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 pho tochemistry. Despite the large uncertainties in these OH fields, the s imulated MCF concentrations at the five ALE/GAGE stations compare reas onably well to the measurements, As a next step, the OH fields are adj usted to fit the measurements optimally. An ensemble (Monte Carlo) tec hnique is used to optimize the OH scaling factor and to derive the lin ear trend in OH. The optimized OH fields and trend imply a MCF lifetim e in the troposphere of 4.7 years in 1978 and of 4.5 years in 1993. Fo r CH4 these lifetimes (due to OH destruction only) are 9.2 and 8.6 yea rs in 1978 and 1993, respectively. Uncertainties in these estimates ar e discussed using box-model calculations. The optimized OH concentrati on is sensitive to the strength of other MCF sinks in the model and is constrained to 1.00(-0.15)(+0.09) x 10(6) molecules cm(-3) in 1978 an d to 1.07(-0.17)(+0.09) x 10(6) molecules cm(-3) in 1993. The deduced OH trend is sensitive to the trend in the MCF emissions and is confine d to the interval between -0.1 and +1.1% yr(-1) with a most likely val ue of 0.46% yr(-1). Possible causes of a global increase in OH are dis cussed. A positive OH trend is calculated due to stratospheric ozone d epletion, declining CO concentrations, increased water vapor abundance , and enhanced NOx emissions. Although the changes in the atmospheric composition are to a large extent unknown, it seems that the observed changes are consistent with significant increases in OH over the past decades.