Table 1.1 summarizes the programs in operation or terminated at MLO during 1996-1997. Relevant details of note on some of the respective programs are as follows:

Gases

Carbon Dioxide. The CMDL Siemens Ultramat-3 IR CO₂ analyzer and the Scripps Institution of Oceanography (SIO) Applied Physics IR CO₂ analyzer (see listing under Cooperative Programs in Table 1.1) were operated in parallel without major problems throughout 1996-1997. Routine maintenance and calibrations were undertaken on both instruments as scheduled. SIO upgraded their CO₂ data acquisition system in 1994 but had problems with power surges at MLO that caused frequent computer downtime. An uninterruptible power supply (UPS) was installed in 1996 to correct that problem. Data are recorded on a strip chart recorder and stored on a PC hard disk and a floppy disk that are mailed to SIO weekly. The CMDL CO₂ data acquisition system worked through 1996-1997 without major problems, except that the network card in the computer was damaged in the August lightning strike.

The weekly CO₂, CH₄, and other gas sampling programs, using flasks at MLO and at Cape Kumukahi, were carried out according to schedule throughout the year without major problems. An AIRKIT sampling unit at Cape Kumukahi, upgraded from the MAKS unit, uses flask types and sampling procedures that are the same as for the MAKS method. Both the MAKS and the AIRKIT sampled simultaneously next to each other through 1996-1997 without problems.

Carbon dioxide emissions from the Mauna Loa volcano measured at MLO, resumed their steady decline in 1996-1997 after a brief factor-of-2 rise in 1994-1995 that was probably due to new emissions from the upper southwest rift. The ratio of SO₂ to CO₂ measured in the plume remained steady at about 4 Ž 10^-5 between mid-1994 and the end of 1996. Since magma exsolves CO₂ at a much greater depth than SO₂, this observation suggests that there has been no magma ascent to shallow depths since 1994. The temporary increase in CO₂ in 1994-1995 must have come from a small intrusion of magma at a great depth.

Because outgassing from the volcanic vents at the Mauna Loa caldera and along the northeast rift zone at Mauna Loa has fallen to negligible levels, the tables for MLO outgassing that appeared in prior reports have been discontinued. They will be reinstated should MLO outgassing become active again, as is the case following major eruptions.

Carbon Monoxide. The Trace Analytical RBA3 reduction gas analyzer for the continuous measurement of CO mixing ratios developed a major problem in 1997. The unit was shipped back to the Boulder laboratory on March 26, 1997, and has been out of service through December 1997.

Methane. Chromatograms from the HP6890 methane gas chromatograph (GC) system are stored on the Carbon Cycle Group (CCG) hard disk. Since April 1996, instead of shipping all the required gases needed for this program from the Boulder laboratory, MLO has been purchasing the nitrogen carrier gas and the oxidizer in Hilo at discounted prices. The price for grade 5 ultra pure nitrogen (99.999%) and oxidizer (40% oxygen in balanced nitrogen, with analytical accuracy of +2% and analysis precision of +1%) in size 200 cylinders is about $100 per cylinder. The amount of these gases used is one cylinder of nitrogen per 5 weeks and one cylinder of oxidizer per 2.5 weeks.

The CH₄ data continued to show variations of clearly defined frequency. The typical diurnal cycle was well correlated with up- and downslope winds, with the marine boundary layer air having the higher CH₄ concentrations. Multi-day or synoptic-scale CH₄ cycles were also observed, which apparently relate to different air mass source regions.

Sulfur Dioxide. In February 1997 the TECO SO₂ analyzer was returned to the NOAA Air Resources Laboratory ending 2.5 years of measurements at MLO. Another analyzer was lent to MLO from the University of Miami in June, and a new system that overcomes most of the problems encountered with the earlier program was developed in the following months. Sampling lines and solenoid valves in the new system are entirely made of Teflon. The lines are heated and regulated by computer to allow for constant temperature or constant relative humidity samples. Air is alternately drawn froms of 4, 10, 23, and 40 m. By measuring the SO₂ profile above the ground, we can determine if enhanced levels at night are coming from beneath the surface temperature inversion (Mauna Loa volcano) or outside the temperature inversion (Kilauea volcano), or are uniformly mixed (e.g., long-range transport of anthropogenic pollution). In conjunction with existing multilevel wind and temperature measurements, this information will be useful in studying the properties of the downslope wind and air circulation above MLO.

During system development and testing in the Hilo office from September through December 1997, prior to deployment at MLO, a quasi-continuous record of SO₂ was obtained in Hilo for that period. Concentrations as low as a few tens of a part per trillion were sometimes measured in clean downslope air at night. Episodes of volcanic pollution from Kilauea had concentrations that could exceed 200 parts per billion (ppb). In comparison, up to 50 ppb were observed at MLO in upslope winds contaminated by Kilauea volcano emissions.

Ozone Monitoring. The 1996-1967 MLO ozone monitoring program consisted of three measurement focii: continuous MLO surface ozone monitoring using a Dasibi model 1003-AH UV absorption ozone monitor; daily total and Umkehr ozone profile measurements using a computer-based automated Dobson instrument (Dobson no. 76); and ozone profile measurements based on weekly ascents of balloonborne electrochemical concentration cell (ECC) ozonesondes released from the NWS station at the Hilo airport.

Dobson no. 76, the MLO instrument, was operated daily during weekdays throughout the period including AM/PM Umkehr profiles. A new computer was installed in June 1996, and data acquisition upgrades and network connections were finalized at that time. Summer intercomparisons with the world standard Dobson no. 83 occurred in both 1996 and 1997. The instrument was shut down from March 1997 to May 1997 because of dust and vibrations from the construction of the NDSC building.

The Dasibi program operated normally throughout the period and the data transmission software was upgraded in August 1996. The Dasibi was calibrated and yearly maintenance was carried out by Boulder-based staff in March 1997, and absorption tubes were cleaned by other Boulder-based CMDL employees in November 1997 at which time analog/digital checks were also completed.

Ozonesondes were launched weekly whenever supplies were available from Boulder. In 1996 there were 47 ozonesonde flights and in 1997 there were 45.

Halocompounds and Nitrous Oxide. The Nitrous Oxide and Halocompounds (NOAH) system had its semiannual maintenance by Boulder staff in June 1996. Besides making the normal maintenance checks, the staff installed an electronic actuator to replace an air-actuated model for the stream select valve. In August round-robin tank measurement and precision checks were run by MLO. The UPS unit started to have problems in December. Minor repairs and adjustments and new batteries in March 1997 returned the system to normal operation. The lightning storm in August 1997 damaged the serial card, the network card, and a CPU fan. These were replaced and the system placed back on line.

Radon. The CMDL-Department of Energy (DOE) radon program operated normally in 1996 and during the first half of 1997. The lightning strike in August destroyed the instrument electronics and computer. In early 1997, reorganization within DOE ended its involvement in radon monitoring. Without spare parts, schematics, or documentation, the DOE radon program was terminated at MLO. Radon continues to be measured continuously with the Australian Nuclear Science and Technology Organization (ANSTO) instrument, which has a similar time response and a four times greater sensitivity than the DOE detector. The ANSTO program (see Table 1.1, Cooperative Programs) now has an 8-year record of radon measurements at MLO.

Aerosols

Condensation Nuclei. The Thermo Systems Incorporated (TSI) unit is a continuously operating CN counter (CNC) in which condensation occurs in butyl alcohol vapor with light scattering detection and single-particle counting statistics as a basis for determining CN concentrations. The Pollock CNC continues to be used as a primary daily calibration.

Aerosol Light Scattering. The four-wavelength nephelometer for determining aerosol optical properties continues to run without considerable down time. A three-wave TSI nephelometer with much better resolution now operates in parallel with it, sampling the same air stream.

Aerosol Absorption. The aethalometer performed satisfactorily during most of 1996-1997. On September 12, 1996, it was sent back to the manufacturer to be upgraded. It was returned on January 9, 1997. The new instrument has a built-in computer and an automatic filter-changing system.

Stratospheric and Upper Tropospheric Aerosols. Weekly observations continued with both the CMDL and NDSC ruby and Nd:YAG lidars throughout 1996 and 1997. The ruby laser (694-nm wavelength) had various electronic problems but no optical problems except for the usual flashlamp changes. Two of three high-voltage transformers burned out in the power supply rebuilt by the prior lidar operator. The laser still works well with only one transformer to charge the large capacitors. A source of replacement transformers was located in the event the last unit on hand fails. A significant change was made in the data acquisition electronics in the spring of 1997. After many years of service, the 8-bit Biomation was replaced with an Analogic FAST 16 board (16-bit conversion at 1 MHz). The new board plugs into the PC. The 16-bit conversion captures the entire altitude range from about 7 to 45 km at one detector sensitivity. Previously the laser shots (typically 200) were spread between several different detector sensitivities requiring splicing of the altitude ranges.

The Nd:YAG laser power supply also had a few failures. The problems were corrected without a service call from California. The four flashlamps were replaced on two occasions when the power dropped significantly. In preparation for the move into the new NDSC building some of the lidar control functions were automated. General Purpose Interface Bus (GPIB) control of some of the lidar electronics was added as well as remote tuning of the laser power. Polarization measurements of cirrus clouds at 532 nm were taken on occasion to demonstrate the technique for future studies. An Avalanche Photodiode (APD) was tried for the aerosol measurement at 1064-nm laser wavelength using the FAST 16 board, but the response was far too slow for the lidar signal. A photomultiplier tube (Hamamatsu R632) with an S-1 cathode was then installed. The sensitivity of the tube at 1064 nm is low, but the laser emits most of its power at that wavelength. The detector must also be cooled (-30°C) to reduce the dark noise. The first profiles were taken in October 1996. The accuracy is already better than that of the ruby lidar system.

A time-dependent atmospheric model was developed from Hilo radiosonde data and upper stratospheric models for the aerosol calculation that requires molecular density as a function of altitude. The ruby analysis has always used a fixed model, but the higher accuracy and altitude range of the YAG 532-nm channel required a better molecular density profile. The 532-nm observations have all been processed using the appropriate daily radiosonde values extrapolated to high altitudes with the new model. The temperature analysis was improved by comparisons with the Goddard and Jet Propulsion Laboratory (JPL) lidars that were operated at the observatory during the Mauna Loa ozone (MLO3) campaign. Nonlinearities in high count rates were noticed, and it was found that initializing the calculation at 75 km produced better agreement than at 80 km. The temperature record has been reanalyzed with these corrections.

The lidar laboratory in the new building has been designed with separate control, laser, and telescope rooms, providing about twice the space of the current building. A large rollaway hatch, ~1.8 m Ž 2.1 m (9 ft Ž 9 ft) for the telescopes was installed with a screw drive for automatic operation. The smaller laser hatch ~0.6 m Ž 1.8 m (2 ft Ž 6 ft) is hinged and driven by an electric actuator. An optical rack for multiple telescopes and both lasers was built from existing aluminum components.

Solar Radiation

Excavation of the NDSC building foundation in the spring of 1997 caused a heavy loading of fine dust that was cleaned off of the radiation windows daily. Plumes of diesel exhaust from heavy equipment on the site caused occasional intermittent increases in aerosol optical depth determined to be as great as 0.2. Even after construction was completed, high winds were lifting dust from areas of the site that had been back-filled by fine sand.

In August 1997 a new computerized tracker was installed on the radiation tower. This tracker carries the primary MLO normal incidence pyrheliometer (NIP), a second NIP, a diffuse pyranometer, and a diffuse pyrgeometer. It uses an altitude-azimuth mount that is controlled by a computer.

The lightning strike in mid-August damaged the solar dome computer, the new tracker, and the Campbell data acquisition system. These were repaired in the following weeks. In November 1997 new grid tables were built on the new solar radiation platform above the NDSC building, and most of the instruments and data systems were relocated there. The only instrument that remained in its original place was the diffuse pyranometer located on the solar radiation wall on the walkway above the station.

Meteorology

A computer-based "New Met System" measures temperatures at the 2-, 9-, and 37-m levels, dewpoint at the 2-m level, and wind speeds and directions at the 8.5-, 10-, and 38-m levels of the MLO Observation Tower. This new system has remained in operation with high reliability to date. MLO meteorological data are presented and discussed in section 1.5 of this report.

Precipitation Chemistry

The MLO modified program of precipitation chemistry collection and analyses was continued throughout 1996-1997 within the basic MLO operational routine. This program consists of collections of a weekly integrated precipitation sample from the Hilo NWS station and the collection of precipitation event samples at MLO. Analyses of these samples are undertaken in the Hilo laboratory for pH and conductivity.

Cooperative Programs

MLO Cooperative programs are listed in Table 1.1. New programs and changes not discussed in the NDSC section (next), are presented here.

In March 1996 MLO began calibrating automated sunphotometers for the NASA Aerosol Robotics Network (AERONET) program which will provide ground-truth optical depth measurements for the upcoming Earth Observing System (EOS) satellites. The photometers are self-contained units powered by solar panels that use a robot to make direct sun and sky measurements. Data is transmitted to a Geostationary Operational Environmental Satellite (GOES) once every hour. In September 1997 the Federal Aviation Administration (FAA) installed high quality GPS and meteorology sensors on the MLO Observation Tower in a study of atmospheric effects on GPS positioning. From this program MLO will receive nearly continuous column water vapor measurements. In November 1997 the Colorado State University-U.S. Department of Agriculture (CSU-USDA) UVB monitoring instruments were installed.

Network for the Detection of Stratospheric Change

In the past 4 years the amount of NDSC equipment and the number of NDSC programs installed at MLO are approaching those of the long-established MLO activities. A short description of current NDSC programs and activities at MLO with relevant dates of installation and modifications to the programs follows. The NOAA lidar, ozonesonde, and Dobson operations, which are also part of the MLO NDSC facility, are described in other sections of this report.

UV Spectroradiometer. The UV spectroradiometer described in the previous Summary Report (No. 23) continued to operate satisfactorily through October 1997, at which time it was replaced by a new model. This new instrument was also developed by the National Institute for Water and Atmosphere (NIWA) in Lauder, New Zealand, and was put into operation in the new MLO NDSC building in November 1997. Routine operations for the new instrument are quite similar to those of the old one. The instrument uses a double monochromator to measure UV irradiance over the interval 280-450 nm, with a resolution of about 1 nm, and is programmed to perform a scan every 5° of solar zenith angle. Weekly quality control calibrations are performed with a mercury lamp and a 45-W quartz lamp. An absolute-standard 1000-W FEL lamp calibration is performed several times each year. The first 2 years of data from the original UV spectroradiometer are presented in section 3 of the present Summary Report.

Ozone Microwave Spectrometer. The University of Massachusetts microwave instrument measures the vertical profile of ozone from 20 to 70 km with a vertical resolution of 10 km or less up to 40 km, degrading to 15 km at 64 km. The ozone altitude distribution is retrieved from the details of the pressure-broadened line shape. The instrument has been well validated.

This instrument is the only ground-based ozone profile instrument capable of making measurements in the upper stratosphere and mesosphere. Its measurements are unaffected by aerosols, an important property following a major volcanic eruption such as that of Mount Pinatubo in June 1991.

UV Lidar. Much of the short-term work of NDSC revolves around providing correlative and validation measurements for satellite and aircraft instruments, in particular the Upper Atmosphere Research Satellite (UARS). Other recent validation efforts have included those for instruments onboard the NASA DC8 during the Tropical Ozone Transport Experiment/Vortex Experiment (TOTE/VOTE) mission. In August 1997 the JPL lidar made coordinated measurements of ozone and temperature profiles in support of the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) Space Shuttle mission, which was launched August 7, 1997. Preliminary intercomparisons of temperature profiles obtained by lidar at MLO and CRISTA show very good agreement over the altitude range from 20 to 60 km.

During the last 2 years the JPL UV lidar has conducted several extensive studies of the dynamics of the middle atmosphere above MLO. Using the lidar, one can observe gravity waves, planetary waves, atmospheric tides, and mesospheric inversions. Temperature climatologies have also been developed using these results. In 1996 and 1997 the JPL system made 177 and 259 lidar measurements, respectively, at MLO. Each measurement provides three profiles: one ozone, one temperature, and one aerosol.

It is assumed that the more stable concrete floor in the new NDSC building will allow JPL to slightly improve data quality. JPL will also use the opportunity offered by the move and the rebuilding of the lidar to try to extend the range of its measurements to lower altitudes. The additional space will make the lidar easier to operate and maintain.

Water Vapor Millimeter-Wave Spectrometer. The water vapor millimeter-wave spectrometer (WVMS) at Mauna Loa has been measuring middle-atmospheric water vapor nearly continuously since March 1, 1996. As expected, the observed seasonal variations in water vapor profiles at MLO are smaller than those observed by WVMS instruments at higher latitude sites at Table Mountain, California (34.4°N), and Lauder, New Zealand (45.0°S). Although the data record at MLO is as yet too short to provide much useful information on multi-year trends, it is expected that the small seasonal variation and excellent observing conditions at the MLO site will make it the prime location for estimating long-term trends.

NO₂ UV/Vis Spectrometer. Since July 9, 1996, total column nitrogen dioxide (NO₂) has been measured at MLO using the twilight zenith technique with a NIWA ultraviolet/visible (UV/Vis) spectrometer. Continuous data have been obtained except for a short period in mid-November 1996 when a viewing window failed and a 4-week period in September-October 1997 when a computer problem occurred. Data through December 31, 1997, have been analyzed and quality confirmed, and will be submitted to the NDSC archive by June 30, 1998. It is planned to upgrade this spectrometer so that in addition to the NO₂ and O₃ currently measured, stratospheric BrO and OCIO, as required to meet the NDSC UV/Vis specification, will also be measured. November 1998 is the target date for this upgrade.

Brewer Ozone/UV Spectrophotometer. A single monochromator Brewer instrument was installed by the Canadian Atmospheric Environment Service (AES) at MLO and began routine measurements of O₃ and UV-B radiation on March 24, 1997. A second instrument was added in November 1997. The measurements are supplemented by all-sky images that are recorded every 10 minutes in order to assist in the analysis of the UV-B data. Overviews of the automatic operation of the instrument and data retrievals have since been carried out remotely from AES in Toronto over the Internet.

The data are archived at the World Ozone and Ultraviolet Data Centre (WOUDC) in Toronto. Up-to-date preliminary data are available over the Internet from AES. Publication of some new results is planned after thorough analysis of a longer data record is completed.

Solar FTIR Spectrometer. The University of Denver FTIR spectrometer routinely monitors total column concentrations of HCl, HNO₃, O₃, N₂O, F-22, HF, CH₄, NO, HCN, CO, C₂H₂, and C₂H₆. Because of the automatic nature of the instrument, the program is able to investigate diurnal variations in the species. Data are not collected on Sundays or Monday mornings unless special operators are on site to load liquid nitrogen into the instrument.

TABLE 1.1. Summary of Measurement Programs at MLO in 1996-1997

All instruments are at MLO unless indicated.

*MLO and Kumukahi.

†Data from this instrument recorded and processed by microcomputers.

‡Kumukahi only.

§Kulani Mauka.

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