ORIGIN OF TROPOSPHERIC NOX OVER SUB-ARCTIC EASTERN CANADA IN SUMMER

The origin of NO(x) in the summertime troposphere over subarctic easte rn Canada is investigated by photochemical modeling of aircraft and gr ound-based measurements from the Arctic Boundary Layer Expedition (ABL E 3B). It is found that decomposition of peroxyacetyl nitrate (PAN) ca n account for most of the NO(x) observed between the surface and 6.2 k m altitude (aircraft ceiling). Forest fires represent the principal so urce of PAN in the region, implying the same origin for NO(x). There i s, however, evidence for an unidentified source of NO., in occasional air masses subsiding from the upper troposphere. Isoprene emissions fr om boreal forests maintain high NO(x) concentrations in the continenta l boundary layer over eastern Canada by scavenging OH and NO3, thus sl owing down conversion of NO(x) to HNO3, both in the daytime and at nig ht. This effect is partly compensated by the production of CH3CO3 radi cals during isoprene oxidation, which slows down the decomposition of PAN subsiding from the free troposphere. The peroxy radical concentrat ions estimated from concurrent measurements of NO and NO2 concentratio ns during ABLE 3B are consistent with values computed from our photoch emical model below 4 km, but model values are low at higher altitudes. The discrepancy may reflect either a missing radical source in the mo del or interferences in the NO2 measurement.