Ultrahigh Resolution Infrared Solar Spectra
From Mauna Loa Observatory: New Results
A modified ultrahigh spectral resolution (0.0035 cm-1) Fourier transform interferometer (FTIR) solar spectrometer system was installed at Mauna Loa Observatory, Hawaii (MLO) in May 1991 and was operated routinely by the observatory staff from November 1991 to November 1995. This instrument provided data on the column abundance of several atmospheric chemical compounds for the Network for Detection of Stratospheric Change (NDSC). Normal operation was one sunrise set of data each week, usually collected on Wednesdays.
During August 1995, a new, almost completely automatic FTIR system was installed at MLO. The solar tracker has a weather-tight hatch that is computer controlled. The new system requires a person to fill the detector dewars with liquid nitrogen; it then records solar spectra when weather conditions are acceptable. Weather permitting, data are collected 5 days-a-week at both sunrise and sunset. Data cartridges need to be changed weekly and desiccant changed monthly.
Here, data are presented from the complete 4-year run of the original interferometer, and comparisons between the two systems are made for data from the overlap period.
To date, we have analyzed the infrared spectra for total column amounts of several gases over a 4-year period at MLO. Gases included in this data base are O3, N2O, HNO3, F22 and HCl.
The ozone spectral lines were chosen to be isolated and only weakly temperature
dependent. The two fitted spectral regions, around 1146 and 1163 cm-1,
included two prominent ozone absorptions and a strong line due to N2O.
Nitrous oxide is reported to be increasing slightly, but is essentially constant
on our scale. Infrared ozone column amounts are very consistent with the Dobson
observations [David et al., 1993]. Figure 1 shows the total columns measured
for O3 and N2O. We see ozone has an annual cycle with
lows in the winter and highs in the late spring to early summer. It is worth
noting that O3 hit a low in early 1995 that is consistent with a
record low measured by the Mauna Loa Dobson spectrophotometer.
Fig. 1. Total column amounts of ozone and N2O over Mauna Loa Observatory.
The HNO3 spectral interval chosen contains three P-branch manifolds of the n5 band located between 868.3 and 869.6 cm-1; see David et al.  for a description of the HNO3 analysis. Figure 2 shows the total column measured for HNO3. Currently, we are seeing a decrease of approximately 9% yr-1. Our measurements started just after Mount Pinatubo erupted in July 1991; therefore, the decrease is expected, however, HNO3 should level off soon.
We analyzed HCl using a two-step process. First, we looked at 2925.5 to 2926.5
cm-1 which included H2O and CH4 in the fit. Then we zoomed
in on the H35Cl (1-0) band of the R(1) line at 2925 89 cm-1.
Our measurements show an increase in the column of HCl of approximately 2.5%
Fig. 2. Total column amount of HNO3 over Mauna Loa Observatory.
Finally, we measured the 2n2 Q-branch of CHF2Cl (F22)
at 829.05 cm-1. Figure 3 shows the total columns measured for F22.
Basically, we measured an increase of 20% yr-1 which slowed down
between spring and summer of 1994.
Fig. 3. Total column amount of F22 over Mauna Loa Observatory.
Both instruments operated at MLO for several months. This resulted in four to six same-day data comparisons. If we look at the average of the new-to-old ratio, we find approximately 1% difference for O3 and N2O with the new instrument slightly higher, approximately 2% difference for the F22, and approximately 5% difference for the HNO3 with the old instrument running slightly higher. Overall, these results have been very encouraging.
Acknowledgments. This research was partially supported by NASA under grants NSG 1432 and NAG2-351. The collection of the data was done with the support of CMDL. We are especially grateful to Bob Uchida for collecting most of this data very early each Wednesday morning.
David, S.J., S.A. Beaton, M.H. Anderberg, and F.J. Murcray, Determination of total ozone over Mauna Loa using very high resolution infrared solar spectra, Geophys. Res. Lett., 19, 2055-2058, 1993.
David S.J., F.J. Murcray, A. Goldman, C.P. Rinsland, and D.G. Murcray, The
effect of the Mt. Pinatubo aerosol on the HNO3 column over Mauna
Loa, Hawaii, Geophys. Res. Lett., 21, 1003-1006, 1994.