capping layer on pollutant transport by upslope flows. The experimental results indicated at least two mechanisms for the return of polluted air to the valley below. Under weakly stable conditions, the often noted return of pollutants via night-time downslope winds was observed. In addition, under more strongly stable conditions, a daytime recirculation mechanism consistent with that described by Vergeiner (1982) was observed. The tests also indicated that the slope flow layer depth and speed decreased with increasing atmospheric stability. In addition, the rate of transport and dilution of pollutants in a slope flow appears to be influenced by the entrainment and growth of the boundary layer, limiting the applicability of models that neglect these processes.