This paper examines the change in air pollutant concentrations between 2005 and 2010 occurring in the City of Hamilton, Ontario, Canada. After analysis of stationary air pollutant concentration data, we analyze mobile air pollutant concentration data. Air pollutants included in the analysis are CO, PM[subscript 2.5], SO[subscript 2], NO, NO[subscript 2], and NO[subscript X]. Stationary monitoring indicates a continuous reduction in air pollutant concentrations. Stationary monitors only cover a small spatial extent of Hamilton. Mobile monitoring of air pollutant concentrations, averaged over census tract boundaries, indicates both improvement and decline in air quality. These improvements and declines in air quality are spatially clustered throughout Hamilton. Mobile data indicated significant decline in median pollutant concentration for CO, SO[subscript 2], PM[subscript 2.5], and NO[subscript 2]; but significant increase for NO and NO[subscript X]. Air quality change in Hamilton is spatially heterogeneous, and is not captured based on the current stationary monitoring network. Coupling of mobile and stationary air pollutant concentration monitoring provides a more accurate spatial assessment of local air quality.