Modality in ambient particle size distributions and its potential as a basis for developing air quality regulation

Current ambient air quality standards are mass-based and restricted to PM2.5 and PM10 fractions. The major contribution to both PM2.5 and PM10 fractions is from particles belonging to the coarse mode and generated by mechanical processes. These standards are thus unable to effectively control particle concentrations from combustion sources, such as motor vehicles and power plants, which tend to emit very small particles that are almost entirely respirable and in the submicron range, and dominate the nucleation and accumulation modes, which contribute much less to particle mass concentration. The aim of this work was to examine whether PM1 and PM10 would be a more effective combination of mass standards than PM2.5 (dominant in the nucleation and accumulation modes) and PM10 (dominant in the coarse mode) in controlling combustion-related ambient particles, as well as those originating from mechanical processes. First, a large body of data oil particle size distributions in a range of environments in South East Queensland, Australia, was analysed, with an aim of identifying the relation between modality in the distributions and sources of particles belonging to different modes. The analyses included a matrix of the following elements: particle volume and number distributions, type of environment and locations of the modes in the range of PM1, PM2.5 and PM10 fractions. Second, with the same aim, 600 published modal location values relating to number, surface area, volume and mass size distributions for a range of environments worldwide were analysed. The analysis identified a clear and distinct separation between the location of the modes for a substantial number of environments worldwide and particle metrics, which suggests that modality in particle size distributions may be a parameter that has potential to be used in the development of PM, air quality guidelines and standards. Based oil these analyses, implications for choosing different mass standards for airborne particulate matter are discussed in the paper. (C) 2007 Elsevier Ltd. All rights reserved.