Organic and black carbon in PM2.5 and PM10: 1 year of data from an urban site in Helsinki, Finland

The results from a 1-year measurement period concerning the diurnal PM2.5 and PM10 organic carbon (OC) and black carbon (BC) concentrations are presented for a traffic-influenced site in the Helsinki metropolitan area. The measurements were based on aerosol sampling using a virtual impactor and the subsequent thermal–optical analysis to distinguish between OC and BC. Backup filters were used to estimate and correct for the positive sampling artefact. Daily-average concentrations in PM2.5 varied between 1.0 and 8.5 μg C m−3 for OC, and between 0.3 and 5.7 μg C m−3 for BC. Annual-average concentrations of OC and BC were 3.0 and 1.2 μg C m−3, respectively, in PM2.5, and 4.2 and 1.3 μg C m−3 in PM10. On an annual level, particulate organic matter (POM=1.6×OC) accounted for 50±14% and 36±8% (average±1σ) of the total PM2.5 and PM10, respectively, whereas BC stayed lower at 14±8% and 7±4%. Typically more than 90% of BC resided in the PM2.5 size fraction. The contribution of coarse particles (>2.5 μm) to the overall OC varied between the 0% and 67% (median 27%). The effect of meteorological conditions on the variability of OC and BC concentrations was examined, and the contribution of secondary organic aerosol to the total fine organic aerosol was estimated.