Interhemispheric gradient of hydroxyl in the troposphere

The hydroxyl radical (OH) in the atmosphere plays a significant role in air pollution chemistry by modulating the oxidizing capacity of the atmosphere and removal of important non-CO2 greenhouse gases. Although the global total OH abundance in the troposphere and its interannual variability have been estimated from observations, the interhemispheric (meridional) gradient remains poorly constraine. Because of the high reactivity of OH, direct observations are challenging and therefore limited in space and time. Emissions of CH3CCl3 have been a small fraction of the atmospheric burden since the late 1990s, and during the 2000s the north-south distribution of CH3CCl3 has been determined mainly by NH/SH OH gradients with little uncertainty contributed by remaining emission estimates. We use an atmospheric transport model that compares very well with HIPPO measured SF6 for interhemispheric transport throughout the troposphere to show that on an annual basis, the NH/SH OH ratio is close to one. This disagrees markedly from commonly used global atmospheric chemistry models that estimate OH ratios as high as 1.4. The NH/SH gradient in CH3CCl3 is decreasing over time. We show that for each year from 2004-2011, the model-predicted annual-mean NH/SH gradient in CH3CCl3 is a tight linear function of the NH/SH gradient in annual-mean OH. Optimizing the model to fit two AGAGE instruments, NOAA network and HIPPO campaign data in each year gives ratios of 0.92×0.08.