Trace gas measurements from air samples collected weekly at a globally distributed network of sampling sites revealed sharp increases in the growth rates of CH4 and CO in the tropics and high southern latitudes immediately following the eruption of Mt. Pinatubo on June 15, 1991. The eruption emitted ∼20 Mt SO2 into the lower stratosphere. Calculations made with a radiative transfer model show that UV actinic flux in the wavelength region 290–330 nm was attenuated by ∼12% immediately after the eruption due to direct absorption by SO2, and that it was perturbed for up to 1 year after the eruption due to scattering by sulfate aerosols. We suggest that the decreased UV flux decreased the steady-state [OH] and led to the observed anomalously large growth rates for CH4 and CO during late-1991 and early-1992.