Sulphur dioxide (SO2) is a colourless gas with a characteristic acrid smell, like that of a struck match. When levels reach between 0.3 to 1.0 parts per million (ppm), most people can detect its acidic taste in the air.

Sulphur dioxide is emitted by burning oil and coal which contain sulphur, for example in power generating stations, pulp mills, metal smelters, refineries and heating boilers.

People with asthma, or chronic lung and heart disease, are the most sensitive to SO2. High concentrations also damage leaves on trees and agricultural crops. Sulphur dioxide (along with NOx) has a number of other environmental effects including lake acidification due to acid rain, and associated corrosion of stone and metalwork. Acid rain is formed when sulphur dioxide mixes and dissolves with the water in clouds, eventually forming dilute sulphuric acid.

SO2 contributes to the formation of acid aerosols (fine acidic particles), which may also have detrimental effects on human health. Fine particles present in the air as a result of sulphur dioxide emissions can cause haze over large regions. Researchers believe such haziness, in addition to being a visual blight, can significantly reduce average temperatures in affected areas. In this way, SO2 could have a significant effect on the earth's climate. This possibility is being examined in studies of climatic change around the world.

The one hour standard for SO2 was 340 parts per billion until November 16, 1995 ?? when a new standard of 170 ppb was legislated (see page ??). Industries in the greater Saint John area are active partners in striving to maintain acceptable levels of ambient SO2. This is achieved by continual monitoring and by switching fuels and reducing industrial output as necessary. The requirement to take such preventative measures is mandated in the Approvals to Operate issued to major industries in the Saint John region.

As part of an acid rain agreement signed in 1987 and strengthened in 1992, New Brunswick pledged to reduce total sulphur dioxide emissions from all emission sources in the province to 175,000 tonnes per year by 1994. This target was met one year early, in 1993, by which time emissions had fallen to 156,000 tonnes. Emissions in 1994 were 128,000 tonnes and provisional figures for 1995 show a further reduction.

The biggest single source of sulphur dioxide in the province is the NB Power plant at Coleson Cove, 15 km southwest of Saint John, which emitted 40,500 tonnes of SO2 in 1994. The two next highest emitters in the Saint John area are the Irving Oil Refinery (7,600 tonnes in 1994) and the Courtenay Bay Generating Station (4,700 tonnes in 1994). The tall stack at Coleson Cove and episode response plans at the oil refinery and Courtenay Bay generating station are examples of methods that have been used to minimize the impact of these emissions on local air quality.

Sulphur dioxide concentrations were variable between each of the four sites reporting in 1994. Generally, monthly mean concentrations ranged from about 3 to 15 ppb with occasional higher values. Annual mean concentrations at all sites were all well below the annual standard of 23 ppb. The Forest Hills and Champlain Heights locations experienced the highest mean and peak readings. One-hour concentrations exceeded the 340 ppb standard at Champlain Heights ??times in 1994. With reference to the updated one hour standard of 170 ppb introduced in 1995, this was exceeded in 10 out of 12 months of 1994 at Champlain Heights, and in two months at Forest Hills.

In 1995, monthly SO2 concentrations were similar to those seen in 1994, except that levels were lower at Champlain Heights. Whereas five months of 1994 had mean values of 10 ppb or higher, no month of 1995 exceeded 8 ppb at this site. The tendency for highest monthly means in the summer months at Forest Hills, apparent in 1994, was repeated in 1995. There is little sign of any seasonal cycle at Champlain Heights, whereas at the Customs building, concentrations tend to be higher in the winter months. This could be due to the influence of local heating boilers in winter affecting the monitor at the Customs Building.

The seasonal effect at Forest Hills is probably due to seasonal changes in wind direction. In the warmer months, southwesterly winds are dominant, which bring air to the site which has passed over the major emissions sources of the Courtenay Bay generating station and the oil refinery. In winter, westerly and northwesterly winds are most common, which usually often do not pass over these sources.

During 1995, three additional SO2 monitors were operating in the city and data are reported here in chart form. These sites were located specifically to monitor more closely the effect of emissions from the oil refinery on ambient sulphur dioxide concs.

The Forest Products Irving monitoring site is located to the south of the oil refinery and is most affected by refinery emissions in the winter months due to more frequent north to northwesterly winds at that time. Values in summer are much lower. Therefore the seasonal cycle is opposite to that at Forest Hills, but due to the same seasonal shift in wind directions. Peak hourly concentrations at the Forest Products location exceeded the 170 ppb standard in February 1995 and in each of the last four months of the year. The 340 ppb standard was not exceeded during the year at any of the SO2 monitoring sites.

The 1-hour 170 ppb standard was exceeded once ?? during 1995 at the Silver Falls monitoring site (in July). SO2 levels peaked during the summer months at this site. At the Three Mile Irving site, monthly means remained low throughout the year in 1995, with no peak values approaching ambient standards.

At the Hillcrest site, SO2 readings were moderate to low in all months with a minimum in summer and no exceedances of standards.

Taking a longer term view, annual average sulphur dioxide levels in Saint John fell from the mid 1970's until the mid 1980's, Since then, the trend has been mixed, with some sites showing a continued decline, and others such as Forest Hills, showing a reversion to higher values in the early 1990's. However since 1992, SO2 at Forest Hills has fallen progressively, and in 1995, annual mean concentrations were similar at all sites. The annual means recorded in 1995 were approximately 60% lower than those prevalent in the late 1970's and early 1980's, and close to the lowest recorded since 1976. Considering a three year, multi-station moving average, SO2 levels appear to be still falling slightly, but have not fallen significantly since 1985.