Summertime distribution and relations of reactive odd nitrogen species and NO_y in the troposphere over Canada

We report here large‐scale features of the distribution of NO x , HNO 3 , PAN, particle NO 3 − , and NO y in the troposphere from 0.15 to 6 km altitude over central Canada. These measurements were conducted in July–August 1990 from the NASA Wallops Electra aircraft as part of the joint United States‐Canadian Arctic Boundary Layer Expedition (ABLE) 3B‐Northern Wetlands Study. Our findings show that this region is generally NO x limited, with NO x mixing ratios typically 20–30 parts per trillion by volume (pptv). We found little direct evidence for anthropogenic enhancement of mixing ratios of reactive odd nitrogen species and NO y above those in“background”air. Instead, it appears that enhancements in the mixing ratios of these species were primarily due to emissions from several day old or CO‐rich‐NO x ‐poor smoldering local biomass‐burning fires. NO x mixing ratios in biomass‐burning impacted air masses were usually <50 pptv, but those of HNO 3 and PAN were typically 100–300 pptv representing a twofold‐threefold enhancement over “background” air. During our study period, inputs of what appeared to be aged tropical air were a major factor influencing the distribution of reactive odd nitrogen in the midtroposphere over northeastern North America. These air masses were quite depleted in NO y (generally <150 pptv), and a frequent summertime occurrence of such air masses over this region would imply a significant influence on the reactive odd nitrogen budget. Our findings show that the chemical composition of aged air masses over subarctic Canada and those documented in the Arctic during ABLE 3A have strikingly similar chemistries, suggesting large‐scale connection between the air masses influencing these regions.