Gravitational settling in the Ar/N2 Ratio observed during the START08 and HIPPO-Global Airborne campaigns

The Ar/N2 ratio may be used as a tracer of atmospheric mixing and transport because of its inert nature and tendency to settle gravitationally. Recent papers demonstrate enrichment of the Ar/N2 ratio in low-altitude night-time inversions (Adachi et al. 2006), and a distinct profile in the arctic from surface to lower stratosphere (Ishidoya et al. 2008). Presented here are the Ar/N2 ratio data from the START08 and HIPPO-Global(I) campaigns in April-June 2008, and January 2009, respectively. An averaged profile based on 270 HIPPO samples distributed between 82 North and 67 South and from 1 to 14 km shows consistency within 10 per meg to roughly 6 km, but a steadily decreasing Ar/N2 ratio above that. Average Ar/N2 ratio at 14 km are roughly 45 per meg lower than near-surface values. The trend above 6 km is roughly consistent with Ishidoya et al. especially if allowing for larger gradients at the poles. An averaged profile of 140 START08 samples taken over the Midwest United States and Canada, distributed between 26 and 62N and 1:14 km shows a slightly different profile, with consistency within 10 per meg until 10 km, and a steadily decreasing Ar/N2 ratio above this. We will discuss these data in the context of possible analytical artifact and in the context of their potential as an indicator of atmospheric mixing. io at the tropopause