National Pollutant Release Inventory: sulphur dioxide

Background

Sulphur Dioxide (SO₂) is the sixth most common air pollutant released to the environment in Canada. SO₂ is a precursor to smog and haze, which effect public health and impair visibility.

The Government of Canada tracks SO₂ emissions from the industrial sector through the National Pollutant Release Inventory (NPRI). By combining the emissions data of SO₂ (reported to the NPRI) with data from ambient air monitoring stations from the National Air Pollution Surveillance Program (NAPS), researchers can better understand and improve air quality in Canada.

The NPRI is a registry that tracks pollution released to air, water and land across Canada. All industrial facilities that release pollutants are required to report to the NPRI if they meet certain reporting requirements (PDF). Since 1993, the NPRI has collected data from more than 21,000 facilities from a variety of industrial sectors. These reports are used to identify and monitor pollution sources in Canada. The map shows the releases of SO₂ reported in 2019.

The National Air Pollution Surveillance Program (NAPS) is a program responsible for monitoring air quality across Canada. NAPS has been operating ambient air monitoring stations since 1969. Today, there are 286 stations in operation in 203 communities. The map below shows where the stations were located in 2019.

Location of NAPS stations in 2019

By combining NPRI and NAPS data, governments can perform scientific analyses and adopt more effective regulations and policies to better manage the impacts of pollutants.

Impact of sulphur dioxide on ecosystems and health

Sulphur dioxide (SO₂) is a colourless gas that has a strong, stinging odour. It is the most common of, and the component of greatest concern in, a group of gases called sulphur oxides (SO_x).

SO₂ is a threat to both health and the environment. Short-term exposure to high concentrations of SO₂ can harm the respiratory system of humans and animals. The risk is higher among people with reduced lung functions, such as those who have asthma. When transformed into other aerosols (like sulphate) in the ambient air, the reactivity of SO₂ increases. When reactivity is increase, SO₂ may combine with other compounds (for example, ammonia) and develop into substances that are harmful for human and environmental health such as fine particulate matter (PM_2.5) (small particles found in air that are measured equal or less than 2.5 microns (μm)).

High concentrations of SO_X in the atmosphere can harm agricultural crops, forests and ecosystems by damaging foliage and reducing growth. As such, SO_X is a threat to agriculture (food supply) and biodiversity and contributes to the loss of habitats.

When dissolved by water vapour to form acids (for example, sulphuric acid) SO₂ contributes to acid rain, which can harm a large variety of aquatic and terrestrial ecosystems, such as lakes, streams and wetlands, where it can be detrimental to fish and other wildlife. Since everything is interconnected in an ecosystem, impacts to one species or plant can affect other levels of the food chain, if not all of them. Acid rain also damages human-made materials, which can result in the need for premature repairs to buildings and infrastructure, thus increasing costs.

The impacts of SO₂ are numerous, which is why it is part of the Criteria Air Pollutants (CAP) and listed under CEPA 1991 Schedule 1-List of toxic substances as a precursor to particulate matter. The Canadian Ambient Air Quality Standards (CAAQS), which is responsible for setting safe thresholds for air pollutants in Canada, sets the maximum hourly concentration of SO₂ at 70 ppb (parts per billion) peaks and the maximum annual average at 5 ppb.

Sulphur dioxide hotspots

The map below combines data from NPRI releases (in blue) and NAPS concentrations. Large concentrations of SO₂can be found in areas with mining and smelting activities, pulp and paper production, and fuel processing and burning. Most of these activities take place in facilities located outside of major population centers.

Map of sulphur dioxide "hotspots" in 2019

The pie chart below shows the breakdown of reported SO₂ releases by province and territory in 2019. The releases reported in Alberta accounted for one-quarter of the national total.

Main contributors

In Canada, the main sources of elevated concentrations of SO₂ in the atmosphere are industrial processes and the burning of fossil fuels such as:

• Metal smelting (non-ferrous metal smelting, nickel, zinc, and aluminum)
• Fossil-fuel electricity generation
• Pulp and paper
• Petroleum refining
• Chemical manufacturing
• Cement and natural gas processing

In 2019, the electricity sector was responsible for 34% of SO₂ emissions in the country, followed by oil and gas extraction at 23%, and metals at 10% and aluminum 9%. Collectively, these four sectors account for more than half of the total SO₂ emissions reported to the NPRI in 2019.

SO₂ is highly reactive with other compounds which means that it transforms into derived substances when in contact. As such, it does not remain long in its original state in the atmosphere. Unlike ozone and fine particulate matter (PM_2.5), which can persist for long periods of time in the ambient air and travel long distances, SO₂ concentrations tend to decrease rapidly with distance. Therefore, when SO₂ is measured by ambient air monitoring stations, it implies that a nearby source has directly released it into the atmosphere.

Overall, SO₂ emission have been steadily decreasing since the 1970s. The decrease was initially as a result of the conversion to natural gas for heating purposes across the country. In the 1980s, Canada began to address acid rain by setting emission ceilings for each province from Manitoba eastward. In 1991, the Canada-United States Air Quality Agreement was signed to address transboundary air pollution leading to acid rain. Both countries agreed to reduce emissions of sulphur dioxide (SO₂) and nitrogen oxides (NO_x), the primary precursors of acid rain, and to work together on acid rain related scientific and technical cooperation. So₂ emissions have continued to decrease in the last decade as can be seen in the below chart from 2002 to 2019.

Limits and considerations

Monitoring ambient air quality is complex because gaseous pollutants are dispersed rapidly in the atmosphere and therefore can, move from one location to another in a short period of time depending on wind speed and wind direction. That is why ambient air monitoring stations often measure both local concentrations and airborne contaminants brought in by prevailing winds.

The image below shows the main wind currents in Canada and the United States responsible for carrying SO₂ from various sources. High SO₂ concentrations normally occur close to the emission sources (<10km) but can be impacted by chimney height and surrounding terrain. SO₂ is almost never observed in high concentrations in distances greater than 25 km from the source of emissions.

In addition, certain meteorological conditions can increase the concentrations of SO₂, including inversions and low wind conditions.

Case study: Fort McMurray, Alberta

Using NPRI and NAPS data, we can identify hotspots for SO₂ concentrations across Canada.

The Fort McMurray region in Alberta is one of the biggest SO₂ hotspots in the country. In 2019, there were two facilities declaring SO₂ emissions to the NPRI in this area, both in the oil and gas sector. These facilities accounted for 29% of the total SO₂ releases from the oil and gas sector including conventional and non-conventional extraction, and for 8% of the total annual SO₂ emissions reported to the NPRI across all sectors.

As shown on the map below, there are two NAPS ambient air monitoring stations in the city. The maximum SO₂ concentrations measured in 2019 was 8.9 ppb and the average was 0.64 ppb.

Map of NAPS stations and sulphur dioxide concentrations in Fort McMurray Alberta

In recent years, SO₂ emissions have been gradually decreasing. This reduction is due to improved operations, including increased use of alternative fuels, preventative maintenance, and the transformation of SO₂ emissions into an ammonium sulphate that can be sold as a commercial by-product (fertilizer). It can also be attributed to Alberta’s industrial air quality management system for environmental assessment, approval and enforcement.

The bar chart below shows that the total number of facilities reporting to the NPRI in Fort McMurray has stayed between two and three in the last ten years, but the total amount of releases has significantly decreased over the same time period.

Case study: Estevan, Saskatchewan

In 2019, the city of Estevan, Saskatchewan reported the second highest releases of SO₂ reported to the NPRI. There were three facilities reporting releases, two of which were from the fossil-fuel electric power generation (coal) and one from the oil and gas extraction. Together, these facilities accounted for 8% of the total SO₂ reported to the NPRI for that year.

As shown on the map below, there is only one NAPS station in Estevan. The maximum SO₂ concentration measured in 2019 was 20.6 ppb and the average of 1.1 ppb.

Map of NAPS stations and sulphur dioxide concentrations in Estevan, Saskatchewan

The bar chart below shows that the quantity of releases have tripled in Estevan since 2002. The SO₂ total releases have peaked in 2011 (when a new reporting facility was added) and in 2012.  The releases have been constant since. Also, there was only one reporting facility in 2002 and 2006.

Case study: Trail, British Columbia

In 2019, Trail, British Columbia was the Canadian city with the highest SO₂ concentrations measured by the four NAPS stations in the area, as shown on the map below.

Map of NAPS stations and sulphur dioxide concentrations in Trail, British Columbia

However, data from the NPRI shows that releases reported by two facilities were similar to previous years (as seen on the chart below) and they accounted for less than 1% of the SO₂ emissions reported to the NPRI in 2019. Both were from the non-ferrous metal smelting and refining sector.

The high concentration in Trail in 2019 can be explained by the fact that 2019 was registered as one of the worst years (PDF) in history for wildfires, for both British Columbia and Washington State. Wildfires are known to increase the amount of SO_X released in the atmosphere and, therefore, can add to the concentrations measured by ambient air monitoring stations. This could explain why the SO₂ concentrations measured by NAPS stations were significant in the area, while the NPRI releases in Trail were not unusual.

Pollution prevention

SO₂ emissions have drastically reduced in the past 50 years. The largest impact on SO₂ emissions throughout the years has been the closure of several emissions sources, such as smelters, petroleum refineries, pulp and paper plants and coal-fired electricity units. Also, improvements to industrial processes, for example for aluminum smelters, scrubber improvement, and regulations on marine emissions in port areas have also had great results in decreasing SO₂ emissions.

In addition, the following regulations and agreements have also had a positive impact for improving air quality and reducing SO₂  concentrations:

• Canada-United States Air Quality Agreement (1991)
• Environmental Emergency Regulations (2019)
• Base metals smelters and refineries and zinc plants: pollution prevention planning notice
• New source emission guidelines for thermal electricity generation
• Sulphur in Diesel Fuel Regulations
• Sulphur in Gasoline Regulations

Many voluntary measures have also been taken by industry to reduce their SO₂ emissions. For example, three facilities in Sudbury, Ontario have opted to take part in an Emissions Reduction Program (ERP), a program which requires them to adapt their SO₂ emissions to meteorological conditions by reducing them on days with high atmospheric dispersion conditions depending on the wind speed, direction and temperatures.

Ambient levels of SO₂ have decreased (PDF) by 96% in Canada since 1970 as a result of the use of alternative (low-sulphur) fuels and pollution reduction programs. Furthermore, between 2002 and 2016 (PDF), average concentrations and peak concentrations of SO₂ have decreased for all regions around Canada. Finally, since 2004 (PDF), SO₂ concentrations in all regions across Canada have consistently been below the annual standard of 5 ppb.  

Average ambient concentration of SO₂ 2002-2016 (ppb)

What you can do

The SO₂ data integration has shown how NPRI and NAPS data can be used together to inform Canadians about the releases and concentrations of pollutants in the atmosphere.

Feel free to use the information available in the NPRI or NAPS databases, as well as the CESI interactive maps and reports to further research on this topic or about other air pollutants of interest to you. NAPS and NPRI databases are both free and public!

For more information on how to use the data, please refer to the next section.

How to use our data

NPRI data can provide a general portrait of pollution, but, like all inventories, they do have some limitations to keep in mind when you are analyzing the data. First, NPRI data does not consider all of the SO₂ sources, including those coming from across the borders, accidental releases or natural SO₂ sources like wetlands or wildfires. Some potentially active volcanoes in Canada can also release SO_X.

Second, SO₂ concentrations measured by NAPS ambient air monitoring stations and the emissions data collected by NPRI are not directly comparable. NAPS collect ambient concentration data throughout the year using continuous monitoring equipment and the NPRI collects estimates of industrial releases (tonnes per/yr) based on emission factors, source testing, mass balance, site-specific emission factors, published emission factors or other technical calculations. All of these methods have their own limitations and different types of precision.  

Third, some meteorological factors directly affect the ambient measured concentration, such as wind direction, wind speed, the distance between the source of SO₂ and the ambient air monitoring station, as well as the source chimney height. Also, some emitters of SO₂ in Canada may not have NAPS stations nearby or meet the NPRI’s reporting standards. That is why this analysis of SO₂ focuses on hotspots rather than making a direct comparison between the two datasets. These limitations and considerations can explain the variations between the reported emissions to the NPRI and the ambient concentrations measured by NAPS stations.

Although not included in the NPRI reports, there is another Air Pollutants Emissions Inventory (APEI) which focuses on non-point sources of SO₂ releases such as marine emissions, transportation and agriculture that could also be measured by NAPS stations.