Air Pollution Flow Patterns in the Mexico City Region

According to the Mexico City Emissions Inventory, mobile sources are responsible for approximately 86% of nitrogen oxide emissions in this region, and correspond to a NOx emission of 51 and 58 kilotons per year in Mexico City and the State of Mexico, respectively. Ozone levels in this region are often high and persist as one of the main problems of air pollution. Identifying the main scenarios for the transport and dispersion of air pollutants requires the knowledge of their flow patterns. This work examines the surface flow patterns of air pollutants (NO2, O-3, SO2, and PM10) in the area of Mexico City (a region with strong orographic influences) over the period 2001-2010. The flow condition of a pollutant depends on the spatial distribution of its concentration and the mode of wind circulation in the region. We achieved the identification and characterization of the pollutant flow patterns through the exploitation of the 1-hour average values of the pollutant concentrations and wind data provided by the atmospheric monitoring network of Mexico City and the application of the k-means method of cluster analysis. The data objects for the cluster analysis were obtained by modeling Mexico City as a 4-cell spatial domain and describing, for each pollutant, the flow state in a cell by the spatial averages of the horizontal pollutant flow vector and its gradients (the divergence and curl of the flow vector). We identified seven patterns for wind circulation and nine patterns for each of NO2, O-3, PM10, and SO2 pollutant flows. Their seasonal and annual average intensities and probabilities of occurrence were estimated.