Ultrahigh Resolution Infrared Solar Spectroscopy From Mauna Loa Observatory: Update
Shelle J. David and Frank J. Murcray
University of Denver, Department of Physics, Denver, Colorado 80208
An ultrahigh resolution Fourier Transform InfraRed (FTIR) solar spectrometer was installed at Mauna Loa Observatory, Hawaii (MLO) in May 1991. It was operated routinely by observatory staff from November 1991 to October 1995. In September 1995 we installed a new FTIR system that is almost completely automated. This more versatile automated system allows us to: (1) change filters and setup conditions to cover more spectral regions on a routine basis; and (2) collect data 5 days a week by starting the instrument and turning the solar tracker on and off on a computer-controlled scheduled basis.
Data collected and analyzed is reported to the Network for Detection of Stratospheric Change (NDSC). The original NDSC instrument allowed us to monitor a limited number of molecules (HCl, O3, N2O, HNO3, and CFC-22). The new automated system has allowed us to add molecules such as HF, CH4, NO, NO2, CO, HCN, C2H2, C2H6, and N2 to the list of routinely monitored molecules.
On June 15, 1991, Mt. Pinatubo in the Philippines erupted, injecting large amounts of aerosol and gases into the stratosphere. Lidar measurements at MLO indicated the arrival of a finger of the Mt. Pinatubo volcanic plume above the station on July 1, 1991, followed by two other pulses in July and one in August.
Approximately 6 years of spectra have been analyzed for the total vertical column amount of atmospheric nitric acid vapor (HNO2) above MLO. We see an effect of the volcanic aerosol on HNO3 over MLO, specifically the HNO3 column has slowly decreased since the start of our observations.
The HNO3 total vertical column densities obtained between November 1991 and November 1997 are presented in Figure 1, fit with an exponential decay. From November 1991 to October 1995 our original instrument collected data 1 day a week, weather permitting, at sunrise. Starting September 1991 our new system collected data 5 days a week, sunrise and sunset. The HNO3 data collected between September 1995 and November 1997 is presented in Figure 2. The larger quantity of data collected by the automated system weighed the fit of all the data, making it necessary to give average values for the sunrise and (or sunset) data approximately 1 day a week (Figure 1).
Fig. 1. The HNO3 total vertical column densities obtained between November 1991 and November 1997.
Fig. 2. The HNO3 data collected between September 1995 and November 1997.
The highest HNO3 total vertical column density was observed on December 4, 1991, at 8.8 × 1015 molecule cm-2. The high HNO3 column amounts at the end of 1991 coincide closely with the time of maximum aerosol loading deduced from SAGE II monthly mean measurements integrated from 15 to 35 km and 10°N to 20°N in latitude. Our fit indicated an increase in the HNO3 column of 55% due to Pinatubo and a 1/e decay time of 0.75 year since the eruption. New Zealand observed systematically higher HNO3 amounts between September 1991 and May 1993 that were typically 1.5 to 3.5 × 1015 cm-2 or 10 to 30%.
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