Stratospheric Sources of Surface Ozone and Methane during Arctic Winter
S. Kirkwood, E. Nisbet, J. Hatakka
High ozone concentrations at ground level are a potential source of damage for human health, for ecosystems and for materials. In urban areas, local pollution can lead to high ozone concentrations. In addition to this, particularly at middle and high latitudes, down-folding of naturally ozone-rich air from the stratosphere into the troposphere occurs. However, despite many experimental and modelling studies, it is not yet well established whether this contributes significantly to surface ozone. Stratospheric air is also characterised by lower methane concentrations compared to tropospheric air, with higher 13C/12C and D/H ratios due to processing in the upper atmosphere. Stratospheric methane is transported into the troposphere by the same processes as stratospheric ozone. Using measurements by ESRAD and from the Pallas GAW station, we can show that turbulent down-mixing of air with high ozone and low methane concentrations affects surface air in Arctic Scandinavia throughout the winter, from September to March. Due to this process, surface ozone concentrations increase from ~30 ppmv to ~45 ppmv (90 μg/m3) over the course of the winter, reaching 75% of the WHO target for health protection. The same process is likely to occur over much of northern Europe which implies that goals to reduce surface ozone by pollution control must take account of this substantial and unavoidable natural background. Turbulent mixing also reduces the surface methane concentration by 10 ppmv between January and March, accounting for more than 30 % of the annual cycle. The results therefore imply that the stratospheric influence must be taken into account in interpreting methane and methane-isotope records at the surface, for example in ice-core records.
Created 2006-05-30 09:13:24 by Mats HolmstrÃ¶m Last changed 2006-05-30 09:39:46 by Mats HolmstrÃ¶m