Partitioning of reactive atmospheric nitrogen oxides at an elevated site in southern Quebec, Canada

Abstract

Measurements of the major reactive nitrogen oxides including NO, NO2, HNO3, particulate nitrate, PAN, and NOy, as well as O3 and supporting meteorology, were made at an elevated site (845 masl) in the rural eastern townships of Quebec, Canada, from February 1998 to October 1999. NOx (and NOy) mixing ratios exhibited a distinct seasonal cycle with median NOx levels in winter more than a factor of 4 greater than in summer. Seasonal partitioning indicated that NOx was the dominant fraction of NOy during winter (>70%), with HNO3 and PAN fractions of <18 and 9%, respectively. The NOy reservoir in summer consisted of average contributions from NOx, HNO3, and PAN of 35, 20–25, and 13%, respectively. A NOy deficit of the order of 25–30% was observed during spring and summer that could not be explained by measurement uncertainties or potential interferences. It is possible that there were other reactive nitrogen species in the atmosphere that were not separately measured that contributed to the NOy deficit. Indications are that these compounds were photochemically produced and present in highly processed air masses.