Synergistic effect of the occurrence of African dust outbreaks on atmospheric pollutant levels in the Madrid metropolitan area

The occurrence of African dust outbreaks over specific areas of the Mediterranean basin has been associated with increases in the PM10 concentration levels and also in the mortality rates. Different hypothesis have been proposed in the last years to explain the processes by which African dust storms generates negative health effect over urban areas in southern Europe but are still not clear. Some authors have suggested the existence of an interaction between air pollutants from local sources and the occurrence of African dust outbreaks, with a consequent increase in the risk of mortality due to exposure to these anthropogenic emissions. This study sought to identify such a synergistic effect in the Madrid metropolitan area. To this end, an assessment of the influence of African dust on air quality levels, the vertical structure of the atmosphere over Madrid and daily mortality was carried out. Our results indicated that African dust caused a reduction of the mixing layer height and the surface wind speed, by reducing the amount of solar radiation reaching the ground. These facts favored the accumulation of air pollutants emissions from local anthropogenic sources. Moreover, when the dust contribution to PM10 levels exceeded a threshold value (8 mu g/m(3)), particulate matter mass (PM10, PM2.5) and number (ultra-fine particles) concentration as well as levels of gaseous air pollutants (CO, NO and NO2) registered at urban-background and urban-traffic monitoring sites, increased with statistical significance. In these conditions, it was found a statistically significant increase in risk of daily mortality in the PM10 exposure. Hence, ambient air in Madrid was more toxic during African dust events of increasing intensity due to this synergistic effect. Because it can be envisaged that the frequency, duration and intensity of dust storms will increase in the north of Africa due to climate change, it will be a priority to put forward and assess proposals to mitigate the adverse effects on health, focused on the reduction of air pollutant emissions from local sources, as well as proposals regarding the adaptation of the population in urban areas across the Mediterranean basin.