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 NOx 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.
Xuanyi Zhang et al.
Status: open (until 22 Apr 2023)
- RC1: 'Comment on acp-2023-26', Anonymous Referee #1, 21 Feb 2023 reply
Xuanyi Zhang et al.
Xuanyi Zhang et al.
Viewed (geographical distribution)
This paper uses tower and balloon observations and a 1-D chemical transport model to study ozone over a forest near an region producing oil sand. The manuscript is generally well-written and robust, but a few changes would further strengthen the paper.
The introduction needs some restructuring. The information is rich and almost complete, but poorly connected, thus does not provide the motivation of the study well enough. A paragraph explicitly talking about “Why ozone concentration and deposition over AOSR” is highly recommended.
L 38 – 40: NO does not ALWAYS dominate the in-canopy chemical sink of ozone (e.g. Wolfe et al. (2011) propose BVOC to be the dominant chemical sink in a warm pine forest)
L 149: Need citation for the observed “shelf shape”
L 114: The maximum height of profile measurement was 300m and most of the paper discuss about near-surface turbulent mixing and sinks. Would 1000 vertical layers be an overkill and potentially introducing unnecessary error from vertical transport? Please explain and discuss.
L 115, 156 – 157: This approach looks weird, or underexplained at best. In the model, what height does z = 0 correspond to? Assuming z = 0 refers to soil surface, this is not most of the deposition occurs (since leaf surface is mostly the major sink over healthy forest), nor what typical big-leaf model (displacement height) takes. The choice of which layer to put the “big-leaf” foreseeably affect the modelled in-canopy ozone profile. A sensitivity run to explore how the choice of level where the big-leaf is placed, or at least argument for why choosing to put the big-leaf at soil surface is needed.
L 230: Does GEM-MACH have enough resolution to resolve these regional details?
L 310: When the wind speed is higher, there should also be more vertical turbulent mixing generated by horizontal wind shear. This factor should also be considered and discussed in comparing the ozone gradients.
L 393: Does the model explicitly consider the strong diurnal variation of ozone deposition velocity? If not, explain how this might affect your result.
L 408: How long did snow cover last? Since snow and the compounding low temperature during early season can also significantly reduce ozone dry deposition. This might be a worth-discussing point.
L 464: Direct ozone (and to a lesser extent NO2) flux measurement would also help tremendously to constrain the deposition velocity/flux.