Airborne boundary layer flux measurements of trace species over Canadian boreal forest and northern wetland regions

Abstract

Airborne heat, moisture, O3, CO, and CH4 flux measurements were obtained over the Hudson Bay lowlands (HBL) and northern boreal forest regions of Canada during July–August 1990. The airborne flux measurements were an integral part of the NASA/Arctic Boundary Layer Expedition (ABLE) 3B field experiment executed in collaboration with the Canadian Northern Wetlands Study (NOWES). Airborne CH4 flux measurements were taken over a large portion of the HBL. The surface level flux of CH4 was obtained from downward extrapolations of multiple‐level CH4 flux measurements. Methane source strengths ranged from −1 to 31 mg m2− d−1, with the higher values occurring in relatively small, isolated areas. Similar measurements of the CH4 source strength in the boreal forest region of Schefferville, Quebec, ranged from 6 to 27 mg m−2 d−1 and exhibited a diurnal dependence. The CH4 source strengths found during the ABLE 3B expedition were much lower than the seasonally averaged source strength of 51 mg m−2 d−1 found for the Yukon‐Kuskokwim delta region of Alaska during the previous ABLE 3A study. Large positive CO fluxes (0.31 to 0.53 parts per billion by volume (ppbv) m s−1) were observed over the inland, forested regions of the HBL study area, although the mechanism for the generation of these fluxes was not identified. Repetitive measurements along the same ground track at various times of day near the Schefferville site also suggested a diurnal dependence for CO emissions. Measurements of surface resistance to the uptake of O3 (1.91 to 0.80 s cm−1) for the HBL areas investigated were comparable to those observed near the Schefferville site (3.40 to 1.10s cm−1). Surface resistance values for the ABLE 3B study area were somewhat less than those observed over the Yukon‐Kuskokwim delta during the previous ABLE 3A study. The budgets for heat, moisture, O3, CO, and CH4 were evaluated. The residuals from these budget studies indicated, for the cases selected, a moderate net photochemical production of O3 present in the boundary layer over the HBL that coincided with an in situ destruction of CO, although the mechanism responsible for the destruction of CO was not identified. Results from the O3 budget analysis indicate the importance of in situ photochemical production and its possible dominance over surface deposition to the local O3 budget at the Schefferville site. Measurements of the in situ production of O3 indicated a direct relationship between the presence of biomass burning or large‐scale pollution effects. Residuals from budget calculations for conserved quantities (heat, moisture, and CH4) were compared with their respective surface fluxes to provide a measure of the internal selfconsistency of the flux measurements.