Reactive nitrogen oxides and ozone above a taiga woodland

Abstract

Measurements of reactive nitrogen oxides (NOₓ and NO{sub y}) and ozone (O₃) were made in the planetary boundary layer (PBL) above a taiga woodland in northern Quebec, Canada, during June-August, 1990, as part of NASA Arctic Boundary Layer Expedition (ABLE) 3B. Levels of nitrogen oxides and O₃ were strongly modulated by the synoptic scale meteorology that brought air from various regions to the site. Industrial pollution from the Great Lakes region of the US and Canada appears to be a major source for periodic elevation of NOₓ, NO{sub y} and O₃. We find that NO/NO₂ ratios at this site at midday were approximately 50% those expected from a simple photochemical steady state between NOₓ and O₃, in contrast to our earlier results from the ABLE 3A tundra site. The difference between the taiga and tundra sites is likely due to much larger emissions of biogenic hydrocarbons (particularly isoprene) from the taiga vegetation. Hydrocarbon photooxidation leads to relatively rapid production of peroxy radicals, which convert NO to NO₂, at the taiga site. Ratios of NOₓ to NO{sub y} were typically 2-3 times higher in the PBL during ABLE 3B than during ABLE 3A. This is probably the result of high PAN levels and suppressed formation of HNO₃ from NO₂ due to high levels of biogenic hydrocarbons at the ABLE 3B site.