Global OH trend inferred from methylchloroform measurements

Methylchloroform (MCF) measurements taken at the Atmospheric Lifetime Experiment / Global Atmospheric Gases Experiment (ALE/GAGE) measurement stations are used to deduce the tropospheric OH concentration and its linear trend between 1978 and 1993. Global three-dimensional fields of OH are calculated with a transport model that includes background photochemistry. Despite the large uncertainties in these OH fields, the simulated MCF concentrations at the five ALE/GAGE stations compare reasonably well to the measurements. As a next step, the OH fields are adjusted to fit the measurements optimally. An ensemble (Monte Carlo) technique is used to optimize the OH scaling factor and to derive the linear trend in OH. The optimized OH fields and trend imply a MCF lifetime in the troposphere of 4.7 years in 1978 and of 4.5 years in 1993. For CH4 these lifetimes (due to OH destruction only) are 9.2 and 8.6 years in 1978 and 1993, respectively. Uncertainties in these estimates are discussed using box-model calculations. The optimized OH concentration is sensitive to the strength of other MCF sinks in the model and is constrained to 1 an+o.09 0 6 ... o.•s x 1 molecules cm -a in 1978 and to 1 07+0.09 ... o. •7 x 106 molecules cm -a in 1993. The deduced OH trend is sensitive to the trend in the MCF emissions and is confined to the interval between -0.1 and +1.1% yr -• with a most likely value of 0.46% yr -•. Possible causes of a global increase in OH are discussed. A positive OH trend is calculated due to stratospheric ozone depletion, declining CO concentrations, increased water vapor abundance, and enhanced NO• emissions. Although the changes in the atmospheric omposition are to a large extent unknown, it seems that the observed changes are consistent with significant increases in OH over the past decades.