Nitrogen oxides in the Arctic troposphere

Abstract

Nitrogen oxides play a critical role in tropospheric photochemistry. In order to characterize these compounds in the arctic troposphere, ground-level concentrations of total reactive nitrogen (NO$\sb{y}$) and NO were determined over an extended period at a site near Barrow, Alaska. A high-sensitivity instrument developed for this purpose was used in three measurement campaigns: summer 1988, spring 1989, and March-December 1990. During the 1990 campaign, the detection limit for NO was 3-10 pptv (depending on averaging period), and the NO$\sb{y}$ uncertainty was $\pm$26%. A screening algorithm was applied to the data to eliminate effects from local (Barrow) sources, and the remaining data were divided into "background periods" (unaffected by local or regional NO$\sb{x}$ sources), and "events" (periods when emissions from a regional NO$\sb{x}$ source--the Prudhoe Bay oil-producing region--apparently impacted Barrow). These measurements revealed a sharp seasonal cycle of background NO$\sb{y}$ concentrations, with high values in early spring (median 560-620 pptv) and $\sim$70 pptv (median) during summer. This cycle is similar to that of other compounds in arctic haze but is partially attributed to a reduction in NO$\sb{y}$ lifetime due to organic nitrate decomposition as temperatures and insolation increased. Evidence indicates that the springtime arctic NO$\sb{y}$ reservoir was primarily composed of stable removal-resistant species, including PAN and other organic nitrates. PAN decomposition as temperatures rose in late spring likely caused an observed pulse of NO to $\sim$35 pptv (maximum hourly average); hourly-average NO concentrations were otherwise generally $<$8 pptv. NO$\sb{x}$ production from PAN decomposition due to the onset of spring or southward advection may affect springtime O$\sb3$ levels both in the Arctic and in the northern mid-latitudes. NO$\sb{y}$ and O$\sb3$ concentrations were positively correlated during summer, possibly indicating long-range transport of both and/or the presence of a mid-tropospheric NO$\sb{y}$ reservoir combined with a stratospheric O$\sb3$ source. A number of events with substantially elevated NO$\sb{y}$ concentrations (to 16 ppbv) were observed in air not impacted by emissions from the town of Barrow. Substantial evidence indicates that these events were a result of NO$\sb{x}$ emissions from the Prudhoe Bay region ($\sim$300 km to the ESE), which is also expected to affect measurements of other compounds at the Barrow site.