Measurements of reactive oxidized nitrogen at eight Canadian rural sites

The partitioning of reactive oxidized nitrogen (NOy) across eastern Canada was studied through the measurements of total NOy and the major individual species comprising NOy (NO, NO2, PAN, PPN, HNO3, and particle-NO3−) during 14 short-term (18–50 days) field campaigns at eight selected rural sites from 2001 to 2005. Significant differences were found between populated and remote regions in the budget and partitioning of NOy and in their seasonal and diurnal variations. Median NOy mixing ratios were higher than 5 ppbv at rural sites located in populated regions and lower than 1 ppbv at remote locations. NOy mixing ratios were 30–40% higher during the cold seasons compared to the warm and hot seasons at all locations. Seasonal variations of NOx were much larger than NOy in populated regions, although their seasonal variations were of a similar magnitude at remote locations. On average, NOx contributed 50–80% to total NOy during the cold seasons and 30–60% during the warm and hot seasons; PAN, HNO3 and NO3− also contributed an appreciable amount (each up to 19%) to total NOy. PPN mixing ratios were 10–15% of PAN during the warm and hot seasons and were 15–25% during the cold seasons. The NOy deficit was found to be 15–42%, depending on location, during the warm and hot season. The NOy deficit was generally smaller than 10% during the cold season, although a 25% deficit was also observed during one cold-season campaign. The maximum NOy deficit seemed to appear in early spring instead of mid-summer at locations where nearby sources had little impact.