| Summary: | This paper discusses in situ measurements of O3, aerosol number density, and aerosol size obtained during NASA Arctic Boundary Layer Expedition aircraft flights over the Alaskan Arctic region. The major source of summer O3 for the troposphere in the intrusion of stratospheric air and subsequent transport to lower altitudes. Photochemistry of mixed layer emissions and O3 transported from high northern latitude urban/industrialized areas do not appear to play major roles as sources of O3 for the Alaska region. O3 gradients reflect the loss at the surface and supply from the stratosphere. Free tropospheric O3 averaged 74 ppbv compared to 32 ppbv for the mixed layer. O3 loss mechanisms are a combination of the destruction via photochemistry, chemical reaction with surface emissions, and direct loss through deposition to the surface. The boreal forest in the most efficient of the O3 sinks and has the largest increase in aerosol number density relative to the free troposphere.
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