Spectroscopic measurements of the vertical column, abundance of hydroxyl (OH) in the earth's atmosphere

Abstract

Extensive new information on the daytime column abundance of atmospheric OH is presented from Spectroscopic observations of sunlight absorption by the P1(I)A2Σ+ – X2π(0, 0) transition of OH at 3081.7 Å, taken with a Pepsios spectrometer. The observations were made from the NOAA Aeronomy Observatory at Fritz Peak, Colorado, between December 1976 and December 1979. An overall seasonal dependence on the depth of penetration of the solar ultraviolet flux, which is believed to initiate the photochemistry of daytime OH, was found. The overall averaged data is represented by an empirical curve, N(sec χ), where χ is the solar zenith angle, showing an overhead sun maximum abundance of 7.1 × 10l3/cm2, decreasing to 4.9 × 1013/cm2 at sec χ = 2.0. The OH abundance and the sec χ variation agree reasonably well with predictions given by recent theoretical models of the stratosphere and mesophere and with the analysis of earlier in situ measurements of stratospheric and mesospheric OH by Anderson. An OH abundance increase of about 1 × 1013/cm2 per year is found for the 1976–1979 period. There was a seasonal variation in 1978 with a 25–30% decrease from a springtime maximum to a fall minimum. The 1978 abundances also show a 30–40% diurnal oscillation whose cause is as yet undetermined. This effect is shown to be dependent on solar flux on both a diurnal and an annual basis, and the 1978–1979 observations exhibit additional long‐term changes, possibly related to the solar cycle, which will be monitored in future work.