Hydroxyl column abundance measurements: PEPSIOS instrumentation at the Fritz Peak Observatory and data analysis techniques

Abstract

We review the observations and spectral analysis for daytime measurements of the vertical column abundance of hydroxyl (OH) in the earth's atmosphere from the Fritz Peak Observatory in Colorado (40°N) and from a concurrent series of observations from New Mexico Institute of Mining and Technology (34°N). These are high-resolution measurements of solar ultraviolet absorption by atmospheric OH in the P 1(1) 2Π→ 2Σ electronic transition at 3081.7 A ̊ . The Fritz Peak OH database, initiated in 1977, consists of over 19,000 measurements and shows distinct diurnal, seasonal, and long-term variations. New Mexico OH observations began in 1996 using an instrument of comparable design and operation. Data from both locations are in conflict with OH abundances calculated by photochemical models for solar zenith angles less than about 60°. In addition, OH variations observed with respect to latitude, season, and long-term changes are not explained using current models. We present a critical examination of instrument characteristics and spectral analysis, one which indicates no tendency for systematic or interference effects that could contaminate observed OH abundances or their variations. This includes results of sensitivity analyses on synthetic spectra, taking into account temperature and pressure variations along the atmospheric optical path, and effects of absorption by SO 2 and CH 2O.