JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. D15, PAGES 16,383-16,394, OCTOBER 30, 1992 The Arctic Boundary Layer Expedition (ABLE 3A)' July-August 1988 R. C. HARRISS,• S.C. WOFSY,2 D. S. BARTLETT, M. C. SHIPHAM,3 D. J. JACOB, J. M. HOELL, JR., 3 1 2 R. J. BENDURA,3 J. W. DREWRY,3 R. J. MCNEAL, 4 R. L. NAVARRO,5 R. N. GIDGE, 5 AND V. E. RABINE5 The Arctic Boundary Layer Expedition (ABLE 3A) used measurements from ground, aircraft, and satellite platforms to characterize the chemistry and dynamics of the lower atmosphere over Arctic and sub-Arctic regions of North America during July and August 1988. The primary objectives of ABLE 3A were to investigate the magnitude and variability of methane emissions from the tundra ecosystem, and to elucidate factors controlling ozone production and destruction in the Arctic atmosphere. This paper reports the experimental design for ABLE 3A and a summary of results. Methane emissions thetundra from landscape varied widely from-2.1 to 426mgCH4 m-2 d-1 . Soil moisture and temperature were positively correlated with methane emission rates, indicating quantitative linkages between seasonalclimate variability and soil metabolism. Enclosure flux measurement techniques,tower-based eddy correlation, and airborne eddy correlation flux measurementsall proved robust for application to methane studies in the tundra ecosystem. Measurements and photochemical modeling of factors involved in ozone production and destruction validated the hypothesized importance of low NOx concentrations as a dominant factor in maintaining the pristine Arctic troposphere as an ozone sink. Stratospheric intrusions, long-range transport of mid-latitude pollution, forestfires,lightning, aircraft all potential and are sources NOx andNOy to Arcticandsub-Arctic of regions. ABLE 3A results indicate human that activities havealready may enhanced NOy inputs the to region to the extent that the lifetime of 0 3 againstphotochemicalloss may have already doubled. A doublingof NOx concentrationfrom presentlevels would lead to net photochemicalproductionof 03 during summer months in the Arctic (Jacob et al., this issue (a)). The ABLE 3A results indicate that atmospheric chemical changes in the northern high latitudes may serve as unique early warning indicators of the rates and magnitude of global environmental change. INTRODUCTION The Arctic Boundary Layer Expedition (ABLE 3A) was conducted in Arctic and sub-Arctic regions of North America and Greenland during July and August 1988. This was the first comprehensive investigation of the sources, sinks, and distribution of trace gas and aerosol chemical species in a northern high-latitude region during summer months. The ABLE 3A experimental design placed emphasis on the role of biosphere-atmosphere interactions in determining the chemical composition of the troposphere and on processes Tropospheric Chemistry Program [McNeal et al., 1983]. Previous ABLE expeditions have reported on the chemistry of North African dust and marine air over the tropical Atlantic [e.g., Ferek et al., 1986; Talbot et al., 1986] and on air chemistry over the tropical rain forests of Guyana and Brazil [e.g., Gregory et al., 1986; Harriss et al., 1988, 1990]. A second expedition to the northern high latitudes (ABLE 3B) was conducted jointly with the Canadian Northern Wetlands Project during July-August 1990. This paper reports the overall experimental design for ABLE individual 3A and studies. includes a brief overview of results. A which influence the tropospheric03 budget (Figure 1). The suite of chemical species measured included the following gases:methane (CH4) , carbon monoxide (CO), carbon dioxide (CO2), nonmethane hydrocarbons(NMHC), acetic acid (HA), formic acid (HFo), nitric oxide (NO), nitrogen dioxide following series of papers report the detailed results of ARCTIC AND BOREAL REGION AIR Arctic and boreal regions (>50øN) are uniquely important to tropospheric chemistry for at least two reasons: (1) these regions include approximately 27% of the world's soil carand size distribution. bon [Post et al., 1982]. The exchange of this carbon between The ABLE 3A is a component of the NASA Global soilsand the atmosphere, as CO2 and CH4, is influenced by Tropospheric Experiment (GTE) sponsored by the NASA climate variability [e.g., Billings, 1987]. In a "global warming" era these environments may be "feedback" regions l Institute the Study Earth,Oceans Spate,University which influence rates of climatic change. (2) Even the most for of and of New Hampshire, Durham. remote wilderness areas of the region are showing indica2Harvard University, Cambridge, Massachusetts. 3Atmospheric Sciences Division, NASA Langley Research Cen- tions of air pollution derived from long-range transport from ter, Hampton, Virginia. mid-latitude source emissions. During late winter and early 4Earth Science Applications and Division, National Aeronautics spring, meteorological conditions are particularly favorable and Space Administration, Washington, D.C. for midtropospheric air masses to track across industrialized 5NASAWallops FlightFacility, Wallops Island, Virginia. regions and into the Arctic (see Bartie [1986] for a review). Copyright 1992 by the American Geophysical Union. It is particularly important to understand both direct and indirect impacts of long-range transport of pollutants on the Paper number 91JD02109. 0148-0227/92/91JD-02109505.00 chemistry of high-latitude air masses. Direct impacts could (NO2), total "reactive" nitrogengas (NOv), nitric acid (HNO3), peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), ozone (03), and aerosol chemical composition 16,383 16,384 HARRISSET AL.: ARCTIC BOUNDARYLAYER EXPEDITION, ABLE 3A peratures in water-saturated organic soils. Soil drying decreasesCH 4 emissions.Experimental studieson tundra soil cores indicate the oppositebehavior for CO2 flux; aerobic decomposition which producesCO2 is the dominantprocess 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 reducedCH 4 emissions and enhancedCO2 flux. Potential trace gas feedbacks to climate change in northern tundra environments can be expected to operate on at least three time scales: (1) Changesin CH 4 flux from the Fig. 1. A schematicillustration of interactionsof the biosphere, near-surface "active" soil layer in response to seasonalor climate, and long-range transport of pollutants as sources and/or interannual climate variations will be the initial signal of a sinks for atmospherictrace gasesin northern high-latituderegions. biosphericfeedback. (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 include detrimental effects on Arctic ecosystems. Or increased deposition of nitrogen could enhance biosphere- increase the depth of the seasonal active layer. (3) A atmosphere gas exchange with subsequenteffects on atmo- long-term dramatic warming of the Arctic (e.g., an ice-free sphericgases like CO2 and CH 4. Another concernis changes condition) could lead to a release of CH 4 from presently depth below in the chemistry of "baseline" Arctic air, which is a major frozen methanehydratesfound at considerable the surface. The ABLE 3 is focused on understandingthe source region for north-to-south flow across North America and other regions. Thus, information on Arctic and sub- "early warning" responseof the near-surface, organic active Arctic air chemistry is essential to resolving controversial layer to climate variability. Several preliminary modeling studies have also been conissues like natural versus human contributions to acid rain ducted to explore potential interactions between climate and ozone pollution in the mid-latitudes. changeand atmosphericchemistry [e.g., Hameed and Cess, 1983; Khalil and Rasmussen, 1989]. These studies also Chemistry-Climate Connection indicate that climate-induced feedbacks from natural soils The ABLE 3A program was focused on high-latitude (>50øN) regions because almost all climate models and paleoenvironmental studies indicate that these regions are couldpotentiallyinfluencethe global CH 4 budget. The ABLE 3A has obtained regional-scaleempirical data on trace gas exchanges between northern ecosystems and the atmosphere, which will permit a more detailed analysis of potential biosphere-atmosphere feedback processesin response to climate variability. Three independent approachesto CH 4 flux measurementwere used to define the characteristictemporal and spatial variability for the YukonKuskokwim tundra ecosystem, Alaska. Ground-based enclosure and eddy correlation measurements were used to characterizetemporal variability, individual landscapeelements as sources or sinks, and integrated flux from the local area. An airborne eddy correlation measurementprogram was usedto characterizespatialvariability in CH 4 flux at the regional scale. especially sensitiveto 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 atmospherictrace gas concentrations [e.g., Dickinson, 1986; Ramanathan et al., 1987]. There are also some empirical data which indicate increasingpermafrost temperatures [Lachenbruchand Marshall, 1986], suggestingthat certain regions of the high- latitude biospheremay be experiencingthe early stagesof 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 documentedby direct measurementof trace gas exchange rates in response to seasonalclimate 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 biosphericorigin suchas CO2 and CH 4 in ice coressuggest that atmospheric composition has been closely coupled to atmospheric temperature in polar regions for at least the past 160,000years [e.g., Raynaud et al., 1988;Chappellaz et al., 1990]. A more direct indication of initial feedbacks to climate High-Latitude Air Pollution: Magnitude and Impacts changecan be derived from field and experimentalstudiesof CH 4 exchangerates between northern peatlandsoilsand the atmosphere in response to seasonalvariations in soil temperature and moisture. In both tundra and boreal environ- ments CH 4 flux to the atmospherehas 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., this issue]. Methane flux increases in response to increasing soil tern- 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, significantperturbations 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 comprehensivestudiesof the chemistry of the Arctic troposphere during summer months. Observations during the summerare critical to an assessment the full impact of the of accumulatedwinter/springpollutant loadings, and to determine if significant long-range transport and injection of pollutants occur during these months. HARRISSET AL..' ARCTIC BOUNDARYLAYER EXPEDITION, ABLE 3A 16,385 Observations at a few ground-basedmonitoring sites have indicated that concentrations of aerosols are at a minimum during summer periods [e.g., Bodhaine, 1986]. However, ground-based monitoring stations at Arctic sites are influenced by frequent stratuscloud cover and may not be a good indicator of overall tropospheric air chemistry. Evidence gathered in ABLE 3A indicates that the stratus cloud decks common over the Arctic during summer months may filter out soluble aerosol species before they reach ground level [e.g., Talbot et al., this issue]. The observation of a possible increasing trend in surface 03 at Barrow, Alaska [Oltmans and Komhyr, 1986] is a potential indicator of an increasing degree of Arctic pollution. However, the 03 concentrationat any individual site will be influenced by a variety of meteorological and chemical factors. The ABLE 3A placed special emphasis on identifying the range of variables which might have a significant influenceon the tropospheric03 budget in the Barrow region during the summer period. A componentof ABLE 3A 03 studieswas to determine the sources of nitrogen oxides (NOx) and total reactive was conducted during July and August 1988. A complimentary program of surface-based biogeochemical studies, termed the Biospheric Research on Emissions from Wetlands (BREW), supported by the NASA Biospherics Research Program, was conducted in Bethel, Alaska, during the period of the ABLE 3A. Investigators sponsored by the NASA Interdisciplinary Program also participated in the expedition. A list of principal investigators, institutions, and measurements is presented in Table 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 speciesover tundra environments to explore the qualitative effects of biosphere-atmosphere exchange versus atmospheric transport processeson the chemical composition of the atmosphericmixed layer and overlying free troposphere. (2) Flux measurements were conducted over tundra environnitrogen (NOy) to the Arctictroposphere. Previous studies ments to quantify exchange rates for CH4, CO, and 03 at indicated that primary production in many biological envi- incremental scales of approximately 50-150 km over a total ronments in the Arctic is limited by inadequate levels of of up to 2000 km per experiment. (3) Several missionswere available nitrogen during summer months [e.g., Van Cleve devoted to determining the large-scale distribution of gas and and Alexander, 1981]. These results suggest that surface aerosol species over ice and oceanic environments upwind environments bea netsink NOxand should for NOy.Urider of tundra, with flight lines along a sea or ice to land gradient. natural conditions, the Arctic region should be an important These missions also provided an excellent qualitative indilow-NOx regionfor testingphotochemicaltheory on the role cation of gas and aerosol source/sink processes associated of NO• in 03 production and destructionprocesses.How- with different surface environments. (4) Several missions, ever, the alternate possibility existed that a reservoir of and transit flights between bases, were devoted primarily to atmospheric reactive nitrogen accumulated during winter characterizing mid-tropospheric variability of gas and aeroand spring months from mid-latitude pollution sourcescould sol species for investigation of long-range transport of polprovide a source of NO• to influence photochemical 03 lutants to the study regions and tropospheric photochemical chemistry during summer months. Enhanced deposition of processes. A schematic illustration of these generic flight nitrogen to the Arctic biosphere from mid-latitude pollution patterns is shown in Figure 3. sources could also stimulate primary production and alter The areas studied by intensive aircraft missions are shown biosphere-atmosphere exchange rates of other trace gases in Figure 4. The characteristics of each mission are summarized in Table 2. like CO2 and CH4. Another characteristic of the Arctic tundra ecosystem In situ measurements of most of the trace gas and aerosol important to ABLE 3A objectives is the paucity of plant chemical species discussedin the above sections are availspecies known to emit isoprene and other reactive non- able for all of the flights listed in Table 2. The twomethane hydrocarbon species which are important in 03 dimensionaldistribution of aerosol and 03 from the surface chemistry. As a long-term strategy, the ABLE missionsare to the tropopausewas measured along each flight path using designedto study 03 production and destructionprocesses a UV Differential Absorption Lidar (DIAL) described by in atmospheric boundary layer environments which have Browell et al. [this issue]. The UV DIAL also provides characteristics low NO•/low NMHC (tundra), low NOx/ information on cloud distribution and on mixed layer dynamof high NMHC (boreal forest), low NOx/high NMHC (wet ics. Aircraft research missions were conducted from bases in seasontropical forest), intermediate NO•/high NMHC (dry season,unpolluted tropical forest), high NO•/high NMHC Barrow, Alaska (flights 6-12), Bethel, Alaska (flights 14-21, (polluted tropical and boreal forests), and high NOx/low 25-26), Cold Bay, Alaska (flights 22-24), and Thule, GreenNMHC (polluted tundra environments). Results from sev- land (flight 29). Flights 30-33 from Thule, Greenland, to eral of these categories are reported in ABLE 2 publications Wallops Island, Virginia, on August 15-16, 1988, included (Journal of Geophysical Research, volume 93, pages 1349- vertical profiles along the flight track to determine latitudinal 1624, 1988; and volume 95, pages 16,721-17,050, 1990) and in distributionsof trace gas and aerosol speciesas a function of the present issue. altitude. Flights 1-5 were constant altitude transits between Wallops Island, Thunder Bay, Churchill, Thule, Fairbanks, and Barrow (Figure 4). APPROACH The scientific objectives of ABLE 3A were accomplished through a coordinated program of chemical and meteorological measurements at surface sites in Alaska and on the Ground-Based Experiments NASA Lockheed Electra research aircraft. The expedition The extensive peatland environments in the YukonKuskokwim Delta region of Alaska, overflown during mis- 16,386 HARRISSET AL.: ARCTIC BOUNDARYLAYER EXPEDITION, ABLE 3A TABLE Investigator 1. Principal Investigators Participating in ABLE 3A Institution Investigation John Barrick John Bradshaw Edward V. Browell David R. Fitzjarrald Gerald L. Gregory Robert C. Harriss Paul Kebabian Enio Pereira John Ritter F. Sherwood Rowland Glen W. Sachse Hanwant Singh Robert W. Talbot Steven C. Wofsy Tropospheric Chemistry Program NASA Langley Research Center Airborne meteorological/positiondata (a) Georgia Institute of Technology Nitric oxide,nitrogen dioxide,Nay (a) NASA Langley Research Center Aerosols, ozone profiles (a) State University of New York at Albany Micrometeorogical studies (s) Ozone, aerosol size (a) NASA Langley Research Center NASA Langley Research Center Carbon dioxide/Mission Scientist (a) Methane (s) Aerodyne Research, Inc. Radon (a) Instituto de PesquisasEspacials, Brazil NASA Langley Research Center Eddy correlationflux (CO, CH4, 03, H2, H2a ) (a) University of California at Irvine Nonmethane hydrocarbons (a) NASA Langley Research Center Carbon monoxide, methane (a) NASA Ames Research Center PAN, PPN, CC14 (a) NASA Langley Research Center Aerosol composition, nitric and organic acids (a) Carbon dioxide (a) Harvard University Nitrogen species (NO, Na 2, Nay) (s) Eddycorrelation (03, ca2, Nay) (s) flux Biospheric Research Program (s) David S. Bartlett Michael Hardisky Karen B. Bartlett Mark Hines NASA Langley ResearchCenter University of Scranton College of William and Mary University of New Hampshire University of Maine University of Delaware University of North Carolina CO2 exchange/Mission scientist Below-groundbiomass/radar Methane flux Sulfur flux Methane oxidation Below-ground biomass/radar Gary King Vic Klemas Christopher Martens Patrick Crill Radon/CH4 Isotopes Interdisciplinary Program (s) University of New Hampshire Methane flux (a), airborne; (s), surface. 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). Data on the distribution of vegetation types and other environmental characteristics were available at a number of 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 speciesexamined 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. 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-basedresearch program were (1) measurements of fluxes, total storage, and isotopic characteristics of biogenic trace gasesusing enclosures and soil sampling techniques. (2) Flux and ambient atmospheric concentrations of trace gases were also measured using micrometeorological and eddy correlation techniquesto obtain time seriesof surface/atmosphere exchange. The objective of enclosure measurementswas to quantify relationships of biogenic gas sources and sinks along major environmental gradients representative of the YukonKuskokwim tundra. Topography and resulting surface and soil water conditions exert major controls over vegetation and soil composition (Figure 5a). These relationshipswere also hypothesized to mediate trace gas fluxes. Satellite remote sensingof the spatial distribution of surfaceenvironments was used to extrapolate point measurementsof flux to the regional tundra environment [e.g., Bartlett et al., this The ABLE 3A micrometeorological tower facility is shown in Figure 6. The tower was locatedapproximately50 km WNW of Bethel (61øN, 162.5øW)and was accessibleby float plane. Climatological data were used to site the tower in a location which would be subjectto minimal local pollution effects. Ground-based measurements were carefully coordinated with aircraft overflights to provide data for comparing estimates of CH 4 emissions from local to regional scalesbased on enclosure, tower, and airborne eddy correlation methods. In addition to ABLE 3A investigators, the ground-based program included investigators sponsoredby BREW and the NASA Interdisciplinary Research Program (Table 1). METEOROLOGICAL MEASUREMENTS Meteorological forecasts during the ABLE field expedition were provided by a team of meteorologistsstationed at the Anchorage National Weather Service Office. The Anchorageoffice 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. HARRISSET AL.: ARCTIC BOUNDARYLAYER EXPEDITION, ABLE 3A 16,387 NASA-429 Laboratory 0 3, Aerosolsize FCO, CH 4 (NASA, LaRC) (NASA, LaRC) PAN NO,NO NOy 2, (Ga. Inst. of Tech.) (NASA, Ames) Rn NMHC LIDAR (INPE) Aerosols TAMMS (Univ.of Calif.at Irvine) & MET Oa, Aerosols profiles (NASA, LaRC) Organic Acids, HNO3 (Univ. of New Harnp.) (NASA, LaRC) Fig. 2. The NASA Electra research aircraft and a diagram of the location of instrumentation during ABLE 3A. In addition• a Micro-VAX-II computer from NASA was used to receive domestic, international, and model data through a satellite downlink from Zeypher Weather, Inc. This allowed a host of additional products to be generated and stored in near real time, including soundings, potential 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. temperature time-height cross sections, and 12- to 48-hour forecast wind fields. All generated products were faxed to the aircraft location on a twice daily basis. Forecasts were updated via telephone and were available on an as needed basis. Methane 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 concentrations below ambient concentrations in the atmo- Postmission meteorological analyses included a comparison of weather during the study period to climatological means, calculation of isentropic trajectories for air massflow and source regions associated with each aircraft mission, and compilation of the active forest fires in Alaska during ABLE 3A. A summary of meteorological methods and results for ABLE 3A is provided by Shipham et al. [this issue]. OVERVIEW OF RESULTS spheric mixed layer, acting as a weak sink for tropospheric CH4 [Whalen and Reeburgh, 1990]. In the YukonKuskokwim Delta environments studied in ABLE 3A, CH4 exchange ratesranged widelyfrom -2 mg m-2 d-] (net consumptionof atmosphericCH4) to net emissionsas high as400mgm-2 d-] [Bartlett al., thisissue]. et A synthesisof publishedCH 4 flux data from high-latitude tundra sites by Bartlett et al. [this issue] was used to 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. calculate an annual flux of approximately 11 --+4 Tg CH 4 from the global tundra ecosystem. Scaling up the CH4 flux data from the ABLE 3A micrometeorological tower pro- duced estimate approximately Tg CH4 yr-• from an of 11 global tundra [Fan et al., this issue]. Most previous esti- mates centered between and40Tg yr -• [e.g.,Sebacher 20 et al., 1986; Whalen and Reeburgh, 1988]. It is significant to 16,388 HARRISSET AL.' ARCTICBOUNDARYLAYER EXPEDITION,ABLE 3A 3 - 5 km AGL (a) SURFACE EXCHANGE STUDIES (c) LAND-OCEAN-ICE GRADIENT STUDIES (d) PHOTOCHEMICAL STUDIES Fig. 3. Simplifiedillustrationsof flight profilesused in ABLE 3A. note that these new, lower estimatesof the tundra CH 4 source are very compatible with the preferred estimate derivedwith a globalmodelingtechnique[Fung et al., 1991]. The dominantfactors determining magnitude CH 4 the of flux were soil moisture and soil temperature. Watersaturatedsoilstypically emitted CH• at rates more than an order of magnitude greater than dry soils (Figure 7). The sensitivity of CH• flux rates from both saturated and moist THULE CHURCHILL THUNDER BAY i r•--•RESEARCH ..... TRANS• AREAS FLIGHTS Fig. 4. Map of the regions studiedby ABLE 3A. HARRISS AL..'ARCTIC ET BOUNDARY LAYEREXPEDITION, ABLE 3A TABLE 2. Mission 16,389 Summary of the Flights Conducted During the ABLE 3A Expedition Departure Arrival Number 1 2 3 4 5 6 7 8 9 10 II Flight Date July 7 July 7 July 8 July 9 July 10 July 12-13 July 13-14 July 15-16 July 17 July 18-19 ,.,,., July ,• -,n 17--/.•J Time 1312 1813 1356 1250 1951 2332 1945 2033 1756 1925 2024 Location NASA Wallops Island Thunder Bay Churchill Thule Fairbanks Barrow Barrow Barrow Barow Barrow • ......u 1•o.•11 Itv Time 1700 2108 1845 1844 2329 0304 0043 0046 2309 0048 n• • uxdJ Location Purpose Thunder Bay Churchill Thule Fairbanks Barrow Barrow Barrow Barrow Barrow Barrow Barrow Barrow Bethel Bethel Bethel Bethel Bethel Bethel Bethel Bethel Bethel Mid-troposphere Mid-troposphere Mid-troposphere Mid-troposphere Mid-troposphere Correlations distributions distributions distributions distributions distributions Boundary layer composition Boundary layer composition Vertical distributions Flux measurements Vertical distributions Vertical distributions 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Time July 21-22 July 24 July 26-27 July 27-28 July 28-29 July 29-30 July 31 Aug. 2-3 Aug. 3 Aug. 4 Aug. 7 Aug 7-8 Aug. 8 Aug. 9 Aug. 9-10 Aug. 11-12 Aug. 12 Aug. 13 Aug. 15 Aug. 15 Aug. 16 Aug. 17 is GMT. 2303 1801 2007 2351 1955 1859 1707 1855 1800 0001 1902 2329 2206 0131 2057 2136 1723 1330 1200 1719 1333 1340 Barrow Barrow Bethel Bethel Bethel Bethel Bethel Bethel Bethel Bethel Bethel Cold Bay Cold Bay Bethel Bethel Bethel Barrow Thule Thule Frobisher Bay Goose Bay Portland 0349 2343 0033 0503 0107 0016 2214 0010 2220 0404 2157 0419 2331 0645 0156 0015 2224 1836 1636 2108 1750 1708 Mid-troposphere distributions Vertical distributons Vertical distributions/correlations Flux measurements Land-sea interface Flux measurements Land-sea interface Vertical distributions Vertical distributions Cold Bay Cold Bay Bethel Bethel Mid-troposphere distributions Vertical distributions Mid-troposphere distributions Land-sea interface Flux measurements Bethel Barrow Thule Thule Frobisher Bay Goose Bay Portland NASA Langley Mid-troposphere Mid-troposphere Mid-troposphere Mid-troposphere Mid-troposphere Mid-troposphere Mid-troposphere distributions distributions distributions distributions distributions distributions distributions tundra to variations in soil temperature are also shown in Figure 7. In wet meadow tundra a 2øC increase in temperature at 10- to 20-cm soil depth increasesthe CH 4 flux to the atmosphere approximately 120%. These results together with similar characterizations at other Arctic and boreal sites concentrations.A long-term monitoring programfor ambient CH 4 at sites downwind of extensive tundra could possibly provide an "early warning" of climate change effects in the Arctic. [e.g., Crill et al., 1988; Whalen and Reeburgh, 1988] indicate that a warming of several degreescentigrade during summer months in northern high latitudes could possibly produce a detectable increase in regional tropospheric ambient CH 4 Sources and Chemistry of Nitrogen Gases The ABLE 3A results indicate that pollutant emissions from human activities in mid-latitude regions and emissions WET MEADOW 30%of Delta 0 -' UPLAND TUNDRA (50%of Delta) LAKE (20%of Delta) I - 2 VERTICAL SCALE, 3 meters 4- 5 - Fig. 5a. Schematiccrosssectionof the type of environmentsstudiedby aircraft and ground-based investigationsin the Yukon-Kuskokwim Delta region of Alaska during ABLE 3A. 16,390 HARRISSET AL..' ARCTIC BOUNDARYLAYER EXPEDITION,ABLE 3A Fig. 5b. An aerial perspectiveof the Yukon-KuskokwimDelta, includingthe ABLE 3A ground site. HARRISSET AL.' ARCTIC BOUNDARYLAYER EXPEDITION, ABLE 3A EDDY CORRELATION MEASUREMENTS FLUX OF 16,391 3- COMPONENT SONIC ANEMOMETER AND •' • NOy- Au CONVERTER/LUMINOL CHEMILUMINESCENCE 12 .• m 11 m 03, CO2, CH 4, THC, NOy HIGH SPEED TEMPERATURE ••10m 9m 8m PROFILES OF: FAST I-I RESPONSE rI r..n•.• NO - CHEMILUMINESCENCE 1%11'tVIVI NO - PHOTOLYTIC 2 CONVERTER/ CHEMILUMINESCENCE 5m 03 - DASIBI 4m'• CO2 - IR ABSORPTION THC FLAME IONIZATION DETECTOR W-COMPONENT SONIC ANEMOMETERS AND HIGH SPEED TEMPERATURE CH 4 -ZEEMAN-TUNEDHe - Ne LASER ABSORPTION Fig. 6. Ground-basedmicrometeorologicaltower used for conductingflux measurementsin a tundra environment near Bethel, Alaska. from sub-Arctic forest fires are sources of reactive nitrogen gasesto the sub-Arctic and Arctic troposphere during summer months. The tundra ecosystem is a net sink for atmospheric nitrogen species. A brief synthesisof the ground and airborne nitrogen measurementsis presented here as a guide to the detailed results presented in other papers in this issue [Bakwin et al., this issue; Sandholm et al., this issue; Jacob et al., this issue (a); Singh et al., this issue (a, b); Talbot et al., this issue; Wofsy et al., this issue]. Both ground and airborne measurements indicate that the sub-Arctic tundra ecosystem is a net sink for atmospheric nitrogen species. Bakwin et al. [this issue] report a near- continuous series NO, NO2, andtotalNOy measuretime of ments at the Lake ABLE ground site for July and August. The fluxesof NOx and NOy determined from thesedata indicate an emission rate for NO from the tundra surface to the atmosphere 0.17(_+.10) 10 molecules -1 s-•. of x 9 cm The meandry deposition NOy to the tundrawas 2.0 of (----- X 10 molecules -1 s-1. Themean deposition 1.0) 9 cm wet rate for NOj- to the Lake ABLE region during the ABLE study period approximately x 10 molecules -1 was 3.9 9 cm s-• [Talbotet al., thisissue]. Thus,thetundra wasa netsink for atmospheric nitrogen speciesduring this summer period. lOO 0 ß lO UPLAND TUNDRA TUNDRA Enhanced NOx andNOy concentrations observed the at Lake ABLE ground site during the study correlated with the long-range transport of emissions from forest fires into the Yukon-Kuskokwim Delta region. Forest fire emissions polluted a significantportion of the tropospheric column during episodes of westerly flow from the areas of active burning which were centered around the Yukon Flats region north of Fairbanks [Wofsy et al., this issue; Shipham et al., this issue; Harriss et al., this issue; Bakwin et al., this issue (a)]. The NOx levels associatedwith emissionsfrom forest fires were often greater than 30 pptv, a level which would promote photochemical 03 production [Jacob et al., this issue; Singh et al., this issue (b)]. However, studies of haze WET MEADOW SOILTEMPERATURE, - 20 cm (øC) 10 Fig. 7. Fluxes of methane from two ABLE 3A study sites [Bartlett et al., this issue]. 16,392 HARRISSET AL.: ARCTICBOUNDARY LAYER EXPEDITION,ABLE 3A 400 I I I I ß layers derived from biomass burning indicate a relatively rapid conversionof NOx to PAN in the Alaska troposphere, with consequent low 03 enhancements comparedto tropical haze layers [Jacob et al., this issue (a); Wofsy et al., this issue]. T 200 The vertical distributionsof PAN, NO, NO2, HNO3, and NOy indicate theprimary that sources these of gases the to North American sub-Arctic and Arctic troposphere are a combination of stratospheric intrusions, long-range transport of pollutants from mid-latitude sources, and warm season biomass burning in sub-Arctic environments. The concentration of PAN increases with altitude, with highly variable concentrations above 3 km (e.g., <50 to >700 ppt). Singh et al. [this issue (a)] attribute the origin of PAN to a group of diverse sources,including injectionsof mid-latitude pollution during winter-spring "Arctic haze" events, forest fires, and stratospheric intrusions. It is likely that lightning -200 - -400 0 I 10 I 20 I 30 I 40 50 NO, ppt andaircraft also are potentially significant sources NOy to of this region. The relative stability of PAN in the cold middle and upper Arctic troposphere promotes accumulationand a lifetime determined primarily by the dynamics of downward transport. In the atmospheric mixed layer (0-3 km), PAN concentrations are typically 0-50 ppt. PAN, and an as yet unidentified suite of organic nitrate gases (alkylnitrates and pernitrates?), has the potential to control the summerNOx availability in the high-latitude troposphere and thus to determine 03 concentrationsand distribution [Singh et al., this issue (b); Jacob et al., this issue (a)]. Photochemical modeling indicated that decomposition of PAN alone could accountfully for the NOx concentrationsobservedat 0- to 2-km altitude, but for only 20% of the NOx at 5-6 km. Ozone' Distribution and Variability Fig. 8. Photochemicalproduction minus loss rate of 03, (P L)03, as a function of NO concentration.Results are shownfor 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 0.1-6.2 km and the temporal range 0600-1915 solar time. produceNOx in the Arctic are derived, in part at least, from human activities, the lifetime of 03 against photochemical loss may have already increased significantly. Increased NOx inputs would further reduce the capacity of the Arctic as a region for 03 destruction; in particular, a doubling of NOx concentrations from presentlevels would lead to net 03 photochemical production in the Arctic (Figure 8). Acidic Gases, Aerosols, and Precipitation The Arctic and sub-Arctic tropospheric regions studied during ABLE 3A were acidic. Formic and acetic acids were The stratospherewas the dominant sourceof 0 3 to lower the principal acidic gases,the aerosol acidity was due to the tropospheric altitudes in the ABLE 3A study regions. Well- presence of "excess" sulfate [Talbot et al., this issue]. The definedintrusionsof O3-rich air from the upper troposphere rainwater-free acidity (average pH = 4.69) in the Bethel area and stratospherewere directly observed with remote sensing was derived from the carboxylic acids and H2SO4. Sources to influence03 concentrations altitudesof 1 km and lower of these acids included marine and continental biogenic at [Browell et al., this issue]. The dynamical characteristicsof emissions, forest fires, and to a lesser extent, long-range stratosphere/troposphereexchange at high latitudes has transport of industrial pollutants. been discussedby several authors [e.g., Gidel and Shapiro, Nitric acid is a major component of the nitrogen cycle in 1980; Shapiro, 1980; Shapiro et al., 1987;Raatz et al., 1985]. the boundary layer. Decomposition of PAN and biogenic The concentrations of NOx in the region studied were emissions of NO are precursors for HNO 3 production, sufficientlylow that photochemicalprocesseswere typically biomass burning, and long-range transport of industrial a net sink for tropospheric03 [Sandholmet al., this issue; pollutants to the region contribute to episodic increases Jacob et al., this issue (a)]. The extensive biomass burning in Alaska during summer 1988 had little impact on the [Talbot et al., this issue; Bakwin et al., this issue; Jacob et al., this issue (a); Singh et al., this issue (a, b)]. The results observedtropospheric03 distributions[Gregory et al., this issue; Browell et al., this issue; Jacob et al., this issue (a); Wofsy et al., this issue]. Long-range transport of photo- chemicallyderived 03 from the mid-latitudesinto the study area was difficult to detect due to the relatively high "background" of 03 derived from upper atmosphericsources. Jacob et al. [this issue (b)] combined the aircraft 03 measurements estimate an average 0- to 7-km 03 column to of photochemicalmodel simulationspredict an HNO3 concentration of 50 ppt for the boundary layer [Jacob et al., this issue (a)], the measured mean concentration was 59 _+ 25 ppt [Talbot et al., this issue]. Nitric acid is the primary component NOy dry deposition the tundra of to ecosystem [Bakwin et al., this issue]. IMPLICATIONS FOR FUTURE STUDIES of approximatelyx 102!molecules -2. Using depo6 m the sition flux average measured at the Lake ABLE tower of was calculated. 60øN is 2-3 If the time scale for ventilation months the Arctic cannot of air north of as an The results of ABLE 3A confirmed two major hypotheses tundra ecosystem are sensitive to changes in climate variables such as soil moisture and temperature. Enclosure flux measurements, eddy correlation flux measurements from a ground-based tower, and airborne eddy correlation tech- -1.1 x 10 molecules -2 s-• an03 lifetime 8 months which generatedthe study. First, emissionsof CH 4 from the • cm of be viewed ultimate sink for 03. If the PAN and organic nitrates which decompose to HARRISSET AL.: ARCTIC BOUNDARYLAYER EXPEDITION, ABLE 3A 16,393 niquesfor CH 4 flux measurement,all proved to be robustfor use in studying emissionsfrom the tundra landscape. Airborne flux measurementsare most useful for surveying large areas to characterize relationshipsbetween major ecosystem parameters (e.g., distribution of vegetation type) and CH 4 flux. Tower-based eddy correlation and enclosure flux techniques can be best used to quantify specific responsefunc- tions relating changesin CH 4 emissionsto changesin soil climate. and G. L. Gregory, Large-scale variability of ozone and aerosols in the summertime Arctic and sub-Arctic troposphere, J. Geophys. Res., this issue. Chappellaz, J., J. M. Barnola, D. Raynaud, Y. S. Korotkevich, and C. Lorius, Ice-core record of atmospheric methane over the past 160,000 years, Nature, 345, 127-131, 1990. Crill, P.M., K. B. Bartlett, R. C. Harriss, E. Gorham, E. S. Verry, D. I. Sebacher, L. Madzar, and W. Sanner, Methane flux from Minnesota peatlands, Global Biogeochem. Cycles, 2, 371-384, 1988. Second, concentrationsof NOx are critical to 03 production/destruction processes in the relatively pristine highlatitude regions studied. At present, 03 destruction processesdominate; however, NOx pollution from mid-latitude sourcesmay have already reduced the capacity of the region to act as an 03 sink [Jacob et al., this issue(a)]. These ABLE 3A results indicate that atmospheric chemical changesin the Arctic environment may serve as a unique early warning indicator of global change. If northern hemisphere NOx emissionscontinue to increase, particularly in the newly industrializing nations (e.g., Korea, China, India), Arctic 03 levels could increase rapidly with significant 320, 1986. implications for the northern hemisphere and, perhaps, the Fung, I., J. John, J. Lerner, E. Matthews, M. Prather, L. P. Steele, and P. J. Fraser, Three-dimensionalmodel synthesisof the global global environment. Future studies should emphasize determethane cycle, J. Geophys. Res., 96, 13,033-13,065, 1991. mining the pathways and mechanismsof the transport and Dickinson, R. E., How will climate change?, in The Greenhouse Effect, Climate Change, and Ecosystems, edited by B. Bolin et al., pp. 206-270, John Wiley, New York, 1986. Doolittle, J. A., M. F. Gross, and M. Hardisky, A groundpenetrating radar study of active layer thickness in areas of moist sedge and wet sedge tundra near Bethel, Alaska, U.S.A., Arct. Alp. Res., 22, 175-182, 1990. Fan, S.-M., S.C. Wofsy, P.S. Bakwin, D. J. Jacob, S. M. Anderson, P. L. Kebabian, J. B. McManus, C. E. Kolb, and D. R. Fitzjarrald, Micrometeorological measurementsof CH 4 and CO2 exchange between the atmosphere and sub-Arctic tundra, J. Geophys. Res., this issue. Ferek, R. J., R. B. Chatfield, and M. O. Andreae, Vertical distribution of dimethylsulphide in the marine atmosphere, Nature, fate of NOy to the Arctic from mid-latitude pollution sources. Gidel, L. T., and M. A. Shapiro, General circulation model estimates of the net flux of ozone in the lower stratosphere and the implicationsfor the troposphericozone budget, J. Geophys.Res., 85, 4049-4058, 1980. Methane flux from tundra environments may be one of the most sensitive, integrative indicators of climate change effects on the Arctic biosphere. A long-term monitoring program of CH 4 flux at a network of sites in the tundra ecosystem, in combination with enhanced monitoring of ambient air CH 4 trends, could contribute to early detection of climate change effects in the Arctic. Acknowledgments. The ABLE 3A project acknowledges the assistanceand outstandingcooperation provided by both municipal and federal officials in Barrow, Bethel, and Anchorage, Alaska. The U.S. Fish and Wildlife Laboratory in Bethel provided excellent research facilities. The comments of Shaw Liu were very helpful in improving this manuscript. Diana Wright carefully and patiently typed several versions prior to publication. 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Herlth, D. O'Hara, K. Zahnle, J. D. Bradshaw, S. T. Sandholm, R. Talbot, P. J. Crutzen, and M. Kanakidou, Relationship of peroxyacetyl nitrate to active and total odd nitrogen at northern high latitudes: Influence of reservoir species on NOx and 03, J. Geophys.Res., this issue(b). Stonehouse, B. (Ed.), Arctic Air Pollution, 328 pp., Cambridge University Press, New York, 1986. R. J. Bendura, J. W. Drewry, J. M. Hoell, Jr., and M. C. Shipham,AtmosphericSciencesDivision, NASA Langley Research Center, Hampton, VA 23665. R. N. Gidge, R. L. Navarro, and V. E. Rabine, NASA Wallops Flight Facility, Wallops Island, VA 23337. D. J. Jacob and S.C. Wofsy, Harvard University, Cambridge, MA 02138. R. J. McNeal, Earth Science and Applications Division, National Aeronautics and Space Administration, Washington, DC 20546. (Received December 10, 1990; revised July 31, 1991; accepted August 12, 1991.)