Tropospheric hydroxyl and atomic chlorine concentrations, and mixing timescales determined from hydrocarbon and halocarbon measurements made over the Southern Ocean

During the First Aerosol Characterization Experiment (ACE 1) field campaign, 1419 whole air samples were collected over the Southern Ocean, of which approximately 700 samples were collected in the marine boundary layer (MBL), 300 samples were taken in the free troposphere (FT), and the remainder were collected in the buffer layer (BuL), the layer between the MBL and FT. Concentrations of tetrachloroethene, ethane, ethyne, and propane decayed over the 24 day duration of the intensive portion of the field campaign, which began November 18, 1995. This decline was consistent with what is known about seasonal increase of HO and the seasonal decrease in biomass burning. Using a simple empirical model, the best fit to the observations was obtained when the average [HO] was 6.1 ± 0.3 × 10 5 HO cm −3 , and an average [Cl] of 720±100 Cl cm −3 . The corresponding exchange times were 14±2 days between the MBL and FT, and 49 + 40/−13 days between the MBL in the intensive campaign region and the MBL region to the north (nMBL).