Estimation of dry deposition of trace elements and polycyclic aromatic compounds in the Canadian Oil Sands Region

Environment Canada recently initiated a long-term monitoring plan in the Canadian Athabasca Oil Sands Region to measure polycyclic aromatic compounds (PACs) and selected trace metals. One of the main objectives is to use the monitored air concentrations to produce atmospheric deposition maps so the toxic effects produced by the oil industry emissions to various ecosystems can be assessed. The present study describes the approaches selected for estimating the dry deposition of various PACs and trace element species. An existing big-leaf dry deposition model is modified to handle gaseous PACs for estimating dry deposition velocity values. A size-resolved particle dry deposition model is used for estimating particle dry deposition velocity. Gas/particle partitioning of PACs is also developed. Results from one monitoring site with high-volume PAC sampling suggested that the estimated annual dry deposition for the individual PACs are mostly in the order of 1-100 µg m^-2. Gaseous-phase PACs dominated the deposition of low-molecular PACs and particulate-phase PACs dominated the dry deposition of high molecular PACs. The estimated annual dry deposition for the 40+ trace elements ranged from 0.1 to 10000 µg m^-2 if only considering the contributions from particles smaller than 2.5 µm. The total dry flux depends on the particle size distributions – for example, if coarse particles are included in the dry deposition budget, the flux needs to be adjusted by a factor of 2 to 10. A review of the current knowledge on PAC and metal dry deposition and possible approaches for generating total deposition map will also be discussed.