Ozone in the boreal forest in the Alberta oil sands region

Abstract. Measurements of ozone were made using an instrumented tower and a tethersonde located in a forested region surrounded by oil sands production facilities in the Athabasca Oil Sands Region (AOSR). Our observations and modelling show that the concentration of ozone was modified by vertical mixing, photochemical reactions, and surface deposition. Measurements on the tower demonstrated that when winds are from the direction of anthropogenic emissions from oil sand extraction and processing facilities, the ozone mixing ratio in the forest is as much as 10 ppb lower than when winds are from the direction of undisturbed forest. This finding is supported by previous studies which suggest that surplus NO_x from oil sands emissions results in ambient ozone titration. Gradients of ozone mixing ratio with height were observed using instruments on a tethered balloon (up to a height of 300 m) as well as a pulley system and 2-point gradients within the canopy. Strong gradients (ozone increasing with height between 0.2 and 0.4 ppb m^-1) were measured in the canopy overnight, while daytime gradients were weaker and highly variable. A 1D canopy model was used to simulate the afternoon in-canopy gradient with reduced mixing overnight (suggesting high stability within the canopy), and an ozone deposition velocity of 0.2 cm s-¹. Sensitivity simulations using the model suggest the local NO concentration profile and coefficients of vertical diffusivity have a significant influence on the O₃ concentrations and profiles in the region.