Gerald F. Hill

Airborne tunable diode laser sensor for high-precision concentration and flux measurements of carbon monoxide and methane

An airborne tunable diode laser instrument is described that is capable of operating in two measurement modes. One mode provides high precision (0.1 percent CH4; 1 percent CO) measurements of CH4 and CO with a 5 second response time, and a second mode achieves the very fast response time that is necessary to make airborne eddy correlation flux measurements. Examples of data from atmospheric expeditions of the Global Tropospheric Experiment are presented.

Carbon monoxide and methane over Canada: July–August 1990

Carbon monoxide (CO) and methane (CH4) were measured in the 0.15- to 6-km portion of the troposphere over subarctic and boreal landscapes of midcontinent and eastern Canada during July–August 1990. In the mid-continent region, Arctic air entering the region was characterized by relatively uniform CO concentrations (86–108 parts per billion by volume (ppbv)) and CH4 concentrations (1729–1764 ppbv). Local biomass burning and long-range transport of CO into the area from industrial/urban sources and distant fires did frequently produce enhanced and variable concentrations. Emissions of CH4 from the Hudson Bay lowlands was the primary source for enhanced and variable concentrations, especially at altitudes of 0.15–1 km. In eastern Canada, most of the observed variability in CO and CH4 was similar in origin to the phenomena described for the midcontinent region. However, unexpectedly low concentrations of CO (51 ppbv) and CH4 (1688 ppbv) were measured in the midtroposphere on several flights. Combined meteorological and chemical data indicated that the low CO-CH4 events were the result of long-range transport of tropical Pacific marine air to subarctic latitudes.

Airborne flux measurements of trace species in an Arctic boundary layer

In situ airborne measurements of the turbulent flux and mean values for O3, CO, and CH4 were obtained in the boundary layer over selected wetland systems in Alaska. These measurements were obtained in July–August 1988 as part of the NASA Global Tropospheric Experiment Programs Arctic Boundary Layer Expedition (ABLE 3A). The flux measurements obtained from this study provide information on the source/sink distribution of O3 and CH4 over the Yukon-Kuskokwim Delta (YKD) and Alaskan North Slope (ANS) regions of Alaska. The source/sink distribution over the YKD is qualitatively correlated with surface vegetation type, identified from multispectral scanner imagery. Direct measurements of the spatial variation in the CH4 source strength were obtained over the YKD. The CH4 source strength over the YKD ranged from 25 to 85 mg m−2 d−1 during a flux survey flight which spanned a considerable portion of the YKD. A spatially averaged, seasonally adjusted source strength of 51 mg m−2 d−1 was established for the YKD. Indirect CH4 flux estimates obtained over the ANS indicate a much lower (∼10 mg m−2 d−1) source strength. The global CH4 emissions from tundra were estimated to be 44 Tg/a based on (1) the spatially averaged source strength obtained over the YKD, (2) current estimates of the global coverage of tundra, and (3) assuming a similarity between other tundra areas and that of the YKD. This estimate is taken to be an upper limit due to possible sampling inadequacies and because the spatial distribution of the CH4 source function over the YKD may not extend to all other northern wetland regions. This estimate is, however, in reasonable agreement with previous estimates. Airborne CO flux measurements over the YKD indicated low negative flux values over the coastal areas, while some positive fluxes were observed in the inland, sparsely forested regions. An inspection of the cospectrum of CO with vertical velocity for sample runs in coastal areas indicated a minimum at wavelengths which were noticeably shorter (70–400 m) than where any prominent feature could be found for similar cospectrums of heat, moisture, O3, or CH4 with vertical velocity. Similar analyses for transects over inland areas indicated occasional peaks in this same, short wavelength band. These features indicate the possibility of in situ photochemical destruction/production of CO, although the identification of a possible chemical mechanism was not attempted at this time. Tundra surfaces are estimated to be responsible for ∼32% of the total deposition loss of O3 poleward of 60°N. Data from the airborne O3 flux measurements made during this study indicate similar values of Rc for the ANS and YKD regions.

Carbon monoxide and methane in the North American Arctic and Subarctic troposphere: July–August 1988

Measurements of carbon monoxide (CO) and methane (CH4) were made in the North American Arctic during July–August 1988. The distribution of CH4 was variable in the atmospheric mixed layer (0–2 km), with concentrations determined primarily by interactions of biogenic emissions from wet tundra and turbulent mixing processes. Carbon monoxide exhibited little variation in unpolluted mixed layer environments indicating a minor role for biogenic sources and/or sinks in determining its distribution. In the free troposphere (2–6 km) both CO and CH4 were variable. Concentration gradients were most frequently associated with intrusions of upper tropospheric or stratospheric air into the midtroposphere, emissions from forest and tundra fires, and long-range transport of enhanced concentrations of these gases from unidentified sources. Summertime haze layers exhibited midtropospheric enhancements of CH4 similar to those measured in winter Arctic haze events. However, these summer pollution episodes did not exhibit positive correlations with particulate sulfate. The summer Arctic and subarctic haze events observed during the Arctic Boundary Layer Expedition (ABLE 3) were primarily a result of forest and tundra fires of natural origin. The tendency for relatively high variability of CO and CH4 at altitudes of 3–6 km indicates that ground-based monitoring will not provide an adequate assessment of the chemical composition of the Arctic troposphere to support future global change studies.

Airborne nitrous oxide observations over the western Pacific Ocean: September–October 1991

The Langley tunable diode laser instrument package incorporated an additional channel to measure nitrous oxide (N2O) during the Pacific Exploratory Mission (PEM) West A. These measurements represent the first airborne, fast response (5-s) N2O data set obtained within the troposphere. Most data were recorded over the western Pacific between 0°N and 45°N latitude, 110°E and 180°E longitude, and 0.3 to 12 km altitude. Important observations include a decreasing N2O latitude gradient of approximately 0.4 parts per billion volume (ppbv) from northern midlatitudes to the equator, a decreasing N2O longitude gradient of 0.4 ppbv from the western Pacific to the central Pacific at northern midlatitudes, and an enhancement of 0.2 ppbv in the boundary layer (altitudes below 0.5 km) relative to the rest of the tropospheric vertical profile. Other observations include increased N2O mixing ratios within both urban and biogenic affected air masses and reduced N2O mixing ratios in stratospheric intrusions. These relationships with air mass source characteristics are exhibited in the large-scale correlations between N2O and CO, CH4, and CO2 in the free troposphere. Atmospheric inputs of N2O are examined and the relative strengths of continental biogenic and anthropogenic/industrial sources are estimated. The data set is also examined for evidence of an oceanic source of N2O.

Atmospheric fine structure during GTE TRACE A: Relationships among ozone, carbon monoxide, and water vapor

A major focus of the Global Tropospheric Experiment/Transport and Chemistry Near the Equator-Atlantic (GTE TRACE A) experiment was to determine whether the seasonal tropospheric ozone (O3) buildup over the tropical South Atlantic, observed by both satellites and sondes, is due to photochemical production of O3 from biomass burning effluents or from transport of O3 from the stratosphere. During this expedition, fast response in situ measurements were made of O3, carbon monoxide (CO), and dew point (DP) from aboard the NASA DC-8 aircraft. Numerous vertical profiles were obtained over Brazil, Africa, and the South Atlantic to determine the origin of the observed O3 enhancement. Fine structure in these data was examined by (1) investigating differences in the species concentration (5-s averages) and its 2 km vertical running mean during altitude profiles, (2) calculating Fishman-Seiler coefficients for all vertical profiles with altitude range greater than 3 km, and (3) calculating linear regressions between species across short time intervals (2.5 min) using time series data. The fine structure holds information about the recent history of an air mass and therefore can indicate the “source” of tropospheric O3 enhancement: either transport from the stratosphere (negative correlation with CO and DP) or from photochemistry in the troposphere (positive correlation with CO). All three methods indicate that within the tropical South Atlantic during the TRACE A experiment, net in situ photochemical production is significantly more important than transport from the stratosphere.

Airborne tunable diode laser system for trace gas measurements

An airborne instrument using tunable diode lasers (TDLs) has been developed to make in situ measurements of trace atmospheric gases. The instrument performance for the measurement of ambient CO is characterized. A technique to suppress TDL excess noise is demonstrated that is expected to lead to an increase in the state-of-the-art sensitivity of the TDL differential absorption technique.