Canadian Smog Science Assessment Highlights and Key Messages: chapter 8

Emerging Issues

• Climate Change
• Intercontinental Transport

Climate Change

A changing climate can impact air quality in a number of ways, including: changing chemical reaction rates because of an increase in temperature and water vapour; changing the distribution of meteorological conditions which affect transport of air pollutants and location of air pollution episodes; modifications in global circulation, again affecting distribution of air pollutants; changing emissions of natural precursor gases; decreasing cloudiness which enhances smog production; and changes to the frequency, seasonality and intensity of forest fires. 

The impact of future climate change on regional air quality is not yet well quantified as there are many possible future emissions and climate scenarios, as well as many modelling methodologies. The complexity of the linkages between air quality and climate change, and interactions between the two, is an area of continuing research. Preliminary studies for Canada, however, indicate that climate change may increase O₃ levels with the greatest impact occurring in areas that already experience high levels, such as the Windsor-Quebec City corridor. The effect on O₃ would not affect all areas across the country consistently, as certain regions may experience local reductions in O₃, depending on changes to local emissions and meteorology.

Studies on climate change effects on PM show increases in both PM_2.5 concentration and number of peak days, but a smaller impact on the daily maximum 24-hour PM_2.5 concentration, and a smaller increase in the number of exceedance days in comparison to O₃. A future scenario involving both climate change and projected future emissions suggests an increase in PM concentration, as does a future scenario with climate change only (no changes in emissions), though to a lesser degree.

When the above scenarios were analysed for health impacts, there were clear indications that climate change impacts on air quality would result in increased adverse health effects. However, the scenarios used were relatively simplistic and more complex further analysis is required to determine overall impact and regional variation^{Footnote 16} .

The combined impacts of O₃ and carbon dioxide (CO₂) on crop yields and ecosystems is also an emerging concern since there is uncertainty about the net effect on vegetation of elevated atmospheric CO₂, warmer temperatures, possibly elevated O₃ levels as well as other environmental changes such as changes in precipitation and nutrient availability. Short-term exposure to increased levels of O₃ can decrease plant species’ ability to respond to increased levels of CO₂, decreasing net primary productivity. Recent studies on crop yields have also shown that the joint effects of both elevated O₃ and CO₂ can lead to crop yield reductions, with overall decreases in net biomass. Recent long-term studies also indicate that the differing sensitivity of species to various O₃ and CO₂ levels can alter species composition in plant communities and can either enhance or slow the conversion of communities towards less sensitive species.

Intercontinental Transport

Trans-Pacific transport of pollutants from Asia into North America can occur through the winter and early spring, mainly resulting in increased ambient levels from mid-latitudes to the Arctic where the contribution reaches its maximum.

The contribution of O₃ from Asian anthropogenic emissions to the annual average ambient concentration in North America has been estimated to be in the range of 2-5 ppbv^{Footnote 17}  in 1997. Other similar studies estimate a 10% increase in Asian emissions would increase annual average surface O₃ in the U.S. by 0.1-0.2 ppbv, and more in regions of higher elevation. Short-term episodic influences could increase regional O₃ levels by more than 10 ppbv.

Transportation of dust aerosols across the Pacific has been observed through satellite data for many years and tends to be episodic in nature. However, the contribution of other transported species to local PM_2.5 levels is harder to quantify and observe. The annual average contribution of intercontinental transport to total PM levels in Canada has been estimated to be in the range of 0.1 µg m^-3 in annual PM concentration in the form of sulphate, black carbon and organic carbon. Model simulations show that Asian sulphur emissions have little influence on surface sulphate levels in Canada, with the exception of the west coast; however, the contribution is significant at higher surface altitudes across the country. Ongoing research on the effects of intercontinental transport on domestic air quality includes reconciling the range of model results, and studying the climate change-air quality interactions resulting from the increased loadings of fine particles into the atmospheric column.

Satellite based measurements have emerged in the last decade as a new means of studying air pollution, and their use is expected to grow. Current generation satellites are capable of estimating surface air pollution concentrations, and have been used to track long-range transport events on the regional and