Sulphur dioxide

Sulphur dioxide is a colourless gas that smells like burnt matches. It can also be oxidized in the atmosphere to form sulphuric acid aerosols. In addition, sulphur dioxide is a precursor to sulphates, one of the main components of airborne secondary PM_2.5. Major sources of SO₂ include smelters, industrial processes and electric power generation.

Health effects caused by exposure to high levels of SO₂ include breathing problems, respiratory illness, and the exacerbation of respiratory and cardiovascular disease. People with asthma, chronic lung disease or heart disease are the most sensitive to SO₂. Sulphur dioxide damages trees and crops. Similar to NO₂, SO₂ is also a precursor to acid rain, which contributes to the acidification of soils, lakes and streams, accelerated corrosion of buildings, and reduced visibility.

Smelters in central Ontario are the major sources of SO₂ emissions in Ontario, accounting for approximately 64% of the provincial SO₂ emissions according to 2017 estimates for point, area and transportation sources (excluding emissions from open and natural sources), as shown in Figure 13. Industrial sources (e.g. petroleum refining, cement and concrete manufacturing) accounted for an additional 32%. The transportation sector and miscellaneous sources accounted for the remaining 4% of all SO₂ emissions in the province (Air Pollutant Emission Inventory 1990-2017, 2019).

Figure 13: Ontario SO₂ emissions by sector (2017 estimates for point/area/transportation sources)

There were no exceedances of the provincial one-hour, 24-hour and annual AAQC for SO₂ of 250 ppb, 100 ppb and 20 ppb that were in effect in 2017, respectively, at any of the Ministry’s AQHI air monitoring stations in Ontario. Hamilton Downtown recorded the highest SO₂ annual mean (3.5 ppb) during 2017. Sudbury recorded the highest one-hour maximum concentration (119 ppb) and Sarnia recorded the highest 24-hour maximum concentration (26 ppb) for 2017. A summary of the 2017 SO₂ annual statistics for individual AQHI air monitoring stations in Ontario are presented in Table A7 of the Appendix.

The SO₂ annual mean concentrations from 2008 to 2017 show a decreasing trend of 46% across Ontario in Figure 14. SO₂ 10-year trends for individual AQHI air monitoring stations in Ontario are presented in Table A17 of the Appendix. Overall, provincial SO₂ emissions have decreased by approximately 52% from 2008 to 2017 as shown in Figure 15 (Air Pollutant Emission Inventory 1990-2017, 2019). The reduction of SO₂ emissions over the years is the result of various initiatives, which include, but are not limited to:

• establishing emissions controls at Ontario smelters through site-specific standards under O. Reg. 419/05
• banning of coal use in electricity generation
• low sulphur content in transportation fuels

Figure 14: Trend of SO₂ annual means across Ontario (2008-2017)

Figure 15: Ontario SO₂ emission trend (2008-2017)