THE SOUTHERN ONTARIO OXIDANT STUDY (SONTOS) - OVERVIEW AND CASE-STUDIES FOR 1992

The southern Ontario oxidant study (SONTOS) is one of the scientific r esponses to the problem of ozone exceedences in the Windsor/Quebec Cit y corridor, the strip extending from Windsor in the extreme southwest of Ontario, along the north shores of Lake Erie and Lake Ontario and d own the St Lawrence valley. This corridor includes the two major metro politan areas of Toronto and Montreal and experiences more exceedences of the Canadian ozone guideline per year than any other part of the c ountry. The overall objective of SONTOS is to elucidate the factors wh ich lead to these high levels, The study design is based on intensive measurements made at a site to the NNW of Toronto, with supplementary measurements at other sites and on board an aircraft. This paper, and the seven which follow, describe and interpret the measurements made i n the summer of 1992. Although this summer was noteworthy for generall y very low ozone concentrations, a number of interesting observations have been made which have a bearing on the oxidant phenomenon in south ern Ontario. In particular, this paper describes two specific events. In one, which occurred on 6 August 1992, the intensive measurement sit e experienced the impact of a polluted air mass, which had apparently travelled to the site from Toronto, by way of Lake Ontario. This case has been contrasted with an event in the last week of August, in which large-scale, organised southwesterly flow led to the conditions which more typically give rise to widespread ozone episodes in the province . Events similar to the 6 August episode occurred on several occasions during the study period, and underline the very important role of the Great Lakes in ozone transport and delivery in southern Ontario. Copy right (C) 1996 Elsevier Science Ltd