2.8. Measurements on Tall Towers

The main goals of the tower program are to obtain regionally representative measurements of the mole fractions of CO₂ and other tracers in the continental planetary boundary layer (PBL) and to provide data to improve quantitative understanding of the processes that cause trace gas abundances over the continents to differ from those at "background" stations. For CO₂ these processes include regional fossil fuel combustion, exchange with the terrestrial biosphere (photosynthesis and respiration), and PBL dynamics and exchange of air with the overlying free troposphere (entrainment). In addition there are important feedbacks between these processes we seek to understand through our measurements. For example, vegetation has a strong influence on the surface energy budget, mainly through surface albedo and energy partitioning between sensible and latent heat, and thereby influences the daily development of the PBL and cloud-induced entrainment. Also, surface exchange (photosynthesis and respiration) and PBL mixing co-vary on diurnal and seasonal timescales as was discussed by Denning et al. [1996]. Continuous measurements of CO₂ in the mid-PBL, along with characterization of PBL mixing and cloud processes, are necessary observations for quantifying these effects.

The tower program consists of continuous observations of CO₂ and other tracers on two tall communications towers in the U.S.: the North Carolina (NC) tower (610 m tall, 35.37°N, 77.39ºW, 9 m above sea level) in a rural area of the southeast and the Wisconsin (WI) tower (447 m tall, 45.95ºN, 90.27ºW, 472 m above sea level) located in a sparsely populated area of the upper Midwest (Chequamegon National Forest). The regions around the towers differ greatly in climate, land use, and human population density (Table 2.11).

TABLE 2.11. Meteorological Data and Human Population Densities Near the North Carolina and Wisconsin Towers

*Population densities are given for the counties in which the towers are located: Pitt (NC) and Price (WI).

Measurements began at the NC tower in June 1992 and at the WI tower in October 1994. Observations of CO₂ and basic meteorological variables (wind speed and direction, air temperature, humidity and pressure, etc.) are described by Bakwin et al. [1995]. We also measure CH₄, CO, H₂, CFC-11, CFC-12, CFC-113, CH₃CCl₃, CCl₄, CHCl₃, C₂Cl₄, SF₆, and N₂O by in situ gas chromatography (GC) in collaboration with the NOAH group, as described by Hurst et al. [1997]. The GC observations are of particular value for identifying pollution sources of CO₂ since the CFCs, other halocarbons (except CHCl₃), and SF₆ are only emitted from anthropogenic sources and their mole fractions are highly correlated in pollution plumes [Bakwin et al., 1997]. Further, CO is a key tracer for combustion. Methane, H₂, and N₂O have significant biogenic fluxes.

The WI tower is also a component of the AmeriFlux network for long-term, ecosystem scale studies of atmosphere and surface exchange of CO₂, H₂O, and energy [see Wofsy and Hollinger, 1998]. We measure fluxes of CO₂, H₂O, sensible heat, and momentum using eddy covariance with sensors located 30, 122, and 396 m above the ground. Collaborative investigations (observations and modeling) are underway to characterize the biophysical environment, separate component fluxes (e.g., photosynthesis and respiration, transpiration, and evaporation), understand spatial variability of surface fluxes with reference to varied land cover types, and study dynamics of the PBL and exchange of air between the PBL and free troposphere. The overall program surrounding the WI tower is known as the Chequamegon Ecosystem/Atmosphere Study (CHEAS, see homepage at http://ra.forest.wisc.edu/cheas).

A recent paper [Bakwin et al., 1998] provides an overview of CO₂ data from the WI and NC towers, compares the tower data to "background" (marine boundary layer) sites from the Globalview dataset, and suggests ways in which the results could be used in models to improve understanding of the global carbon cycle. The tower CO₂ mole fractions and meteorological data are available on the CMDL website.

Landscape scale surface fluxes of H₂ and CH₄ at the WI tower have been determined from hourly GC measurements of their vertical profiles in the PBL and from eddy covariance measurements of CO₂ surface fluxes at 30 m above ground [Hurst et al., 1998]. Large vertical gradients of H₂, CH₄, and CO₂ are observed at night when surface fluxes enrich CH₄ and CO₂, and deplete H₂ in the shallow stable layer. Nighttime surface fluxes of CH₄ and H₂ are calculated using surface layer similarity theory from measured CO₂ fluxes and vertical gradients of H₂, CH₄, and CO₂. Daytime fluxes are extrapolated from nighttime data using the observed relationship between fluxes and soil temperature at 10 cm depth. Relationships with soil moisture will be explored in future work. Given the height of CO₂ flux measurements and the stability of the atmosphere at night, surface fluxes integrate over ~30 km² footprints that include both wet, lowland soils (bogs, fens) and dry, upland forest soils.

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