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

Abstract

Abstract. 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. Background air from undeveloped forested areas air showed a peak number concentration for diameters near 70 nm while air mixed with upgrader smokestack plumes had higher number concentrations with peak number between diameters of 70 and 80 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 vd = 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 PM1 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. PM1 fluxes 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.

Ozone in the boreal forest in the Alberta oil sands region

“…In-canopy ozone measurements at heights of 2 m and 25 m indicate a stronger gradient overnight (near 0.4 ppb m -1 ) with a weaker gradient (near 0.16 ppb m -1 ) in the afternoon. This is consistent with increased mixing within the canopy in the afternoon (see our companion paper, Jiang et al, 2022) and with the ozone gradient measured in a mixed temperate forest (Wu et al 2016). Model analysis can reproduce the in-canopy gradients when ozone deposition velocity is modeled as 𝑣𝑣 𝑑𝑑 = 0.2 cm s -1 and the diffusivity coefficients are reduced by a factor of 2 overnight.…”

“…2) does not account for the increased stability within a canopy associated with canopy decoupling. Moderate overnight canopy decoupling was measured at this site as described in Jiang et al (2022). Further investigation is needed to improve how this decoupling is incorporated into the 1D canopy model.…”

“…No ozone measurements were made during this field study, but SO2 measurements (43i, Thermo Scientific and AF22e, Envea) at ground level and a height of 30 m and sizeresolved sub-micron aerosol measurements (UHSAS, DMT) helped to further identify wind sectors bringing polluted air to the site. The SO 2 and aerosols measurements are discussed in our companion papers Gordon et al (2022) and Jiang et al (2022) respectively.…”

“…While our companion paper (Jiang et al, 2022) describes source locations for aerosols with finer angular resolution, this is not possible with the ozone measurements since ozone follows a regional diurnal cycle upon which NO titration events are superimposed. There are not enough data to separate both wind direction and time-of-day into more than 3 sectors.…”

“…Gradient and mixing ratio measurements are compared with a 1D canopy model with various deposition schemes using a "big leaf" approach, which assumes a deposition flux at the lowest model layer. This paper is a companion paper to Jiang et al (2022) and Gordon et al (2022) which respectively investigate aerosol and SO2 deposition at this site.…”