Similarities of boundary layer ventilation and particulate matter roses

Pollution wind sector or rose analyses show that under South-Easterly winds, many areas of the UK experience an increase in mean airborne particulate matter concentration of up to 30% over the average for all directions. This is often attributed solely to long-range transport of pollutants from continental Europe. Here, we present a rose analysis that suggests an additional influence of boundary layer ventilation. The directional increase in pollutant concentration is found to coincide with a 45-55% reduction in ventilation, obtained from ECMWF reanalysis. The reduced ventilation will increase the concentration of locally emitted particulate matter. This effect is explained by low average South-Easterly wind speeds, and advection of high-level warm air from continental Europe, which stabilises the boundary layer, and reduces the boundary layer height. A similar dependence of ventilation on wind direction was found across most of the world, so that a similar effect on pollution roses may be present at most locations. The high number of particulate matter exceedance days for Easterly flow was not found to be associated with a high frequency of extremely low ventilation conditions, suggesting that ventilation alone cannot explain these events.