Comparison of Seasonality of Urban Grime in Three Southern Ontario Cities

Urban grime refers to films made of a complex chemical mixture of organics and inorganics deposited on outdoor surfaces. The specific chemical composition of such films can vary considerably due to geographic and seasonal influences. We compared the seasonal change of grime’s water-soluble inorganic ion content in a rural (Thamesford), suburban (Oshawa), and urban city (Toronto) in southern Ontario, Canada. Sodium, calcium, chloride, nitrate and sulphate amounts in grime was collected and quantified over four seasons in Oshawa, ON and Thamesford, ON and compared with results from a previous study in Toronto to elucidate differences. We found that all three regions displayed a clear seasonality in sodium and chloride consistent with winter road salt inputs and chloride loss in summer. Mole fractions of sulphate were consistently the lowest of the measured ions and NO3-/SO42- ratios showed a winter-spring minimum and summer-fall maximum. Urban – rural differences in the seasonal changes of ionic mole ratios suggests that additional chloride displacement by nitrate occurs within grime in Toronto that is not present in the two other rural/suburban cities in the same region.