A Case Study of Ozone Production in a Rural Area of Central Ontario

An O3 episode observed at Dorset, a rural site in central Ontario, during a stagnant high pressure period of the intensive Eulerian Model Evaluation Field Study (EMEFS), in the summer of 1988, is simulated using a photochemical box model with a two-layer treatment. In the model analysis, natural hydrocarbon chemistry is simulated based on an isoprene-only scenario. Sensitivity tests indicate that local isoprene emissions are an important contributor to local O3 production, relative to anthropogenic hydrocarbons (AHCs), during the event. The model calculated isoprene contribution to the local O3 production, defined as the ratio of the O3 amount formed in the absence of AHCs to that in the presence of AHCs, is characterized by a strong NOx dependence. A minimum value (50%) of the contribution was found at a NOx level of 6 ppbv for the representative hydrocarbon composition during the episode. At this NOx level, O3 production was strongly influenced by the presence of AHCs. At significantly higher or lower NOx levels, isoprene is more important than AHCs in the local O3 production

Summertime NOx NOy and Ozone at a Site in Rural Ontario: Results from SONTOS

Measurements of NOy, NOx NO, SO2, PAN, HNO3, O3, and CO, obtained as part of SONTOS, have been examined to give a picture of the typical chemistry at this location, and for comparison with other North American sites. The total amount of odd nitrogen (NOy) at the Hastings site is less than found at other North American sites, due to the distance of the site from sources and the absence of major anthropogenic sources north west of the site. The air masses also appear younger, containing a higher fraction of NOx, although the chain length for ozone production is similar. The ozone concentrations at the site are lower than reported elsewhere, and the late afternoon maximum shows that it is typically produced elsewhere and transported to the site. A similar conclusion can be drawn from the PAN measurements. There appears to be a significant local NOx source. The flux needed to explain the observations is within the range of values from soil emissions, but a strong correlation with CO, and no correlation with SO2, suggest a mobile combustion source

Observational Evidence For The Impact of the Lake Breeze Circulation on Ozone Concentrations in Southern Ontario

Very rapid increases in the concentrations of ozone and ozone precursors, in the late afternoon, have been observed at a rural and an urban site in southern Ontario. Ozone concentration increases of 30 ppbv in a few minutes have been observed. These increases occur simultaneously with the arrival of a Lake Ontario lake breeze front as identified from meteorological measurements and visible satellite imagery. This indicates that polluted air masses from over Lake Ontario are being transported inland by the lake breeze. Aircraft measurements of ozone, NOx, and hydrocarbons show such an air mass moving inland. Chemical measurements at the sites show that the polluted air masses are not of local origin, but are of similar age to those regularly encountered in rural area

Impact of Toronto Urban Emissions on Ozone Levels Downwind - A Case Study

During the 1992 Southern Ontario Oxidants Study (SONTOS 92), an impingement of the Greater Toronto urban plume was observed in the late afternoon of 6 August at the Hastings monitoring site, 140 km to the northeast of the Toronto urban core. Associated with the start of the impingement, sharp increases in the concentration of O3 and other species were observed. A 1 D photochemical transport model was used to investigate this event further. The 1D model was first exercised in an Eulerian mode to generate two initial chemical systems, one for the air parcel associated with the Greater Toronto urban core and the other characterizing the background. Lagrangian calculations with the 1D model were then conducted following both the urban plume and the background air parcels. When consecutive plume puffs passed over preset virtual receptors, the time of passage and species concentrations, both within the plume and in the background air, were registered. The calculated and measured changes in the concentrations of O3, PAN, NOx and hydrocarbon compounds due to the urban plume impingement are in reasonable agreement. Further sensitivity studies were conducted and the factors affecting the impact of the Greater Toronto urban plume are discussed

Observation of O3 and Precursors Levels at Two Sites Around Toronto-Ontario, During SONTOS 92

Characteristic air quality features observed at two ground level air quality monitoring stations operated during the Southern Ontario Oxidant Study field campaign (SONTOS 92) are described. The two sites, Binbrook and Hastings, are located 60 km SSW and 140 km NE of Metropolitan Toronto, respectively. The analysis of 5-min average concentrations in relation with local wind directions-suggests that a strong contrast exists between conditions of northerly clean air and polluted southwesterly flows at both locations. This contrast is shown to deeply influence the shape and amplitude of the diurnal cycles of O3, NO and NOz. The analysis indicates that the downward transport of either clean or polluted background air during the first part of the day, as the boundary layer develops, is responsible for the alteration of the diurnal patterns with the prevailing wind direction between 08:00 and 10:00 (EDT). At Hastings, NMHC/NOx ratios are found to be relatively elevated ( > 8 ppbC ppb−1) and highly variable for both the clean northerly and the southwesterly polluted regimes. At this site and for the polluted southwesterly flow, an average of 11.2 molecules of O3 appear to be formed for each molecule of (NOx) oxidized. Estimated values for “backgrounde” ozone levels associated with the different sectors, as well as for the contribution of major urban centres in the region are discussed

The Southern Ontario Oxidant Study (SONTOS): Overview and Case Studies for 1992

The southern Ontario oxidant study (SONTOS) is one of the scientific responses to the problem of ozone exceedences in the Windsor/Quebec City corridor, the strip extending from Windsor in the extreme southwest of Ontario, along the north shores of Lake Erie and Lake Ontario and down the St Lawrence valley. This corridor includes the two major metropolitan areas of Toronto and Montreal and experiences more exceedences of the Canadian ozone guideline per year than any other part of the country. The overall objective of SONTOS is to elucidate the factors which 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 in the summer of 1992. Although this summer was noteworthy for generally very low ozone concentrations, a number of interesting observations have been made which have a bearing on the oxidant phenomenon in southern Ontario. In particular, this paper describes two specific events. In one, which occurred on 6 August 1992, the intensive measurement site 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