Abstract: The present invention relates to measuring hydroxyl in an atmosphere, including forwarding sunlight and ultraviolet light into a gas cell; switching between nitrogen gas only, or nitrogen gas and water vapor, into the gas cell; emitting ultraviolet rays into the cell which breaks down the water vapor into hydroxyl; and detecting a difference between two states, including 1) an OFF state where only nitrogen gas does not react to the ultraviolet light or the sunlight and there is no OH filter and the detector detects light that OH absorbs; and 2) an ON state where the water vapor is broken down by the ultraviolet rays to produce hydroxyl, and the gas cell acts as an OH filter and does not detect the light that OH absorbs; where a difference in signals measured by the detector in the two states is proportional to a column abundance of OH in earth atmosphere.