Sources and processes affecting carbonaceous aerosol in central England

Airborne particulate matter (PM) in the PM10, PM2.5 and PM1.0 size fractions has been sampled at three sites within the UK West Midlands, i.e. BROS (roadside), BCCS (urban background) and CPSS (rural). Daily samples were analysed for organic (OC) and elemental carbon (EC) using a technique involving optical pyrolysis correction. The results show a strong roadside to urban to rural gradient in EC, but little if any gradient in OC. The concentrations are consistent with a regional background which contributes most of the OC concentration at all sites, upon which is superimposed a locally generated aerosol with high EC/OC content consistent with that of road traffic emissions. This explains not only the roadside increment above the urban background, but also the urban background above rural concentrations. Estimation of secondary and primary OC concentrations using the EC tracer method reveals a strong seasonality in primary OC paralleling that of EC with higher winter than summer levels, but a much smaller seasonal pattern in secondary OC, with higher average concentrations in winter than summer. A number of explanations for this apparent behaviour are explored, but no firm conclusions are reached. Similarities between the seasonal cycles of secondary OC and nitrate are suggestive of long-range transport and temperature (through semi-volatility) having a major influence on secondary OC concentrations.