The weekday–weekend difference and the estimation of the non-vehicle contributions to the urban increment of airborne particulate matter

Analysis of three separate urban data sets has identified significant weekend reductions of PM10, PM2.5 and PM2.5–10 mass concentrations compared to weekday values. Significant weekday–weekend differences were also found for elemental carbon in the fine size fraction, and calcium in the coarse size fraction. There was no significant weekday–weekend difference in the case of sea salt or compounds of nitrate or sulphate. Given the paucity of day of the week-specific activity data for sources other than road traffic, it has been assumed that non-traffic sources of particles are uniform throughout the week, allowing estimation of the traffic contribution to PM concentrations. While this approach may neglect a weekday–weekend difference in particulate matter from construction sources, the effect of such sources is believed to be small. Two methods (by calculation and regression) of deriving the non-vehicle contribution to the urban increment above the regional background were compared. Both resulted in values of 2.1–2.7 μg m−3 (with an estimated error of around 2 μg m−3) for the non-vehicle urban increment in PM10 measured by TEOM (gravimetric equivalent) at some urban background sites in London. The annual average traffic contribution to the urban increment is estimated as 1.6–4.4 μg m−3 for background sites in London, except Bloomsbury where the estimated traffic contribution is higher and that for non-traffic sources is lower. Use of the same method at the rural Harwell site leads to an estimate of 1.8 μg m−3 for the traffic contribution.