Infrared measurements of HF and HCl total column abundances above Kitt Peak, 1977–1990: Seasonal cycles, long‐term increases, and comparisons with model calculations

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Abstract
Series of high‐resolution (∼0.01 cm−1) solar absorption spectra recorded with the McMath Fourier transform spectrometer on Kitt Peak (altitude 2.09 km, 31.9°N, 111.6°W) have been analyzed to deduce total column amounts of HF on 93 different days and HCl on 35 different days between May 1977 and June 1990. The results are based on the analysis of the HF and H35Cl (1–0) vibration‐rotation band R(1) lines which are located at 4038.9625 and 2925.8970 cm−1, respectively. All of the data were analyzed using a multilayer, nonlinear least squares spectral fitting procedure and a consistent set of spectroscopic line parameters. The results indicate a rapid increase in total HF and a more gradual increase in total HCl with both trends superimposed on short‐term variability. In addition, the total columns of both gases undergo a seasonal cycle with an early spring maximum and an early fall minimum, with peak‐to‐peak amplitudes equal to 25% for HF and 13% for HCl. In the case of HF, the changes over the 13 years of measurement are sufficiently large to determine that a better fit is obtained assuming a linear rather than an exponential increase with time. For HCl, linear and exponential models fit the data equally well. Referenced to calendar year 1981.0 and assuming a sinusoidal seasonal cycle superimposed on a linear total column increase with time, HF and HCl increase rates of (10.9±1.1)% yr−1 and (5.1±0.7)% yr−1 and total columns of (3.17±0.11)×1014 and (1.92±0.06)×1015 molecules cm−2 (2 sigma) are derived, respectively; the corresponding best fit mean exponential increase rates are equal to (7.6±0.6)% yr−1 and (4.2±0.5)% yr−1 (2 sigma). Over the 13‐year observing period, the HF and HCl total columns increased by factors of 3.2 and 1.8, respectively. Based on HF and HCl total columns deduced from measurements on the same day, the HF/HCl total columns ratio increased from 0.14 in May 1977 to 0.23 in June 1990. Short‐term temporal variations in the HF and HCl total columns are highly correlated; these fluctuations are believed to be caused by dynamical variability in the lower stratosphere. The results of this investigation are compared with previously reported measurements and with time‐dependent, two‐dimensional model calculations of HF and HCl total columns based on emission histories and photo‐oxidation rates for the source molecules.