Evaluating the variability of aquatic acidification and photochemical ozone formation characterization factors for Canadian emissions

Background, aim, and scope  The Canadian life cycle impact assessment method LUCAS proposes a characterization of the impact categories aquatic acidification and photochemical ozone formation using a resolution scale based on 15 terrestrial ecozones. Each ecozone represents areas of the country which can be identified easily by general living (biotic) and nonliving (abiotic) characteristics. The three main purposes of this research are to improve the characterization models of both impact categories including regional exposure and effect factors, to investigate what is the best resolution scale between Canadian provinces or ecozones, and to analyze the extent of spatial variability. Materials and methods  A model framework accounting for variability in fate, exposure and effect factors has been elaborated. The same fate factor, based on Advanced Statistical Trajectory Regional Air Pollution matrices, applies to both impact categories. For the aquatic acidification impact category, the fate factor also accounts for the fraction of the deposition transferred to the aquatic ecosystem. The exposure factor for this impact category is considered to be 1 and the effect factor is based on the critical load exceedance, where the potential impacts are only considered in provinces or ecozones in which the critical load is exceeded. For the photochemical ozone formation impact category, the exposure factor is considered to be proportional to the population density in each province or ecozone, and the effect factor is represented by the chemical reactivity estimated with the maximum incremental reactivity model. The calculation of the new characterization factors using both a province-based and ecozone resolution scale was performed using a matrix which converts data from one resolution scale to another. Results  Results with the inclusion of the effect and the exposure factors show that the spatial variability between provinces remains within a factor of 10 and 5 for aquatic acidification and photochemical ozone formation, respectively. Discussion  Analysis of the results show that regionalization by province is preferable to regionalization by ecozone. It is more accurate in regard to atmospheric modeling and more representative of population distribution. However, averaging the fate factor and the population density over a whole province results in a serious limitation. Conclusions  The spatial variability of characterization factors between provinces is in the same order of magnitude as the overall range between chemicals for aquatic acidification while much smaller for photochemical ozone formation. Hence, at this stage of knowledge, province-based regionalization seems to be more relevant for the aquatic acidification impact category than for photochemical ozone formation. Recommendations and perspectives  Research must be pursued to integrate a better transport and deposition model with improved spatial capabilities and a successive modeling step properly describing the cause–effect chain up to the damage level, such as the biotic environment and the human population.