Interpretation of particulate elemental and organic carbon concentrations at rural, urban and kerbside sites

Abstract

Concentration measurements are reported for particulate organic and elemental carbon, measured using an R&P 5400 ambient particulate carbon monitor at four sites in the United Kingdom: one roadside (London, Marylebone Road), two urban (London, North Kensington, and Belfast, Centre) and one rural (Harwell). The measurements were collected on a continuous three hourly average basis between January 2002 and mid-2004. The concentrations show no obvious seasonal cycle, except for an increase in OC/EC ratio at London, North Kensington, during the summer months consistent with a possibly greater relative contribution of secondary organic aerosol. Perhaps surprisingly this is not, however, seen at the rural Harwell site. At Belfast, both organic and elemental carbon show elevated winter concentrations, consistent with important local primary sources. Only at the roadside Marylebone Road site is there a high correlation ( r 2 = 0.59 ) between organic carbon and elemental carbon concentrations which persists when simultaneously measured urban background concentrations from the nearby North Kensington site are subtracted; the OC/EC ratio in the traffic-related concentration increment is 0.88, well below the ratios typical of the urban background. Directional analysis of the data from the Marylebone Road street canyon shows that whilst elemental carbon concentrations are determined primarily by on-road traffic emissions, both organic carbon and PM 10 derive primarily from inputs from outside the street canyon. It therefore appears that at all times of year non-traffic sources of particulate organic carbon, be they primary or secondary, are dominant over traffic emissions in the urban background. Organic compounds account for about 22% of aerosol mass, irrespective of site.