Properties of coarse particles in the atmosphere of the United Kingdom

The recent rapid expansion of PM2.5 monitoring alongside PM10 has allowed estimation of concentrations of coarse particles (PM2.5-10) by difference. This has been carried out for sites using comparable measurement techniques for both metrics, and the properties of the hourly average concentration data have been evaluated. In the UK atmosphere, coarse particles generally represent well under half of PK10 mass. The ratio of PMcoarse to PM10 averages 0.29 +/- 0.14 (n = 43) for FDMS measurements and 0.38 +/- 0.30 (n = 12) for TEOM measurements and does not show a very consistent pattern according to site type. The values are mostly higher for TEOM measurements than for FDMS which is explicable in terms of semivolatile losses predominantly from fine particles in the TEOM. Sites where PMcoarse/PM10 exceeds 0.5 generally have a marine or industrial influence. Correlations between hourly PM2.5 and PM10 are generally very high (mean R = 0.80 for FDMS data and mean R = 0.85 for TEOM data) whilst those between PMcoarse and PM2.5 are very much lower (mean R = 0.09 for FDMS measurements and mean R = 0.41 for TEOM measurement sites). When inter-site correlations are examined between sites of similar characteristics within 30 km of one another, the inter-site correlation of PM2.5 (R = 0.56-0.87) is very much higher than that of PMcoarse (R = 0.25-0.60). Coefficients of Divergence indicate lower spatial homogeneity for PMcoarse than for PM2.5. When hourly data are used to construct pollution roses, it is typical for PM2.5 and PM10 to show a similar pattern dominated either by long-range transport or local traffic sources while PMcoarse can show quite a different pattern indicative of a different range of contributory sources. Plots of particle concentration versus wind speed show a monotonic decline of concentration with increasing wind speed for PM2.5, but an initial decline and an increase at higher wind speeds for PMcoarse. There are relatively few datasets allowing an examination of temporal trend but those available for 4.5-10 years generally show no significant trend with time for fine or coarse particles, or for PM10. Weekday/weekend and diurnal concentration patterns for coarse and fine particles show a strong anthropogenic influence on PMcoarse and a larger weekday/weekend difference for PMcoarse than PM2.5. It is concluded that coarse and fine particles show very different behaviour as a result of their different sources and properties. The lower spatial homogeneity of coarse particles may be a contributory factor in why they appear to be less toxic than fine particles in epidemiological studies. (C) 2011 Elsevier Ltd. All rights reserved.