PM10 has natural and anthropogenic sources, it is an urban air pollutant from desertic areas or emitted by industry and traffic activities, it reduces visibility and threatens human wellbeing mainly in big cities. Casablanca concentrates many industrial units and a large vehicle fleet. The rate of urbanization in the metropolis and the population density are the highest in Morocco. Marrakech is one of the most populated cities in the country where the motorization rate has increased during recent years. The present work is based on PM10 daily measurements between 2013 and 2016. The main objective is to assess the concentrations of PM10 in Casablanca and Marrakech and study their relationship with the atmospheric circulation. First, we assessed PM10 correlations with climate indexes (NAO and MO), then we characterized the contribution of large-scale atmospheric patterns related to PM10 extreme events. The novelty of this research is the creation of a new climate index to characterize the oscillation, in the country’s southern desert, between the Saharan depression and the Azores high. The time series of the new Saharan Oscillation Index (SaOI) were calculated. This study has demonstrated the relationship between MO and PM10 averages and has shown that particulate pollution in the study area is partly induced by continental northeasterly to southwesterly flow. This flow is triggered by the Saharan trough and managed by the high-pressure area in the north. The assessed correlations related to the SaOI confirm the relationship between this index, PM10 averages, and MO and NAO indexes mainly in winter. The Saharan Oscillation is the new relevant key to understand worldwide pollution by fine particles.