Tropospheric ozone and aerosol observations: The Alaskan Arctic

Abstract

NASA's Arctic Boundary Layer Expedition (ABLE 3A) conducted during the summer of 1988 focused on the distribution of trace species in the Alaskan Arctic troposphere (altitudes <7 km) and the relative importance of surface sources/sinks, local emissions, distant transport, and tropospheric/stratospheric exchange. In situ ozone and aerosol number density and size data obtained during aircraft flights from Point Barrow and Bethel, Alaska, are discussed. Data are also presented for the ferry flights between Wallops Island, Virginia, and Point Barrow, Alaska, via Thule, Greenland. The major source of summer ozone for the troposphere is the intrusion of stratospheric air and subsequent transport to the lower altitudes. Photochemistry of mixed layer emissions and ozone transported from high northern latitude urban/industrialized areas do not appear to play major roles as sources of ozone for the Alaska region. Ozone gradients reflect the loss at the surface and supply from the stratosphere. Free‐tropospheric ozone (3‐ to 7‐km altitude) averaged 74 ppbv compared to 32 ppbv for the mixed layer. All four mixed layers studied (water, wet tundra, dry tundra, and boreal forest) are net ozone sinks. Ozone loss mechanisms are a combination of the destruction via photochemistry, chemical reactions with surface emissions, and direct loss through deposition to the surface. The boreal forest is the most efficient of the ozone sinks. Aerosol data showed that, of the mixed layers studied, the boreal forest has the largest increase in aerosol number density relative to the free troposphere. With the exception of the boreal forest, a significant portion of mixed layer aerosols are from the free troposphere. Results also show that while, in theory, free‐tropospheric air can be classified as originating from continental or maritime regions (Siberia, Canada, Pacific Ocean, Gulf of Alaska), little difference was found in the ozone and fine aerosol number density composition of the air. This is attributed, in part, to modification of the air during transport from its source region.