Radical Chemistry at the SONTOS Site in Rural Ontario

A chemical amplifier based radical detector was used to make measurements of radical (ROx = HO2 + HO + RO + RO2) concentrations at a rural site in the summer of 1992, as part of the Southern Ontario Oxidant Study (SONTOS). The average maximum daytime radical concentration was around 7 pptv, with the maximum value recorded being 23 pptv. There was no distinguishable radical signal on any of the cloudy days, whereas there was a clear diurnal variation, maximizing in the mid afternoon on sunny days. The highest radical signals were observed on days of the highest ozone, which were days that showed photochemical ozone production. The local ozone production was calculated from the radical and NO concentrations but, even in the absence of loss terms, was insufficient to explain the ozone increase at the site. This indicates that part of the measured ozone increase is due to ozone production elsewhere followed by transport to the site. Peroxyacetyl nitrate (PAN) decomposition appears to provide about 30% of the radicals measured at the site so that the contribution of PAN as a radical reservoir needs to be considered. Fluctuations in the measured parameters prevented the meaningful determination of radical concentrations from the photostationary state at the low levels encountered.