Seasonal and solar cycle variability of OH in the middle atmosphere

[1] The photochemistry of the hydroxyl radical (OH) in the middle atmosphere involves the action of solar ultraviolet radiation in many respects. The production of stratospheric OH is initiated through the photodissociation of ozone, and in the mesosphere, OH is produced directly from the photolysis of water vapor. We present an analysis of measurements of the vertical column abundance of OH from Fritz Peak Observatory, Colorado, for seasonal and solar cycle variability. The OH column data span over two 11-year solar cycles and include a wide range of diurnal and seasonal conditions. Hydroxyl abundances show distinct annual and semiannual cycles, with amplitudes of 4.4% and 2.5%, respectively. About half of the seasonal variation in OH can be explained by the annual cycle of ultraviolet intensity, and by annual and semiannual cycles in ozone and water vapor in the middle atmosphere. On longer time scales, a positive correlation with the NOAA MgII solar index is observed. The amplitude of 11- year solar cycle variations in OH is 4.2 +/- 2.4%, which is consistent with photochemical model calculations that consider changes in ozone and water vapor photodissociation rates. Changes in the vertical distribution of ozone over the solar cycle are found to have a smaller effect on the OH column.