The Arctic Boundary Layer Expedition (ABLE 3A): July–August 1988

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include detrimental effects on Arctic ecosystems.Or increased deposition of nitrogen could enhance biosphereatmosphere gas exchange with subsequent effects on atmospheric gases like CO2 and CH 4. Another concern is changes in the chemistry of "baseline" Arctic air, which is a major source region for north-to-south flow across North America and other regions.Thus, information on Arctic and sub-Arctic air chemistry is essential to resolving controversial issues like natural versus human contributions to acid rain and ozone pollution in the mid-latitudes.

Chemistry-Climate Connection
The ABLE 3A program was focused on high-latitude (>50øN) regions because almost all climate models and paleoenvironmental studies indicate that these regions are especially sensitive to climatic change.Several review papers have appeared recently which provide excellent summaries of the theoretical basis for predicting a tropospheric warming trend in response to increasing atmospheric trace gas concentrations [e.g., Dickinson, 1986;Ramanathan et al., 1987].There are also some empirical data which indicate increasing permafrost temperatures [Lachenbruch and Marshall, 1986], suggesting that certain regions of the highlatitude biosphere may be experiencing the early stages of a warming.Biospheric responses to a variable climate are feedback processes which could accelerate or modulate rates of climate change.Examples of potential feedbacks have been documented by direct measurement of trace gas exchange rates in response to seasonal climate changes, and by inference from correlations of trace gas concentrations and isotopically derived temperature records in ice cores.For example, studies of variations of trace gases of biospheric origin such as CO2 and CH 4 in ice cores suggest that atmospheric composition has been closely coupled to atmospheric temperature in polar regions for at least the past 160,000 years [e.g., Raynaud et al., 1988;Chappellaz et al., 1990].
A more direct indication of initial feedbacks to climate change can be derived from field and experimental studies of CH 4 exchange rates between northern peatland soils and the atmosphere in response to seasonal variations in soil temperature and moisture.In both tundra and boreal environments CH 4 flux to the atmosphere has been shown to be sensitive to seasonal climatic variations [e.g., Sebacher et al., 1986;Moore and Knowles, 1987;Crill et al., 1988;Whalen and Reeburgh, 1988;Bartlett et al.,

High-Latitude Air Pollution: Magnitude and Impacts
The large-scale pollution of the Arctic troposphere by long-range transport of pollutants from industrial regions during late winter and early spring months is well documented [e.g., Schnell, 1984;Barrie, 1986;Stonehouse, 1986;Lowenthal and Rahn, 1985].During these "Arctic haze" pollution events the buildup of aerosol constituents, sulfur dioxide, and PAN has been observed [e.g., Barrie and Hoff, 1985;Bottenheim et al., 1986].At the time of Arctic sunrise, significant perturbations in the chemistry of the boundary layer have been observed: 03 concentrations decrease, gaseous halogens increase, and aerosol pollutant species decrease [Barrie et al., 1989].To date, there have been no comprehensive studies of the chemistry of the Arctic troposphere during summer months.Observations during the summer are critical to an assessment of the full impact of the accumulated winter/spring pollutant loadings, and to determine if significant long-range transport and injection of pollutants occur during these months.1.

Aircraft Experiments
The centerpiece of ABLE 3A was a series of research flights with the instrumented NASA Electra (Figure 2).The flights were divided into four generic types of experiments: (1) Boundary layer survey studies determined the regional horizontal and vertical distribution of trace gas and aerosol species over tundra environments to explore the qualitative effects of biosphere-atmosphere exchange versus atmospheric transport processes on the chemical composition of the atmospheric mixed layer and overlying free troposphere.

Ground-Based Experiments
The extensive peatland environments in the Yukon-Kuskokwim Delta region of Alaska, overflown during mis-sions conducted from Bethel, were selected for detailed ground-based studies of trace gas exchange between the biosphere and the atmosphere.The region includes approximately 9 million hectares of lowland tundra, underlain by permafrost, and containing a large number of shallow lakes.Vegetated areas are covered by a moss/lichen/dwarf shrub community on relatively well-drained soils and by a herbaceous community on wet soils (Figures 5a and 5b Ground-based measurements were carefully coordinated with aircraft overflights to provide data for comparing estimates of CH 4 emissions from local to regional scales based on enclosure, tower, and airborne eddy correlation methods. In addition to ABLE 3A investigators, the ground-based program included investigators sponsored by BREW and the NASA Interdisciplinary Research Program (Table 1).

METEOROLOGICAL MEASUREMENTS
Meteorological forecasts during the ABLE field expedition were provided by a team of meteorologists stationed at the Anchorage National Weather Service Office.The Anchorage office receives extensive data, including both GOES and NOAA 9 polar orbitor imagery, all National Weather Service products, statewide surface observations, and a host of specialized computer-generated products tailored to the Alaska region.

OVERVIEW OF RESULTS
In this brief overview of results, we highlight selected findings from individual investigations which relate directly to the primary objectives of the expedition.It is hoped that this summary will serve the reader who may not be able to pursue study of the entire collection of ABLE 3A papers.This summary also serves as a guide to individual papers, which discuss the details of specific factors controlling the distribution of any particular trace gas or aerosol constituent.

Sources and Sinks
Water-saturated soils and lake sediments are the primary sources of CH4 in the Arctic and sub-Arctic landscapes studied in ABLE 3A.Dry tundra soils can reduce CH4

Ozone' Distribution and Variability
The stratosphere was the dominant source of 0 3 to lower tropospheric altitudes in the ABLE 3A study regions.Welldefined intrusions of O3-rich air from the upper troposphere and stratosphere were directly observed with remote sensing to influence 03 concentrations at altitudes of 1 km and lower [Browell et al., this issue].The dynamical characteristics of stratosphere/troposphere exchange at high latitudes has been discussed by several authors [e.g., Gidel and Shapiro, 1980;Shapiro, 1980;Shapiro et al., 1987;Raatz et al., 1985].The concentrations of NOx in the region studied were sufficiently low that photochemical processes were typically a net sink for tropospheric 03 [Sandholm et al.,

Fig. 1 .
Fig. 1.A schematic illustration of interactions of the biosphere, climate, and long-range transport of pollutants as sources and/or sinks for atmospheric trace gases in northern high-latitude regions.
this issue].Methane flux increases in response to increasing soil tern-peratures in water-saturated organic soils.Soil drying decreases CH 4 emissions.Experimental studies on tundra soil cores indicate the opposite behavior for CO2 flux; aerobic decomposition which produces CO2 is the dominant process in dry soils [e.g., Billings, 1987].Thus, a warmer, wetter climate might enhance CH 4 flux from northern peatland environments.A warmer, dryer climate might result in reduced CH 4 emissions and enhanced CO2 flux.Potential trace gas feedbacks to climate change in northern tundra environments can be expected to operate on at least three time scales: (1) Changes in CH 4 flux from the near-surface "active" soil layer in response to seasonal or interannual climate variations will be the initial signal of a biospheric feedback.(2) Gradual climate change on decadal to century time scales could alter permafrost, a warming trend would release trapped CH 4 from the permafrost and increase the depth of the seasonal active layer.(3) A long-term dramatic warming of the Arctic (e.g., an ice-free condition) could lead to a release of CH 4 from presently frozen methane hydrates found at considerable depth below the surface.The ABLE 3 is focused on understanding the "early warning" response of the near-surface, organic active layer to climate variability.Several preliminary modeling studies have also been conducted to explore potential interactions between climate change and atmospheric chemistry [e.g.has obtained regional-scale empirical data on trace gas exchanges between northern ecosystems and the atmosphere, which will permit a more detailed analysis of potential biosphere-atmosphere feedback processes in response to climate variability.Three independent approaches to CH 4 flux measurement were used to define the characteristic temporal and spatial variability for the Yukon-Kuskokwim tundra ecosystem, Alaska.Ground-based enclosure and eddy correlation measurements were used to characterize temporal variability, individual landscape elements as sources or sinks, and integrated flux from the local area.An airborne eddy correlation measurement program was used to characterize spatial variability in CH 4 flux at the regional scale.

NASA
ABLE 3A 03 studies was to determine the sources of nitrogen oxides (NOx) and total reactive nitrogen (NOy) to the Arctic troposphere.Previous studies indicated that primary production in many biological environments in the Arctic is limited by inadequate levels of available nitrogen during summer months [e.g., Van Cleve and Alexander, 1981].These results suggest that surface environments should be a net sink for NOx and NOy.Urider natural conditions, the Arctic region should be an important low-NOx region for testing photochemical theory on the role of NO• in 03 production and destruction processes.However, the alternate possibility existed that a reservoir of atmospheric reactive nitrogen accumulated during winter and spring months from mid-latitude pollution sources could provide a source of NO• to influence photochemical 03 chemistry during summer months.Enhanced deposition of nitrogen to the Arctic biosphere from mid-latitude pollution sources could also stimulate primary production and alter biosphere-atmosphere exchange rates of other trace gases like CO2 and CH4.Another characteristic of the Arctic tundra ecosystem important to ABLE 3A objectives is the paucity of plant species known to emit isoprene and other reactive nonmethane hydrocarbon species which are important in 03 chemistry.As a long-term strategy, the ABLE missions are designed to study 03 production and destruction processes in atmospheric boundary layer environments which have characteristics of low NO•/low NMHC (tundra), low NOx/ high NMHC (boreal forest), low NOx/high NMHC (wet season tropical forest), intermediate NO•/high NMHC (dry season, unpolluted tropical forest), high NO•/high NMHC (polluted tropical and boreal forests), and high NOx/low NMHC (polluted tundra environments).Results from several of these categories are reported in ABLE of ABLE 3A were accomplished through a coordinated program of chemical and meteorological measurements at surface sites in Alaska and on the (2) Flux measurements were conducted over tundra environments to quantify exchange rates for CH4, CO, and 03 at incremental scales of approximately 50-150 km over a total of up to 2000 km per experiment.(3) Several missions were devoted to determining the large-scale distribution of gas and aerosol species over ice and oceanic environments upwind of tundra, with flight lines along a sea or ice to land gradient.These missions also provided an excellent qualitative indication of gas and aerosol source/sink processes associated with different surface environments.(4) Several missions, and transit flights between bases, were devoted primarily to characterizing mid-tropospheric variability of gas and aerosol species for investigation of long-range transport of pollutants to the study regions and tropospheric photochemical processes.A schematic illustration of these generic flight patterns is shown in Figure 3.The areas studied by intensive aircraft missions are shown in Figure 4.The characteristics of each mission are summarized in Table 2.In situ measurements of most of the trace gas and aerosol chemical species discussed in the above sections are available for all of the flights listed in Table 2.The twodimensional distribution of aerosol and 03 from the surface to the tropopause was measured along each flight path using a UV Differential Absorption Lidar (DIAL) described by Browell et al. [this issue].The UV DIAL also provides information on cloud distribution and on mixed layer dynamics.Aircraft research missions were conducted from bases in Barrow, Alaska (flights 6-12), Bethel, Alaska (flights 14-21, 25-26), Cold Bay, Alaska (flights 22-24), and Thule, Greenland (flight 29).Flights 30-33 from Thule, Greenland, to Wallops Island, Virginia, on August 15-16, 1988, included vertical profiles along the flight track to determine latitudinal distributions of trace gas and aerosol species as a function of altitude.Flights 1-5 were constant altitude transits between Wallops Island, Thunder Bay, Churchill, Thule, Fairbanks, and Barrow (Figure 4).
).Data on the distribution of vegetation types and other environmental characteristics were available at a number of spatial scales.Land cover of the entire delta region has been mapped at 80-km resolution by the U.S. Geological Survey using Landsat imagery.More detailed characterization of local areas was conducted during the analysis of ABLE 3A data using System Probatoire d'Observation de la Terre (SPOT) imagery, which has 15-m resolution.The two major components of the ground-based research program were (1) measurements of fluxes, total storage, and isotopic characteristics of biogenic trace gases using enclosures and soil sampling techniques.(2) Flux and ambient atmospheric concentrations of trace gases were also measured using micrometeorological and eddy correlation techniques to obtain time series of surface/atmosphere exchange.The objective of enclosure measurements was to quantify relationships of biogenic gas sources and sinks along major environmental gradients representative of the Yukon-Kuskokwim tundra.Topography and resulting surface and soil water conditions exert major controls over vegetation and soil composition (Figure 5a).These relationships were also hypothesized to mediate trace gas fluxes.Satellite remote sensing of the spatial distribution of surface environments was used to extrapolate point measurements of flux to the regional tundra environment [e.g., Bartlett et al., this issue; Whiting et al., this issue].Relationships of flux with important physical variables such as temperature and light level were also studied.The trace gas species examined were CH 4 [Bartlett et al., this issue], CO2 [Whiting et al., this issue], and several sulfur compounds [Hines and Morrison, this issue].Soil profiles and depth to permafrost along selected transects were obtained using ground-penetrating radar techniques by Doolittle et al. [1990].Studies were conducted in undisturbed sites accessible by road from Bethel, and in the area of the ABLE 3A micrometeorological tower.The ABLE 3A micrometeorological tower facility is shown in Figure 6.The tower was located approximately 50 km WNW of Bethel (61øN, 162.5øW) and was accessible by float plane.Climatological data were used to site the tower in a location which would be subject to minimal local pollution effects.

Fig. 2 .
Fig. 2. The NASA Electra research aircraft and a diagram of the location of instrumentation during ABLE 3A.
Fig. 8. Photochemical production minus loss rate of 03, (P -L)03, as a function of NO concentration.Results are shown for 475 individual points from flights 11-25 where detailed aircraft measurements of atmospheric composition are available.The rates were obtained using a photochemical model [Jacob et al., this issue (a)].The ensemble of points covers the altitude range 0.1-6.2km and the temporal range 0600-1915 solar time.