Chapter 2: Influences of Meteorology on Air Pollution Concentrations and Processes in Urban Areas

In urban areas pollutant concentrations are determined by the balance between pollutant emissions, and the state of the atmosphere which determines pollutant transport and dispersion processes. In this chapter we demonstrate that although the nature and characteristics of urban emissions can be variable in time and space, it is changes in pollutant dispersion pathways both locally and regionally that often determine the temporal and spatial patterns of atmospheric composition in urban areas.

Due to the complexity of the urban atmosphere it is useful to look at the relations between urban meteorology and air pollution at a variety of different scales. At scales ranging from individual buildings and street canyons to the entire city, microscale mechanical and thermally driven turbulence dominates local dispersion processes. However, these processes operate within an hierarchy of larger scales which provide the background state of the atmosphere that modulates air quality within urban areas. Day-to-day variations in urban air quality are determined in large part by processes operating at scales greater than the urban environment. These meteorological processes extend from the regional scale flows such as sea breezes, to the synoptic scale (cyclonic and anticyclone systems that give our day-to-day weather), and finally to the hemispheric and global scales which permit long-range transport of pollutants.

Generally, as the ability of the urban atmosphere to disperse pollutants horizontally and vertically increases, pollutant concentrations decrease. However, complex feedback loops exist between pollutant emission rates, urban form and meteorological processes, local and regional pollutant transport rates and atmospheric chemistry such that individual parameters are rarely independent of each other. Thus an understanding of local, urban and regional scale atmospheric processes is fundamental to a comprehensive evaluation of air pollution in urban areas.