Origin of tropospheric NO x over subarctic eastern Canada in summer

The origin of NO x in the summertime troposphere over subarctic eastern Canada is investigated by photochemical modeling of aircraft and ground‐based measurements from the Arctic Boundary Layer Expedition (ABLE 3B). It is found that decomposition of peroxyacetyl nitrate (PAN) can account for most of the NO x observed between the surface and 6.2 km altitude (aircraft ceiling). Forest fires represent the principal source of PAN in the region, implying the same origin for NO x . There is, however, evidence for an unidentified source of NO x in occasional air masses subsiding from the upper troposphere. Isoprene emissions from boreal forests maintain high NO x concentrations in the continental boundary layer over eastern Canada by scavenging OH and NO 3 , thus slowing down conversion of NO x to HNO 3 , both in the daytime and at night. This effect is partly compensated by the production of CH 3 CO 3 radicals during isoprene oxidation, which slows down the decomposition of PAN subsiding from the free troposphere. The peroxy radical concentrations estimated from concurrent measurements of NO and NO 2 concentrations during ABLE 3B are consistent with values computed from our photochemical model below 4 km, but model values are low at higher altitudes. The discrepancy may reflect either a missing radical source in the model or interferences in the NO 2 measurement.