Aerosol deposition to the boreal forest in the vicinity of the Alberta Oil Sands

Measurements of size-resolved aerosol concentration and fluxes were made in a forest in the Athabasca Oil Sands Region (AOSR) of Alberta, Canada, in August 2021 with the aim of investigating (a) particle size distributions from different sources, (b) size-resolved particle deposition velocities, and (c) the rate of vertical mixing in the canopy. Particle size distributions were attributed to different sources determined by wind direction. Air mixed with smokestack plumes from oil sands processing facilities had higher number concentrations with peak number at diameters near 70 nm. Aerosols from the direction of open-pit mine faces showed number concentration peaks near 150 nm and volume distribution peaks near 250 nm (with secondary peaks near 600 nm). Size-resolved deposition fluxes were calculated which show good agreement with previous measurements and a recent parameterization. There is a minimum deposition velocity of v(d) = 0.02 cm s(-1) for particles of 80 nm diameter; however, there is a large amount of variation in the measurements, and this value is not significantly different from zero in the 68% confidence interval. Finally, gradient measurements of aerosol particles (with diameters <1 mu m) demonstrated nighttime decoupling of air within and above the forest canopy, with median lag times at night of up to 40 min and lag times between 2 and 5 min during the day. Aerosol mass fluxes (diameters <1 mu m) determined using flux-gradient methods (with different diffusion parameterizations) underestimate the flux magnitude relative to eddy covariance flux measurements when averaged over the nearly 1-month measurement period. However, there is significant uncertainty in the averages determined using the flux-gradient method.

Automated summary

Measurements were conducted in a forest in the Athabasca Oil Sands Region to investigate particle size distributions, particle deposition velocities, and vertical mixing in the canopy. Different sources were identified based on wind direction. Smokestack plumes from oil sands processing facilities had higher number concentrations at 70 nm, while aerosols from open-pit mine faces had peaks near 150 nm and 250 nm. Deposition fluxes were calculated and showed good agreement with previous measurements. Nighttime decoupling of air within and above the forest canopy was observed, with lag times of up to 40 min. Aerosol mass fluxes underestimated the flux magnitude when compared to eddy covariance flux measurements.