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Comprehensive Measurements

The Southern Ontario Oxidant Study (SONTOS)


Comprehensive measurements were made near Hastings, Ontario in the summer of 1992, with the objective of improving our understanding of the production of ozone in southern Ontario. Although 1992 was noteworthy for the non-occurrence of ozone episodes, very interesting results were obtained, which hold considerable promise for achieving the stated objectives of the program. Planning is well advanced for a follow up field study in the summer of 1993.

The overall objective of SONTOS was to make progress in elucidating the processes which lead to the occurrence of high ambient oxidant concentrations in the Windsor-Quebec City corridor. Detailed objectives included improving our understanding of the role of natural hydrocarbons, and of the relative contributions of NOx and VOC. A very detailed suite of measurements was made at the Hastings site (Figure 1), as listed below:


  • NO, NO2, NOy, NOx
  • O3
  • PAN
  • Carbonyls
  • Organic nitrates
  • Peroxy radicals
  • Hydrogen peroxide
  • Formaldehyde
  • C2 to C10 hydrocarbons
  • SO2
  • Meteorology (including acoustic sounder)

An important feature of the study design was the inclusion in this list of precursors (e.g., NOx and speciated VOC), intermediates (peroxy radicals) and products (e.g., organic nitrates, formaldehyde, hydrogen peroxide, carbonyl compounds, etc.) In addition, measurements of ozone, NOx, and VOC, were made on board an aircraft and at a site upwind of Toronto.

The measurement campaign was a cooperative venture between several groups drawn from the Air Resources Branch of the Ministry of Environment and Energy, Ontario Hydro, York University, and Unisearch Associates, with CIRAC providing administrative coordination. Scientific coordination took place through regular meetings of a committee of all participants, chaired by Dr. Neville Reid of the Air Resources Branch.

Individual research groups have been responsible for quality assuring and storing their own data, and a data catalogue has been published describing data availability. Synthesis of results is currently under way, following a very successful Data Evaluation Workshop held on 25 and 26 February 1993. Although the summer of 1992 may be characterised as the summer without ozone, two very interesting periods were identified during the workshop, and will be reported in detail as case studies.

The first of these was on 6 August 1992, when the ozone concentration at Dorset increased abruptly from about 45 to 65 ppb between 5.30 and 5.50 p.m. (Figure 2). Similar, abrupt changes in most of the other measured parameters were also observed. The preliminary interpretation is that impingement occurred at that time of a plume of aged, polluted air, probably brought down from aloft after passing over Lake Ontario.

The second case study period occurred towards the end of August, and is noteworthy because it had all the hallmarks of a "classical" ozone episode (temperature close to 30 C, air parcels travelling from the south), but the maximum ozone concentration observed during the period 22 to 25 August barely exceeded 80 ppb. At the same time, the concentrations of other oxidised species, such as nitric acid, reached unusually high levels, a circumstance which would normally accompany much higher ozone concentrations. If the reasons why ozone was not produced in higher concentrations can be understood, it will shed important light on the measures which will be required to control ozone production in southern Ontario. For example, if the concentrations of certain hydrocarbons were unusually low during the period in question, it would suggest that controlling the emissions of these hydrocarbons would be an effective way of controlling ozone production.

Plans are well advanced for the Southern Ontario Oxidant Study to continue with a second campaign in the summer of 1993. It is to be hoped that the atmosphere will cooperate in providing ozone to be studied!


The basis of the study plan is similar to what took place in 1992, but important additions are planned. A similar study will also take place in southeastern Michigan (SEMOS - the Southeast Michigan Oxidant Study), and additional meteorological measurements are proposed for southern Ontario, to allow the link to be made between the two studies, both in the interpretation of the data, and in the subsequent modelling of the transport and transformation processes. In addition, it is planned that more extensive measurements of aerosols, their formation, and their participation in the chemistry of oxidants will be made.

Parts of the planned field campaign are already funded by the Ontario Ministry of Environment and Energy, and funding has also been promised by Ontario Hydro and Consumers Gas. The remainder of the required funding is currently being sought.

Updated on December 13th, 2012.