the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
High sulphur dioxide deposition velocities measured with the flux/gradient technique in a boreal forest in the Alberta oil sands region
Dane Blanchard
Timothy Jiang
Paul A. Makar
Ralf M. Staebler
Julian Aherne
Cris Mihele
Xuanyi Zhang
Abstract. The emission of SO2 from the Athabasca oil sands region (AOSR) has been shown to impact the surrounding forest area and human exposure. Recent studies using aircraft-based measurements have demonstrated that deposition of SO2 to the forest is at a rate many times higher than model estimates. Here we use the flux/gradient method to estimate SO2 deposition rates at two tower sites in the boreal forest downwind of AOSR SO2 emissions. We use both continuous and passive sampler measurements and compare both techniques. The measurements predict SO2 deposition velocities ranging from 2.1–5.9 cm s-1. There are uncertainties associated with the passive sampler flux/gradient analysis, primarily due to an assumed Schmidt number, a required assumption of independent variables, and potential wind effects. We estimate the total uncertainty as ±2 cm s-1. Accounting for these uncertainties, the measurements are near (or slightly higher than) the previous aircraft-based measurements (1.2–3.2 cm s-1) and significantly higher than model estimates for the same measurement periods (0.1–0.6 cm s-1), suggesting that SO2 has a much shorter lifetime in the atmosphere than is currently predicted by models.
Mark Gordon et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2022-668', Anonymous Referee #1, 19 Dec 2022
General comments:
This study presented a detailed estimation of SO2 dry deposition velocity in the Alberta oil sands region, and concluded that the possibility of much lower velocity in the model which determines the lifetime of SO2. Because our knowledge of the dry deposition velocity is still limited, I would like to generally agree to the publication of this manuscript. However, it is required to revise several points. Please see the following comments and address my concerns.
Specific comments:
- Line 16 (Abstract): It is better to explicitly express “corrected values” for these estimated velocities.
- Line 19 (Abstract): In the main manuscript, it is stated that the study of Hayden et al. (2021) reported a value from 1.2 to 3.4 cm/sec. The value is different. Please confirm.
- Line 31: Is this sentence indicates deposition “amounts” or “velocity”? Please clarify. In addition, I cannot get these values (1.7 and 5.4 times) from the values shown in Lines 32-33.
- Line 59 (Section 2.1): For readers, I feel the illustration of this location and its surrounding status will be helpful.
- Line 183: Does the subscript for “r” generally stand for “aerodynamic” (www.atmos-chem-phys.org/acp/3/2067/)?
- Line 185: Duplicated equation number. Please correct.
- Line 189: Because this is the key discussion, it will be better to have more explanations for GEM-MACH deposition parameterization. Especially, why this parameterization led to lower deposition velocities?
- Line 241: The maximum value listed in Table 1 is “9.4”. Please confirm.
- Line 261 (Section 4.1): This subsection has been well organized to seek uncertainties, but I have one question. When we see the list in Table 1, Profile 5 showed the distinctive lower deposition velocity. What caused this distinctive lower value?
- Line 272: It will be better to explicitly state that these two values were mentioned as corrected velocities at 40 m.
- Line 347-349: I guess that this corrected range is presented based on the discussion in Section 4.1, but I cannot fully follow this correction. Please add final remarks in Section 4.1 to present which values were corrected from the discussion for a 30% overestimation.
Technical corrections:
Line 30 and elsewhere: Need subscript for “SO2”.
Citation: https://doi.org/10.5194/acp-2022-668-RC1 - AC1: 'Comment on acp-2022-668', Mark Gordon, 16 Feb 2023
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RC2: 'Comment on acp-2022-668', Anonymous Referee #2, 05 Jan 2023
The manuscript reports observations of SO2 gradients in the region downwind of the Alberta Oil Sands, which the authors interpret to derive estimates of the dry deposition velocity. This is motivated by the inference of high rates of SO2 deposition based on earlier aircraft flights in the region.
Overall, the data and analysis are interesting, but there are several assumptions used in the derivation of the flux and deposition velocities that are questionable. I would have found the manuscript easier to follow if some of these assumptions were highlighted in the introduction so that the purpose of some of the additional measurements (e.g. high time resolution data to assess the assumption of independent variables in the long time averages) was clear at the outset.
While the authors addressed the extent to which some of their assumptions introduce uncertainty into their calculations, I still missed an explanation of some aspects:
On lines 297-309, the authors use high frequency data from two heights to assess the assumption of independent variables. Does this exercise test the assumption that the concentration gradient and the momentum diffusion constant are independent values, or that the concentration and the deposition velocity are independent variables? Or both? For this subset of data, the authors find that violations of the assumption lead to a 30% overestimate in deposition velocity, but given the skewness of the data, can they be confident that this is a representative or conservative estimate?
Line 135 states that the flux gradient framework requires that the diffusion coefficient and the vertical gradient are constant throughout the canopy. But I believe is also an implicit assumption that the vertical flux is constant over the height of the measurements. Is that really true in a needleaf canopy where the elements leading to the uptake of the SO2 are distributed over much of the vertical extent of the gradient? One can imagine significant differences in losses at different heights within the canopy that would manifest in the vertical structure differently depending on the timescales of turbulence/diffusion. Can these observations really be compared the deposition velocities obtained from a point high above the canopy in the constant flux layer, which would be most relevant to the aircraft data and the model parameterizations?
The deposition velocity values derived by the authors are very high, implying minimal canopy resistance to deposition. In that case, we might expect the deposition velocity to reflect only atmospheric and quasi-laminar sublayer resistances. Can the authors confirm that such high deposition velocities are possible with estimates of those two constraints?
Specific Comments
Lines 31-34 It would be useful to know how the aircraft data were interpreted to determine dry deposition velocities.
Figure 5 – The legend and the caption label the ground and sonde traces in opposite ways. Based on the text, the caption appears incorrect
Citation: https://doi.org/10.5194/acp-2022-668-RC2 - AC1: 'Comment on acp-2022-668', Mark Gordon, 16 Feb 2023
- AC1: 'Comment on acp-2022-668', Mark Gordon, 16 Feb 2023
Mark Gordon et al.
Mark Gordon et al.
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