Dry Deposition of Polycyclic Aromatic Compounds to Various Land Covers in the Athabasca Oil Sands Region

English Abstract:

Dry deposition is an important pathway inputting pollutants to various ecosystems. The present study provides the estimation of dry deposition of polycyclic aromatic compounds (PACs), including polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs, dibenzothiophenes (DBTs) to various land covers in the Athabasca oil sands region. A framework was first developed to estimate dry deposition of individual PAC species making use of monitored ambient concentrations of 18 PAHs species, 20 alkylated PAHs and 5 DBTs at three sites. The gaseous dry deposition scheme of Zhang et al. (2003) was modified to handle gas-phase PACs and the particle dry deposition scheme of Zhang and He (2014) was applied to particulate-phase PACs. Henry’s law constant and octanol-air partition coefficient of PACs were used as the basis for choosing model parameters needed in the gaseous dry deposition scheme. Modelled dry deposition velocities for various gaseous PACs and over various land covers were in the range of 0.01-0.1 cm s^-1, comparable with literature values obtained from field studies. Annual dry deposition of the sum of PAHs was estimated to range from 610 to 1150 µg m^-2 over forested canopies surrounding the three sites and from 580 to 670 µg m^-2 over agricultural lands, grass and shrubs. The corresponding numbers are 1500 to 4200 and 1900 to 2800 for the sum of 20 alkylated PAHs, and are 130 to 1100 and 370 to 690 for the sum of 5 DBTs. The three monitoring sites are situated nearby the Athabasca River, and the direct atmospheric dry deposition to water surface was estimated to range from 830 to 1000, 2200 to 4100, and 160 to 880 µg m^-2 for PAHs, alkylated PAHs, and DBTs, respectively. In most cases the estimated annual dry deposition is two to four times the measured annual wet deposition. Alkylated PAHs contributed more than the sum of PAHs and DBTs in both dry and wet deposition budget, suggesting the importance of including this parent group of pollutants in atmospheric deposition budget estimation for subsequent ecosystem impact studies.