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 degrees N-55 degrees S latitude from ATMOS profiles measured during 4 shuttle flights, Measured mixing ratios increase with potential temperature (theta) 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 theta level. Evidence for some confinement near 1150 K in the developing November 1994 vortex is indicated from comparison of CO and N2O 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 CH4 as inputs, Differences decrease with latitude at constant theta and are <20% at 800 K and all latitudes, where the CO photochemical lifetime is 40-50 days.