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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-1094
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
26 Feb 2018
Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).
Estimates of Exceedances of Critical Loads for Acidifying Deposition in Alberta and Saskatchewan
Paul A. Makar1, Ayodeji Akingunola1, Julian Aherne2, Amanda S. Cole1, Yayne-abeba Aklilu3, Junhua Zhang1, Isaac Wong4, Katherine Hayden1, Shao-Meng Li1, Jane Kirk5, Ken Scott6, Michael D. Moran1, Alain Robichaud1, Hazel Cathcart2, Pegah Baratzedah1, Balbir Pabla1, Philip Cheung1, Qiong Zheng1, and Dean S. Jeffries7 1Air Quality Research Division, Environment and Climate Change Canada
2Environmental and Resource Studies, Trent University
3Environmental Monitoring and Science Division, Alberta Environment and Parks
4Watershed Hydrology and Ecology Research Division, Canada Centre for Inland Waters, Environment and Climate Change Canada
5Aquatic Contaminants Research Division, Environment and Climate Change Canada
6Technical Resources Branch, Environment Protection Division, Saskatchewan Ministry of the Environment
7Canada Centre for Inland Waters, Environment and Climate Change Canada
Abstract. Estimates of potential harmful effects to ecosystems in the Canadian provinces of Alberta and Saskatchewan due to acidifying deposition were calculated, using a one year simulation of a high resolution implementation of the Global Environmental Multiscale – Modelling Air-quality and Chemistry (GEM-MACH) model, and estimates of aquatic and terrestrial ecosystem critical loads. The model simulation was evaluated against two different sources of deposition data; total deposition in precipitation and total deposition to snowpack in the vicinity of the Athabasca oil sands. The model captured much of the variability of observed ions in wet deposition in precipitation (observed versus model sulphur, nitrogen and base cation R2 values of 0.90, 0.76 and 0.72, respectively), while being biased high for sulphur deposition, and low for nitrogen and base cations (slopes 2.2, 0.89 and 0.40, respectively). Aircraft-observation-based estimates of fugitive dust emissions, shown to be a factor of ten higher than reported values (Zhang et al., 2017), were used to estimate the impact of increased levels of fugitive dust on model results. Model comparisons to open snowpack observations were shown to be biased high, but in reasonable agreement for sulphur deposition when observations were corrected to account for throughfall in needleleaf forests. The model-observation relationships for precipitation deposition data, along with the expected effects of increased (unreported) base cation emissions, were used to provide a simple observation-based correction to model deposition fields. Base cation deposition was estimated using published observations of base cation fractions in surface collected particles (Wang et al., 2015).

Both original and observation-corrected model estimates of sulphur, nitrogen and base cation deposition were used in conjunction with critical load data created using the NEG-ECP (2001) and CLRTAP (2004, 2016, 2017) protocols for critical loads, using variations on the Simple Mass Balance model for forest and terrestrial ecosystems, and the Steady State Water Chemistry and the First-order Acidity Balance models for aquatic ecosystems. Potential ecosystem damage at 2013/14 emissions and deposition levels was predicted for regions within each of the ecosystem critical load datasets examined here. The spatial extent of the regions in exceedance of critical loads varied between 1 × 104 and 3.3 × 105 km2, for the more conservative observation-corrected estimates of deposition, with the variation dependant on the ecosystem and critical load protocol. The larger estimates (for aquatic ecosystems) represent a substantial fraction of the area of the provinces examined.

Base cation deposition was shown to have a neutralizing effect on acidifying deposition, and the use of the aircraft and precipitation observation-based corrections to base cation deposition resulted in reasonable agreement with snowpack data collected in the oil sands area. However, critical load exceedances calculated using both observations and observation-corrected deposition suggest that the neutralization effect is limited in spatial extent, decreasing rapidly with distance from emissions sources, due to the rapid deposition of emitted primary particles dust particles as a function of their size.

Citation: Makar, P. A., Akingunola, A., Aherne, J., Cole, A. S., Aklilu, Y.-A., Zhang, J., Wong, I., Hayden, K., Li, S.-M., Kirk, J., Scott, K., Moran, M. D., Robichaud, A., Cathcart, H., Baratzedah, P., Pabla, B., Cheung, P., Zheng, Q., and Jeffries, D. S.: Estimates of Exceedances of Critical Loads for Acidifying Deposition in Alberta and Saskatchewan, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1094, in review, 2018.
Paul A. Makar et al.
Paul A. Makar et al.
Paul A. Makar et al.

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Short summary
Complex computer model output was compared to and fused with observation data, to estimate potential damage to ecosystems in the Canadian provinces of Alberta and Saskatchewan. The revised deposition was compared to the maximum no-damage ecosystem capacity for sulphur and/or nitrogen uptake. These critical loads were exceeded, for areas between 10 000 to 330 000 square kilometres, depending on the ecosystem, a substantial fraction of the area of the provinces examined.
Complex computer model output was compared to and fused with observation data, to estimate...
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