National Ambient Air Quality Objectives For Ground-Level Ozone- Summary - Science Assessment Document

3. Sources

As ozone is a secondary pollutant, the question of sources of ground level ozone refers to sources of the gaseous precursors, NO_x and VOC. Both NO_x and VOC have anthropogenic (from human) and biogenic (from biological) sources. Anthropogenic NO_x is the product of both stationary and mobile combustion processes. Nitrogen in the fuel source combines with atmospheric oxygen at high temperature to form several NO_x species, of which NO and NO₂ are the most common. The primary anthropogenic sources of NO_x are transportation sources (i.e. vehicles), thermal electrical power plants and certain industrial processes. Emissions of NO_x from natural sources account for a small percentage of total emissions. These come from forest fires, lightening and soil microbial activity.

Many hundreds or even thousands of different organic compounds fall under the umbrella of VOC. Anthropogenic VOC are released into the atmosphere through combustion and evaporation processes. The largest sources are industrial processes (principally solvent use, petroleum refining and distribution, and chemical manufacturing of various kinds) and transportation sources. Biogenic emissions of VOC, principally from vegetation, can be substantial. Isoprenes from deciduous forests and monoterpenes from coniferous forests are the most relevant biogenic VOC involved in ozone formation.

A national emissions inventory is compiled by Environment Canada, in collaboration with the provinces and territories, every 5 years. The inventory provides emission estimates for approximately 80 different sectors, within 5 major source categories: industrial, transportation, non-industrial fuel combustion, incineration and miscellaneous sources. The process of acquiring, compiling and reviewing the emissions data is a lengthy one. The 1995 Canadian Emission Inventory has only recently been finalized (see http://www.ec.gc.ca/pdb/cac/cacdoc/1995e/main95.html (for the tables in English); and http://www.ec.gc.ca/pdb/cac/cacdoc/1995f/main95f.html (for the tables in French). The results of the 1990 Emission Inventory are shown in Figures 1 and 2 (SAD Figures 4.1 and 4.2). It has been estimated that the range of uncertainty in the 1990 Inventory for NO_x emissions is about 20% and probably even higher than that for VOC emissions.

Within Canada, based on the 1990 Emission Inventory, annual NO_x emissions totaled approx. 2,060 kilotonnes, with 61% originating from the transportation sector, 23% from the industrial sector, and 12% from the electric power generating sector. VOC emissions were estimated at approx. 2,580 kilotonnes. The transportation sector produced 31% and the industrial sector 33%. Applications of surface coatings (e.g. paints), general solvent use and other miscellaneous sources accounted for 24% of VOC emissions. Biogenic emissions of VOC were estimated at 14,645 kilotonnes in 1990. Although this amount represents about 5.5 times the total anthropogenic contribution of VOC, in most urban areas of Canada, anthropogenic emissions exceed biogenic emissions. However, biogenic emissions of VOC undoubtedly play an important role at times in both non-urban and urban ozone levels depending on proximity to sources and the prevailing wind speed and direction.

Figure 1: 1990 Canadian NO_x emissions by sector (total = 2,060 kilotonnes)

Figure 2: 1990 Canadian VOC emissions by sector (total = 2,579 kilotonnes)