Processes affecting concentrations of fine particulate matter (PM2.5) in the UK atmosphere

PM2.5 is now subject to a limit value and exposure-reduction targets across the European Union. This has led to a rapid expansion in PM2.5 monitoring across Europe and this paper reviews data collected in the United Kingdom in 2009. The expected gradient between rural, urban background and roadside sites is observed, although the roadside increment is generally rather small except for heavily trafficked street canyon locations. PM2.5:PM10 ratios decline from around 0.8 in southeast England to below 0.6 in Scotland consistent with a higher contribution of secondary particulate matter in southeast England. Average diurnal profiles of PM2.5 differ around the UK but have a common feature in a nocturnal minimum and a peak during the morning rush hour. Central and southern UK sites also show an evening peak following a concentration reduction during the mid afternoon which is not seen at northern UK sites and is attributed to evaporation of semi-volatile components, particularly ammonium nitrate. Concentrations of PM2.5 are typically highest in the winter months and lowest in the mid-summer consistent with better mixing and volatilisation of semi-volatile components in the warmer months of the year. Directional analysis shows a stronger association of PM2.5 with easterly winds associated with air masses from the European mainland than with the direction of local traffic sources.