Measurements of HONO in Southern Ontario using long path absorption photometry

A highly sensitive Long Path Absorption Photometer (LOPAP) was constructed and used to investigate HONO chemistry at rural and urban sites in Southern Ontario. The instrument utilizes liquid core waveguides (LCW's) to achieve part per trillion (pptv) sensitivities. The 30 detection limit of the instrument was determined to be 5 pptv. The only interference of note was NO2 and it was quantitatively removed by the subtraction of the signal due to NO2 in a second channel.;Analysis of the daytime data from both the forested site and BAQS-Met suggests a HONO source with a dependence on UV radiation, NO2 and H20 mixing ratio. The data set at Harrow, a site impacted by different pollution sources depending on wind direction, produced a variety of apparent rate constants for HONO production in the range of 2.9-7.8 x 10-13 cm3molec-1s-1 which suggests that the HONO production was heterogeneous. In contrast at the forested site at Borden the apparent rate constants varying from only 1.2x10 -13 to 1.6 x 10-13 cm3molec -1s-1. These are similar to one reported (but disputed) value for a gas phase process involving photoexcited NO2 in a reaction with water vapour which may yield HONO, but the small variation in rate is more likely a result of the lack of dependence of the nature of the fetch on wind direction at Borden.;The instrument was deployed in Southern Ontario for the measurement of HONO at three different measurement sites, one urban (Toronto), one forested (Borden) and one semi-rural (BAQS-Met, Harrow super site). High concentrations of HONO were observed at the urban site, which was located next to a major highway. These concentrations can largely be explained by traffic emissions from the highway. While the largest HONO mixing ratios measured were found at the urban sampling site, high HONO/NO2 ratios were observed at the forested site (as high as 16%, mean=1.9%). The forested site (Borden, Ontario) was a very clean site with mean NOx concentrations of only 1.5 ppbv. The mean first order conversion rate of NO2 to HONO during the night-time hours at this site was found to be 1.4 X 10-6 s-1, two orders of magnitude smaller than HONO photolysis at midday. Since measurable amounts of HONO were present during the midday hours this lead to the conclusion that there was a very efficient conversion process of NO2 to HONO in the forested environment. High daytime concentrations of HONO were also observed at the Harrow supersite during the BAQS-Met campaign. These concentrations were also well in excess of predicted HONO values using traditional HONO chemistry,...