Ground-based measurements of ClO from Mauna Kea and intercomparisons with Aura and UARS MLS

The ground-based measurements of upper stratospheric ClO, made with a ground-based millimeter wave instrument at Mauna Kea, Hawaii (19.8 degrees N, 204.5 degrees E) starting in 1992, are compared with UARS (Upper Atmosphere Research Satellite) MLS ClO measurements (1991-1998) and the Aura MLS ClO measurements (2004-2009). The ground-based measurements are made as part of the Network for the Detection of Atmospheric Composition Change (NDACC). Intercomparisons between the ground-based measurements and the Aura MLS measurements show that both instruments retrieve similar seasonal variations over Mauna Kea. The seasonal variation in ClO is also compared with measurements of variations in stratospheric CH4, which affects the partitioning of total inorganic chlorine. Using the ground-based instruments as a transfer standard, we find that the agreement between UARS and Aura MLS ClO measurements near the peak of the mixing ratio profile is within similar to 1%. Combining the uncertainties in the biases calculated from the coincident ground-based and satellite measurements, we find that using the ground-based data as a transfer standard allows us to provide a 2 sigma limit to the bias between the UARS and Aura measurements of 3%-4% near the peak of the ClO profile. Given agreement between UARS and Aura MLS of similar to 1%+/- 4%, there is no reason to apply any bias correction in order to use the UARS and Aura MLS ClO measurements as a single data set.