Ultraviolet Spectroradiometer Measurements at Mauna Loa Observatory, Hawaii
Mauna Loa Observatory (MLO), one of the premier atmospheric monitoring sites in the world, is located on the island of Hawaii at an altitude of 3.4 km. MLO is operated by the Global Monitoring Division of the Earth System Reaearch Laboratory (ESRL/GMD) of the National Oceanic and Atmospheric Administration (NOAA). A large number of measurements are conducted at this site, including carbon dioxide, carbon monoxide, methane, ozone, aerosols, and solar and thermal radiation. Clear mornings occur at MLO approximately 60% of the time providing an excellent site for solar radiation measurements, and especially Langley calibrations. The GMD series of annual reports provides an excellent summary of the many activities at MLO. The geographical coordinates of MLO are 19.53 N, 155.58 W, at an altitude of 3.4 km.
A research-grade scanning ultraviolet (UV) spectroradiometer was installed at MLO in July 1995. This instrument was developed and operated by the National Institute for Water and Atmospheric Research (NIWA) at Lauder, New Zealand, and has been included in a number of spectroradiometer intercomparisons.
At the earth's surface, the incoming solar radiation depends on the absorption and scattering of the atmosphere, the earth-sun distance, and the irradiance of the sun. The atmospheric transmission in the UV portion of the spectrum is controlled primarily by Rayleigh scattering by air molecules, scattering by clouds, and absorption by ozone. Aerosol scattering and absorption can also play a significant role. Solar UV irradiance arriving at the earth's surface is controlled by both natural and anthropogenic effects in both the stratosphere and the troposphere. Furthermore, changes in solar UV affect ozone concentration in the atmosphere.
The UV portion of the solar spectrum is generally divided into three regions: 1) UV-A (315-400 nm); 2) UV-B (280-315 nm); and 3) UV-C (< 280 nm). The UV-A is essentially unaffected by ozone absorption; the UV-B is strongly affected by variations in ozone; and the UV-C is almost entirely absorbed before it reaches the surface.
Relative atmospheric transmission in the visible (broad band) has been measured at MLO since 1958, and optical depth since 1982, yielding one of the most important optical depth records in existence. Although solar UV measurements have been made previously at MLO, the data obtained from the instrument described here are the first spectroradiometer measurements.
Because of the long Dobson spectrophotometer ozone measurement record at MLO (1957-present), the opportunity exists to obtain well-calibrated long-term UV spectroradiometer measurements, and to relate them to the long-term ozone record. The Dobson total ozone record has been described in the series of GMD annual reports. Past studies have shown that short-term variations of UV-B irradiance are inversely correlated with variations in total ozone.
The UV spectroradiometer uses a Jobin-Yvon DH10 double monochromator that is interfaced with a Compaq personal computer for control and data acquisition. A diffuser, designed to minimize cosine error and custom-machined from Teflon to a 17-mm diameter, is mounted as a horizontal incidence receptor to view the whole sky. The entrance slit to the spectrometer is located 4.5 cm below the diffuser. A shading disk can be mounted on the instrument in order to separate the diffuse and direct radiative components.
Stepper-motor driven gratings cover the spectral range of 290-450 nm with a bandpass of about 1 nm. A complete scan requires about 200 seconds. The irradiance signal is sampled every 0.2 nm using a photomultiplier detector with variable gain provided by a programmable high voltage power supply. The instrument is enclosed in an insulated weatherproof box (painted white) and located on a concrete pad at the MLO site for automated and unattended operation. The interior of the enclosure is temperature controlled to 20° ± 0.5°C using a Peltier heater/cooler unit. A quartz dome with active air circulation to inhibit condensation protects the diffuser from the weather. The computer control and data logging system are located in a small building near the instrument.
Calibration of the spectroradiometer is performed on site using a standard 1000-W FEL lamp with calibration traceable to the National Institute of Standards and Technology (NIST ). At approximately 6-month intervals the instrument is moved into the building for calibration, and the FEL lamp is situated 50 cm from the diffuser using a precision optical bench and scale. A He-Ne laser is used to achieve precise alignment. The FEL lamp is powered by a stable current-regulated power supply designed expressly for this type of calibration procedure.
A wavelength calibration is performed weekly using a mercury lamp that mounts directly on the instrument while it rests on the concrete pad, permitting a wavelength precision of ±0.1 nm. The wavelength repeatability (±0.02 nm) is excellent because each scan is also aligned against the known Fraunhofer spectrum using a correlation alignment method. The weekly wavelength calibrations are applied during data processing using a nonlinear wavelength stretching routine to compensate for possible long-term drift due to mechanical wear. A stability test is also performed weekly for quality control using a 45-W lamp in a light-tight housing. The expected long-term accuracy of the spectroradiometer system is expected to be better than ±5.
The spectroradiometer is programmed to begin measurements at dawn and perform scans at 5 solar zenith angle (SZA) intervals throughout the day beginning and ending at 95°, except that during the middle of the day the system switches to a scan every 15 minutes. In addition, a scan is performed each midnight to give "dark" values. Raw data are moved to Lauder, New Zealand, and Boulder, Colorado, using file transfer protocol (FTP). Data are then unpacked, calibrations are applied, and analyses are performed.
View of the UV Spectroradiometer at Mauna Loa Observatory, facing towards the NW. The main observatory building is at the upper right of the photograph.
Example of MLO UV spectra, obtained on a clear day (9 May 1996) at various SZAs throughout the morning (solid lines). A point is plotted every 0.2 nm. Also shown is the erythema spectrum (short- dashed) for the 2° scan obtained by weighting the UV spectrum by the Erythemal Action Spectrum (EAS) (long-dashed). The EAS is a dimensionless quantity normalized to 1 for lambda < 298 nm. The inset table shows integrated erythema at the indicated SZAs (10-6W cm-2). The Dobson total ozone measured during this morning was 295 DU. A sample 2-sigma error bar for 3% is shown in the bottom left corner.
Erythemal irradiance calculated from 12,993 spectral scans obtained from MLO during July 1995- July 1996. The envelope shows lowest values at about the winter solstice and largest values near the summer solstice.
Spectral irradiance (1-nm averages) on 132 clear-sly mornings at MLO for selected wavelengths at SZA 45° over the time period July 1995-July 1996. Two-sigma error bars for the maximum and minimum values are shown for each graph. For lambda > 300 nm, the error is about ±3%.
Erythemal irradiance at SZA 45° (bottom) and total ozone (top) on 132 clear-sky mornings at MLO over the time period July 1995-July 1996.