Measurement Of Pollution In The Troposphere (MOPITT) is one of five instruments flown aboard NASA's Earth System Enterprise (ESE )Terra satellite.Terra is one of a series of earth orbiting vehicles which will study the earth's surface and atmosphere. The spectral data measured from this satellite will help us better understand the interaction between the land and oceans, the biosphere, atmosphere and cryosphere; and together enable us to better understand the environmental impact of human activities on our planet. Terra is a polar-orbiting satellite with a morning equatorial crossing. Originally scheduled for June 1998, the launch was delayed until 18 December 1999. MOPITT began collecting scientific data in March 2000.
MOPITT measures the temporal and spatial distribution of carbon monoxide (CO) and methane (CH4) in the troposphere [the troposphere extends from the surface to approximately 15 - 17 km in the tropics and 7-12 km in the high latitudes]. CH4 is an important greenhouse gas (one that absorbs the earth's emitted (thermal) infrared radiation), whose increasing atmospheric abundance is expected to contribute to future climate change. Carbon monoxide, while not a strong greenhouse gas, plays a key role in atmospheric chemistry and may have a significant indirect effect on climate. While the surface concentrations of these two gases have received much study, less is known about their distribution in the middle and upper troposphere. The MOPITT instrument is a multi-channel, nadir (downward) viewing correlation spectrometer which measures upwelling radiation in 3 wavelengths. The 4.7 um thermal channel uses both pressure and length modulated reference cells to determine the amount of CO in discreet layers of the troposphere. The two solar channels (2.3 and 2.2 um) are used to measure the column abundance of CO and CH4 (respectively). MOPITT will provide tropospheric CO and CH4 column amounts with a 22 x 22 km horizontal resolution, and the vertical distribution of CO in layers of 3-5 km altitude (e.g. 0-3km, 3-6 km, 7-12 km) will be resolved. The expected precision for the CO measurement is 10%, and 1% for the CH4 column measurements.
As part of the Terra measurement program, NASA has provided funds for the external calibration of the 5 instruments in its payload. The MOPITT validation plan includes measurements of tropospheric CO and CH4 made using a variety of analytical techniques (including remote sensing from aircraft, solar spectroscopy, and gas chromatography). In support of the MOPITT validation the NOAA/CMDL carbon cycle group is measuring vertical profiles of the 3 most abundant carbon gases (CO, CH4, and carbon dioxide (CO2)) at 6 locations around the world (Alaska, Colorado, Massachusetts, Hawaii, South Africa, and the Cook Islands). Small aircraft and portable sampling equipment are used to collect samples of air (approximately 1 liter volume at 2 atmospheres pressure) in glass containers. Twenty flasks are held in a suitcase-sized container, and collection of air in a single flask at a unique altitude allows a sampling resolution of 400 m. After each flight the flask packages are shipped to the NOAA laboratory in Boulder for trace gas analysis. An automated analytical system, consisting of three instruments and controlled by a Linux workstation, determines the mixing ratios of CO and H2 (gas chromatography (GC) with hot mercuric oxide detection), CH4 (GC followed by flame ionization detection), CO2 (non-dispersive infrared detection), and nitrous oxide (N2O) and sulfur hexafluoride (SF6 , both by GC + electron capture detection). All measurements are referenced to standards maintained at CMDL which are tied to internationally recognized calibration gases. The approach of discontinuous air sampling using flasks with resolution on the order of 400-500 m defines the main features of the vertical structure of the trace gases of interest. The CO and CH4 results can be directly compared to the CO and CH4 vertical profiles retrieved by MOPITT, measurements of the other gases will add in the MOPITT carbon monoxide and methane retrievals.
As part of the MOPITT validation program biweekly measurements over 3 years are planned for five sites: Poker Flats, Alaska; Harvard Forest, Massachusetts; Carr, Colorado; Molokai, Hawaii, and Raratonga, Cook Islands (Table 1).
|Location||Site Category||Lat/Long||Charter Service||Plane||Maximum Altitude(km)|
|Poker Flats Alaska||Boreal Forest||65.1N, 147.5||Warbelows Air||Piper Chieftain||7.6|
|Harvard Forest, Massachusetts||Polluted Boundary Layer||42.5N, 71.2W||General Aviation||Cessna 421||8.7|
|Carr, Colorado||Background Continental||40.1N, 104.1W||Dakota Ridge||Cessna 210||9.2|
|Molokai, Hawaii||NH Tropical Ocean||21.4N, 157.2W||Hawaii Air||Cessna 414 Ambulance||7.6|
|Raratonga, Cook Islands||SH Tropical Ocean||21.2S, 159.6W||Air Raratonga||Bandeirante||7.0|
The long-term measurements made by this program can provide a rare view of trace gas climatology in the remote free troposphere. During the past decade, it has become clear that much the pollution generated in the boundary level is transported into the lower and middle troposphere. However, few programs regularly determine vertical profiles of multiple trace gases in the troposphere. The boundary layer mixing ratios determined from aircraft at the MOPITT sites show trace gas levels comparable to those measured at the surface, and the aircraft data can be integrated with the CMDL long-term cooperative monitoring network. The highest levels of CO, CH4, and CO2 at all sites are typically found near the surface. However, strongly correlated structure in the form of enhanced or diminished mixing ratios (ppb = parts per billion, 1 x 10-9; 1 ppm = 1 part per million, 1 x 10-6) is often found in the free troposphere. This structure reflects the transport and chemistry of different air parcels.
Although more data are needed to evaluate general features of the MOPITT sites, the preliminary data show interesting relationships between altitude and trace gas mixing ratio. Profiles determined above Molokai, Hawaii in late winter/early spring often show layers of strongly elevated trace gases. Isentropic back trajectories (calculated movement of an air parcel from a point backward in time and space along a surface of constant temperature) commonly show an easterly flow component at the altitudes of these enhancements, consistent with transport from Eastern Asia. Lower mixing ratios appear associated with easterly or circular winds. Fewer occurrences of these enhancements are evident in April and May due to less transport from the east. The lowest mixing ratios of CO and other gases are found above Raratonga, Cook Islands. The low concentrations of these gases reflects, in large part, diminished industrial sources in the Southern Hemisphere combined with high removal rates in the tropics.
Mixing ratios above Harvard Forest, MA. were the highest measured: up to 250 ppb near the surface in winter. From a surface maximum, trace gases typically then decreased with altitude (the boundary layer at HFM reflects the regional pollution of southern New England). Combustion of fossil fuels is major sources of these gases, and all have diminished sinks in winter (CO, CH4 - photochemical oxidation; CO2 -biological uptake), permitting a buildup of pollutants in winter. Trace gases above Poker Flats, Alaska are influenced by transport of anthropogenic surface emissions from lower altitudes, and gas distributions determined in late winter show a general decrease from the surface through the middle troposphere.
Carr, Colorado, located in the northern plains, has been studied by NOAA/CMDL for several years. The detailed trace gas history at this site shows the power of long-term measurements. The biweekly profiles, when binned by altitude, provide a time series that can be used to provide a statistical description of the site. The smooth curve (where the seasonal cycle is represented as the sum of 4 harmonics and the long-term trend as a polynomial) and the uncertainty (3-sigma) of the curve show the expected range of mixing ratios at any particular time.
In addition to the biweekly measurements, we will also participate in an intensive validation experiment in South Africa (The Southern African Regional Science Initiative: SAFARI-2000). SAFARI-2000 is an international, collaborative science initiative which will study the linkages between land-atmosphere processes (including surface radiation, hydrology, gas and particulate emissions from technological sources and biomass burning, land use changes, and terrestrial ecology). CMDL will participate in the August 2000 intensive experiment of SAFARI-2000. This mission is scheduled for the dry season, a time of year when there is extensive biomass burning in eastern Southern Africa, including the countries of South Africa, Zimbabwe, and Mozambique. CMDL will fly the automated sampling equipment aboard a Republic of South Africa Aerocommander to determine the vertical profiles of trace gases. The airborne MOPITT simulator (MOPITT-A) will be flown aboard the NASA ER2 during the mission. Flight plans will include collocated flights of the Aerocommander and the ER2, coincident with MOPITT overpasses. In addition to grab sampling, CMDL plans to conduct continuous measurements of CO beside the flask equipment on selected flights. These measurements will provide information on the very fine scale structure of CO in the lower troposphere.
For additional information contact:
DSRC 325 Broadway
Boulder, CO 80303