Reactive nitrogen oxides and ozone above a taiga woodland

Measurements of reactive nitrogen oxides (NO x and NO y ) and ozone (O 3 ) were made in the planetary boundary layer (PBL) above a taiga woodland in northern Quebec, Canada, during June–August, 1990, as part of NASA Artie Boundary Layer Expedition (ABLE) 3B. Levels of nitrogen oxides and O 3 were strongly modulated by the synoptic scale meteorology that brought air from various source regions to the site. Industrial pollution from the Great Lakes region of the U.S. and Canada appears to be a major source for periodic elevation of NO x , NO y and O 3 . We find that NO/NO 2 ratios at this site at midday were approximately 50% those expected from a simple photochemical steady state between NO x and O 3 , 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 NO 2 , at the taiga site. Ratios of NO x to NO y were typically 2–3 times higher in the PBL during ABLE 3B than during ABLE 3 A. This is probably the result of high PAN levels and suppressed formation of HNO 3 from NO 2 due to high levels of biogenic hydrocarbons at the ABLE 3B site.