Determination of source contributions of NMHCs in Helsinki (60°N, 25°E) using chemical mass balance and the Unmix multivariate receptor models

A chemical mass balance (CMB) receptor model and a multivariate receptor model (Unmix) were used for the determination of the contributions of different non-methane hydrocarbon (NMHC) sources in Helsinki. C2–C10 hydrocarbon measurements were conducted in the Helsinki city area during four different seasons in 2001 in order to get a data set for CMB modelling. The samples were collected using 24-h passive canister sampling simultaneously with 2-h pumped adsorbent samples. The emission profiles of the NMHC sources were also determined. Source contributions were estimated for different seasons of the year and weekly and diurnal variations were examined using the CMB model. According to the CMB analysis major sources of C2–C10 NMHCs were traffic (gasoline exhaust 33%, gasoline vapour 9% and liquid gasoline 12%) and emissions from distant sources, advected to the study area in long-range transported air masses (37%). City gas (2%), solvents (3%), biogenic compounds (1%) and diesel exhaust (0.2%) were minor contributions. Unmix was used to confirm that the major sources were included in the CMB calculations. For C6–C10 hydrocarbons the major sources in both CMB and Unmix analyses were gasoline exhaust, accounting for 52% and 53% of the NMHCs, respectively.