The North American Carbon Program Plan (NACP)
A Report of the Committee of the
U.S. Carbon Cycle Science Steering Group

Appendix 1

 Initial Concepts for Atmospheric Observations

Phased Build-Up of the Long-Term Observing System

Current plans for the long-term observing network envision three phases, to be modified and optimized with the aid of model studies and initial data.

Phase 1 (Summer 2004): Preliminary Work

The initial implementation phase will focus on the agricultural area of the upper U.S. Mid-West, centered on the state of Iowa. This area has high agricultural productivity, low population density, and flat terrain. The expected CO2 signals will be sufficiently large, and the dominating CO2 sink due to crops can be estimated independently, allowing for a proof of concept. The expected differences in CO2 across Iowa can be estimated: with a diurnally averaged net sink prior to harvest of 5 mol C m-2 s-1 (from corn and soybeans, 60% of all state land is devoted to these crops), the average CO2 decrease over one day, if confined to the lowest 2 km of the atmosphere, would be about 5 ppm. This is the expected difference across the state, since at an average wind speed in the boundary layer of 5 m/s, the signal has about a day to build up over Iowa (about 400 x 350 km). This is a sufficiently large signal to test the method.

The problem we want to address eventually has a much smaller signal, however. If through optimized soil management the United States could store as much as 0.2 Pg C/year as organic carbon in agricultural land, and if we apportion 8% of that to Iowa (its share of the total U.S. market value of agricultural products), the annually averaged rate of C storage would be 0.3 mol C m-2 s-1, approximately equal to the emission of CO2 from Iowa from combustion of fossil fuels. This estimate suggests that it may be possible to verify this type of carbon storage on agricultural land, but the region must be several times the area of Iowa to obtain a larger average drawdown for CO2.

A relatively large number of very tall transmitter towers are located throughout the Midwest and Southeast United States (especially the Carolinas). We plan to instrument five tall towers in the Iowa study area, one centrally located and four surrounding it at distances of 300 to 800 km (Figure A1.1). In addition, there are the existing tall tower sites in northern Wisconsin (500 km to NE), and central Texas (1,200 km to S). The central Iowa tower (near Des Moines) should measure fluxes as well as concentrations. New aircraft profiles would be established over three of the new towers as well as the existing Wisconsin and Texas towers. Aircraft profiles would be obtained every 2 days at each of these sites and at the existing aircraft sites in Colorado and over Harvard Forest, Massachusetts.

Note that requirements for measuring net fluxes of CH4 and CO are less stringent in general than for CO2, since fluxes of these gases from the surface do not reverse sign seasonally or diurnally.

In addition to the intensive study area, a few dispersed observing stations should be added across the continent and around its perimeter during the early phases of network development. For example, some of the existing AmeriFlux towers could be instrumented with accurate CO2 measurements and extrapolations could be made to mid-PBL mixing ratios from the surface layer flux and mixing ratio data. One or two measurement sites should target an area of hilly terrain to make an exploratory effort toward atmospheric measurements in these areas, which would also be a focus area during the early intensives. Also, CO2 measurements on a few permanently moored buoys ?50 km from the shore should be initiated (see Appendix 3). These relatively inexpensive enhancements to the network would provide a larger scale view of CO2 distribution over the continent and would be important data for modeling studies of network design.

Phase 2 (Fall 2004): Intensive Field Studies

As we expand the long-term network in the central part of the country, we will start to encounter large fossil fuel sources. We envision at least one intensive during this expansion phase to help establish the capability of the experimental design to distinguish urban/industrial emissions from distributed sources and sinks associated with vegetation and soil processes.

Phase 3 (Summer 2005 and Beyond): Build-Out of the Network

To improve the capability for establishing a continental mass balance there should be a ring of stations along the coasts, in northern Canada, and along the southern U.S. border. With spacing of ?1,000 km, there will be about 12 stations in the ring and perhaps 18 inside, in a more or less regular grid east of the Rocky Mountains.

Figure A1.1

Figure A1.1. Evolutionary design for the NACP atmospheric observation network. Note: Final buildout of the Ameriflux network is not represented pictorially. The location of additional sites will be determined in conjunction with the execution of field campaigns.

Figure A1.1 shows maps with (a) locations of existing observing stations, including flask sites operated by NOAA/Climate Monitoring and Diagnostics Laboratory (CMDL) and the Atmospheric Environment Service (AES) of Canada, AmeriFlux sites, and current aircraft sites operated by NOAA/CMDL; (b) phase 1 proposed enhancement of the long-term observing network, adding several tall tower and aircraft profile sites focused in the agricultural belt around Iowa; and (c) a possible full observing network for North America at build-out, including a total of 30 aircraft sites and 15 tall tower sites. (Cities with population over 300,000 are shown with symbol size proportional to population.) Coastal aircraft flights will be over permanent buoys moored offshore, improved from existing instrumentation to provide accurate atmospheric data traceable to international standards. AmeriFlux sites currently measure fluxes, but none provide atmospheric concentration data certified traceable to international standards. Some AmeriFlux sites will be enhanced for accurate measurements of CO2, CH4, and CO mixing ratios and real-time reporting of data.

A few AmeriFlux and other sites will also have column integral measurements using Fourier transform infrared absorption spectrometers to characterize the diurnal cycle and measure the daytime uptake under cloud-free or partly cloudy conditions. Hilly terrain will be addressed with some sites and targeted during the intensive campaigns to develop appropriate sampling methods in these complex areas, and to aid in development of transport models. We envision that full deployment of the network may take several years. Its design will be refined when the data from the intensives in 2005 and the results from at least a full year of sustained observations have been digested and models improved.

Quality Control of the Observations. The information we seek to extract is contained in small concentration differences. It is paramount that all data are directly comparable to a high level of accuracy (e.g., 0.2 ppm for CO2). Well-defined instrument calibration protocols will be followed on all flights, including the intensives, and at the ground stations. All CO2 and CO measurements will be made on the respective WMO mole fraction scales, maintained by the NOAA CMDL, and CH4 on the CMDL scale. However, experience has shown that careful calibration is not enough. There can be problems with gas handling independent of the calibration. All in situ measurements of CO2, CO, and CH4 will be compared to regular (dried) whole air samples in flasks measured in one laboratory. Flask samples will be obtained on a subset of the flights for the sustained observing system, every 10 days or so, once a day in mid-afternoon (when the boundary layer is maximally mixed) on the tall towers, and perhaps one flask per hour per airplane during the intensives. When the flask results differ from in situ measurements, this needs to be investigated immediately. The problem could be with either method. Rapid turnaround (of a few days) of flask samples is essential so that sampling problems do not persist. Sampling with flasks has the further benefit that additional species and isotopic ratios can be measured.

Data management. All data will be made generally available immediately in preliminary form at a single website. Rapid comparison of different data sets is essential for maintaining quality control. Data will be “flagged” and corrections made as the analysis proceeds.

 Intensive Operation Periods (IOPs)

Phase 1 (July 2004): Exploratory Measurements

The first phase will focus on defining distributions of target gases across the continent, and on smaller scale experiments in a selected region where the first stations of the long-term network have been started. Emphasis will be given to testing the integrity of the measurements and analytical concepts.

Large-Scale Distributions. Observed atmospheric concentrations and linkage to vegetation and atmospheric chemistry will be stressed. Large-scale measurements are expected to link up with airborne observations from coordinated atmospheric chemistry missions such as NASA's Global Tropospheric Experiment and NOAA experiments.

Small-Scale Experiments. Data will be collected over a major agricultural region (e.g., Iowa, or the Mid-West corn belt) using regional budget experiments and frequent vertical profiles over tower locations, providing tracer distributions with much higher spatial and temporal coverage than the network. Budget estimates from regional inversions will be evaluated by comparing results from the network stations and the intensive aircraft data, and further compared to independent bottom-up flux estimates, based on agricultural productivity. Additional intensive study may focus on hilly or complex terrain. The intensives will rigorously test the integrity of network design, the analysis tools, and specific issues such as complex terrain.

Phase 2 (Nov. 2004-Jan. 2005): Development Period

Measurements will emphasize determination of emissions from a major source region reasonably separated from other sources (e.g., an urban complex such as Minneapolis/St. Paul). A different season from that of phase 1 will be chosen, and a comprehensive set of large-scale observations will be obtained again, providing a look at cross-continental gradients in fall. The goals are to refine the overall approach, and to gain data characterizing regions where the long-term network is expanding.

An urban complex will be studied in a regional budget experiment in which we will characterize urban emissions of CO2, CO, and CH4 from a fairly isolated metropolitan complex in flat terrain. These observations will provide the tracer-to-tracer ratios characteristic of the urban plume. Urban emissions will be separated from fluxes due to vegetation and soils, which are still significant in the fall, but not dominant as during the summer. Data assimilation with high spatial and temporal resolution requires knowledge of the fossil fuel source of CO2 on small temporal and spatial scales. Intensive regional measurements around an urban complex serve as test bed for analyses of this problem.

Candidate Site: Minneapolis/St.Paul, Fall 2004

There are a number of towers taller than 300 m in the vicinity of Minneapolis/St. Paul. Three are within 15 km of the center, with one at 50 km S, one at 40 km N, one at 40 km W, and seven more between 100 km and 200 km distance. We would instrument at least one tower at 40-50 km distance, and one due east at 100 km. Regular vertical profiles will be flown over the latter and a spectrometer installed.

Large-Scale Cross-Continental Gradients. It is important to determine if the network can distinguish CO2 gradients imposed by global processes from sources (decay) in the dormant season, and separate biogenic from anthropogenic CH4. Since sources of CO from biomass fires and photochemistry are relatively small at this season, the measurements will provide a quantitative test for emissions inventories for CO.

Phase 3 and Beyond

The program envisions additional intensive studies, covering at least the seasons not previously sampled, winter and spring. In winter, fossil fuel sources should dominate biological sources and sinks for CO2, CH4, and CO, allowing strong tests of emission inventories. However, both the network and the intensives will face operational difficulties due to weather, and the intensive phase will emphasize large-scale observations to test the capability of the network. In spring there are strong soil sources of CO2, but the observations will be complicated by strong geographical gradients, with growth of plants commencing in some areas while in others plants are mostly dormant.

 Intensive Field Campaigns

Possible Aircraft and Instruments in the Intensive Field Missions. The atmospheric aircraft platforms for NACP field programs emphasize remote sensing:

  • Atmospheric data. Principal species: CO2 (+/-0.2 ppm), CO (+/-1 ppb), CH4 (+/- 5 ppb), and H2O.

  • ER-2. An airborne simulator for future satellite measurements of biomass and atmospheric CO2, CO, and CH4 profiles. Tests will include direct comparison with data on the ground and from the in situ aircraft listed below, plus AVIRIS and other remote-sensing instruments for vegetation cover, canopy chemistry, and biophysical parameters.

  • Citation aircraft. Lagrangian, survey, and boundary-layer flights. Limited payload includes in situ measurements of CO2, H2O, CO, and meteorological parameters, possibly O3.

  • 5-10 light aircraft. To be used in phase 1 of the long-term network: fixed-point vertical profiles daily (continuous for CO2, CH4, CO, possibly O3) twice per day, alternate days, over continental sites and over marine (CMDL) stations, using the new instrumentation developed for the long-term network.

The NACP IOPs will be combined with atmospheric chemistry programs to obtain additional heavy-lift, long-range platforms for continental-scale measurements. Three likely candidates are the following:

  • DC-8. Large-area surveys, including high altitudes and all along the coasts and borders to define boundary values for the United States and adjacent areas of North America. Instrumentation would include in situ measurements of CO2, H2O, CO, and O2, with a full complement of atmospheric chemistry measurements (free radicals, solar radiation, nitrogen oxides, etc). The mission profile would include observations for NACP purposes as part of a major deployment for studying the chemistry and transport of pollutants over North America.

  • P-3. Eddy flux flights and smaller-area flights, including boundary layer, possibly also in conjunction with atmospheric chemistry payloads and mission profiles.

  • C-130. Similar to Citation or P-3, with full chemistry and/or remote sensing.

INTEX-NA and the NACP First Intensive Measurement Periods. The first opportunity for joint NACP-atmospheric chemistry measurements will be in the Intercontinental Transport Experiment-North America (INTEX-NA) program in 2004. INTEX-NA is a NASA Global Tropospheric Experiment (GTE) aircraft mission focused on quantifying the sources, sinks, and import/export of environmentally important chemicals on the scale of the continental United States. Chemicals of interest include ozone and its precursors, aerosols and their precursors, and long-lived greenhouse gases. INTEX-NA will use two NASA aircraft, the DC-8 (ceiling 12 km) and the P-3B (ceiling 7 km) operating along the Atlantic and Pacific seaboards as well as over the continental United States. Two deployments are planned, in summer 2004 (Phase A) and spring 2006 (Phase B). Sampling strategies will be guided by information from satellite observations and atmospheric models. Coordination will be sought with NACP and with other experimental programs focused on U.S. air quality (NOAA, DOE), and transatlantic transport (EEC). Validation of Aura satellite observations and scientific application of these observations to address mission objectives will represent an important component of the INTEX-NA activity.

INTEX-NA will follow an experimental design in which bottom-up, prior knowledge of chemical sources and sinks on the scale of the United States can be tested and improved in a top-down manner with atmospheric observations. This design requires an integrated approach where synthesis of the aircraft observations with measurements from other platforms (satellites, sondes, surface sites) and 3-D atmospheric chemistry models is pursued at all stages of mission design, execution, and interpretation. The aircraft flights will be directed at optimal sampling of the continental boundary layer (CBL), of the exchange between the CBL and the free troposphere (FT), and of the synoptic-scale flow across the coastlines and over the neighboring oceans. The aircraft will carry high-performance instrumentation for measuring a wide range of chemical species, building on the capabilities developed for previous GTE missions. These include (1) extensive in situ measurements, (2) eddy correlation flux measurements from the P-3B aircraft, (3) remote-sensing (DIAL) measurements of ozone and aerosols aboard the DC-8 aircraft. The optimal sampling strategy to enable top-down analysis will be developed prior to the mission using atmospheric model simulations and prior observational knowledge. It will be implemented during mission execution through the use of 3-D model forecasts and satellite observations, and through coordination with other field programs including NACP.

Phase A of INTEX-NA will prioritize the eastern United States and outflow to the North Atlantic. Phase B will emphasize inflow from the Pacific. However, both phases will extend their scope to the continental scale. Sampling with the DC-8 will focus on the Atlantic and Pacific seaboards to characterize continental-scale inflow/outflow, and will include transcontinental transects aimed at quantifying large-scale chemical gradients over the United States as well as transport involving Canada and Mexico. Bangor and Seattle are planned as the principal operational bases of the DC-8. The P-3B will focus on regional-scale mapping of surface fluxes and characterization of CBL-FT exchange over the United States. It will be based at interior sites in the country; a site in Wisconsin is presently under consideration. The deployment of the P-3B will be conducted with deliberate intent to maximize opportunities for collaboration with NACP and other field programs towards addressing the INTEX-NA mission objectives.

Close coordination between INTEX-NA and NACP is indeed a compelling investment to augment the scientific returns of both programs. A common objective is the characterization of carbon sources and sinks over the United States. The NACP measurement platforms and biogeochemical modeling resources will be of considerable value for INTEX-NA. The extensive chemical tracer observations together with CO2 and methane available from the INTEX-NA aircraft will offer powerful constraints for carbon sources and sinks. The eddy correlation flux measurements and vertical sounding capabilities of the NASA P-3B aircraft will complement the smaller-scale mapping by the NACP aircraft, while the continental-scale observations from the DC-8 will allow an integrated perspective on carbon budgets. Such fruitful coordination between INTEX-NA and NACP needs to be pursued actively at the mission planning stage to lay the groundwork for successful execution.

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