Stratospheric CO at tropical and mid‐latitudes: ATMOS measurements and photochemical steady‐state model calculations

We characterize the spring and fall stratospheric distribution of CO at 49°N‐55°S latitude from ATMOS profiles measured during 4 shuttle flights. Measured mixing ratios increase with potential temperature (θ) from 12 ppbv (10 −9 per unit volume) at 525 K, to 30–40 ppbv at 1750 K with only minor variations with latitude and season at a θ level. Evidence for some confinement near 1150 K in the developing November 1994 vortex is indicated from comparison of CO and N 2 O horizontal gradients. Measured CO mixing ratios at the tropical tropopause are a factor of 10 higher than values calculated with a steady‐state model using standard photochemistry constrained by correlative temperatures and pressures, and ATMOS measurements including CH 4 as inputs. Differences decrease with latitude at constant θ and are <20% at 800 K and all latitudes, where the CO photochemical lifetime is 40–50 days.