The hydroxyl HO radicals is one of the main chemical species controlling the oxidizing capacity of the global Earth atmosphere. This oxidizing reactive species has a major impact on the concentrations and distribution of greenhouse gases and pollutants in the Earth atmosphere. It is the most widespread oxidizer in the troposphere, the lowest part of the atmosphere. Understanding HO variability is important to evaluating human impacts on the atmosphere and climate. The HO species has a lifetime in the Earth atmosphere of less than one second.  Understanding the role of HO in the oxidation process of methane (CH) present in the atmosphere to first carbon monoxide (CO) and then carbon dioxide (CO) is important for assessing the residence time of this greenhouse gas, the overall carbon budget of the troposphere, and its influence on the process of global warming. The lifetime of HO radicals in the Earth atmosphere is very short, therefore HO concentrations in the air are very low and very sensitive techniques are required for its direct detection. Global average hydroxyl radical concentrations have been measured indirectly by analyzing methyl chloroform (CHCCl) present in the air. The results obtained by Montzka et al. (2011) shows that the interannual variability in HO estimated from CHCCl measurements is small, indicating that global HO is generally well buffered against perturbations. This small variability is consistent with measurements of methane and other trace gases primarily oxidized by HO, as well as global photochemical model calculations.