2.6. MEASUREMENTS ON TALL TOWERS

The Carbon Cycle Group initiated the Tall Towers Program as a component of the effort to incorporate regionally representative continental sampling sites into the global network of CO, CH, and CO observations. The CCG approach is to utilize the tallest existing towers (television transmitters up to 610 m) to get away from the influence of sources and sinks in the immediate vicinity of the tower in order to examine the sources of variance of CO, CH_4,, and CO mixing ratios in the continental boundary layer. These sources of variance include atmosphere/biosphere exchange, boundary layer dynamics, horizontal transport, fossil fuel and biomass combustion, and other anthropogenic sources (e.g., landfills, wastewater treatment, and natural gas leakage for CH).

Observations of CO mixing ratio at the WITN TV tower (610 m) in eastern North Carolina began in June 1992 and are ongoing. A description of the site and surrounding area, and of the experimental setup is given and initial results are discussed in Bakwin et al. [1995]. Measurements are carried out at 51, 123, and 496 m above the ground. Daily mean CO mixing ratios at each of the three measurement levels on the North Carolina tower and smooth curve fits to the data [Thoning et al., 1989] are shown in Figure 2.17. A seasonal cycle of 15­20 ppm amplitude is apparent in the daily mean data from 496 m but is damped in measurements made closer to the ground. The seasonal cycle of CO near the ground is masked by a large diurnal cycle driven by photosynthesis and respiration [Bakwin et al., 1995]. The nighttime buildup of CO near the ground due to respiration, is especially pronounced in summer and "fills in" the seasonal drawdown of CO. Observations well above the level of the nocturnal inversion, as can be obtained from tall towers, are necessary to quantify CO mixing ratios typical of the whole planetary boundary layer (PBL).

To determine the annual growth rate for CO mixing ratios at the 496 m level, a trend curve is fitted through the data in Figure 2.17 as described by Thoning et al. [1989]. The annual growth rates for 1993, 1994, and 1995 were found to be 1.7, 2.0, and 2.0 ppm yr, respectively. These growth rates are larger by about 0.3-0.6 ppm yr than those for the whole northern hemisphere in each year. The reason for this accumulation of CO over the region, relative to the whole northern hemisphere, is not known.

Fig. 2.17. Daily mean CO mixing ratios at 51, 123, and 496 m on the North Carolina tower. In the top panel the data for each day are shown as points, and the vertical axes for each observation height are offset. In the lower panel the smooth curve fits are plotted on the same scale for comparison. The smooth curve fit to daily mean data from 396 m above the ground on the Wisconsin tower is also shown.

In October 1994 CCG began observations of CO, CH, NO, and a suite of halocompounds at 51, 123, and 496 m on the North Carolina tower by automated in situ gas chromatography. The GC design and operating parameters are discussed in section 5.2.2 of the 1993 CMDL Summary Report [Peterson and Rosson, 1994]. Measurements of NO and halocompounds are discussed in Section 5.2.4 of this report. Figure 2.18 shows daily mean CH and CO mixing ratios for 496 m plotted with flask data from Bermuda, giving a comparison of the continental tower site with a "background" marine site at approximately the same latitude. Mixing ratios of CO at the tower are consistently 40­60 ppb higher than at Bermuda, likely reflecting fossil fuel combustion sources proximate to the tower. Emission of CO and CO from the average mix of fossil fuel combustion in the United States occurs with a molar ratio of around 0.020 (20 ppb/ppm) [Bakwin et al., 1994; J. Logan, Harvard University, personal communication, 1993], so our observations indicate that CO mixing ratios at the tower are enhanced year­round by roughly 2­3 ppm relative to "background" air due to regional fossil fuel combustion. Mixing ratios of CH at the tower are enhanced by 20­60 ppb throughout the year, probably also mainly due to anthropogenic sources [Bakwin et al., 1995].

Fig. 2.18. Daily mean CH and CO mixing ratios at 496 m on the North Carolina tower (points) and flask data from the CCG Bermuda sites BME and BMW (open circles). Each time series is fit with a smooth curve as described by Thoning et al. [1989].

In October 1994 measurements began at the WLEF TV transmitter tower in northern Wisconsin (45.95^oN, 90.28^oW, base height 472 m above sea level). The tower is 447 m tall and is located in the Chequamegon National Forest. The region is a heavily forested zone of low relief. The Chequamegon National Forest covers an area of about 3250 km, and the dominant forest types are mixed northern hardwoods (850 km2), aspen (750 km), and lowlands and wetlands (600 km). Much of the area was logged, mainly for pine, during 1860­1920 and has since regenerated (J. Isebrands, USDA Forest Service, personal communication, 1994). The regional population density is very low, and there is limited industry.

Carbon dioxide mixing ratios are measured at 11, 30, 76, 122, 244, and 396 m above the ground. Wind speed and direction, temperature, and humidity at 76, 122, and 396 m, and barometric pressure, rainfall, incident photosynthetically active radiation (PAR) and net radiation at the surface. Intermittently (so far) vertical fluxes of CO are also measured at 76 and 396 m using eddy correlation. The flux measurements have been discontinuous because of instrumental problems, but steps have recently been taken to improve reliability.

In June 1995 an automated GC was installed at the Wisconsin tower for measurements of CH and CO. The method of analysis is similar to that used at the North Carolina tower (Table 5.3 of Peterson and Rosson [1994]), and every 30 minutes one measurement is obtained at each of 30, 76, and 396 m above the ground.

The smooth curve fit to the Wisconsin tower CO daily mean mixing ratios from 396 m above the ground is shown in Figure 2.17 to allow comparison with the North Carolina tower. Mixing ratios at the Wisconsin and North Carolina towers are similar in winter, but the summertime draw­down is 3­4 ppm deeper and at least 1 month narrower at Wisconsin. The inner 50% (by month) of daily averages for CO data from 30, 76, and 396 m is displayed in Figure 2.19, and monthly statistics for CH and CO on the Wisconsin tower are presented in Figure 2.20.

Fig. 2.19. Shaded regions indicate the inner 50% of daily average CO mixing ratios for each month from 30, 76, and 396 m on the Wisconsin tower.

Fig. 2.20. Monthly statistics of CO and CH measurements at the Wisconsin tower. Circles and asterisks are means ±1 standard deviation. The crosses indicate medians (horizontal bars) and upper and lower quartiles (vertical bars). The numbers across the bottom of the plot indicate sampling level (2, 3, and 6 refer to 30, 76, and 396 m, respectively).

Figure 2.21 shows an example of CO, CH, and CO mixing ratios, and CO fluxes at the Wisconsin tower for September 1­2, 1995. During the daytime the PBL is well mixed to heights well above the top of the tower (e.g., 1400­1600 m on these 2 days, W. Angevine, NOAA Aeronomy Laboratory, unpublished data) and the trace gas mixing ratios show little vertical gradient. At night a shallow inversion forms and CO and CH mixing ratios increase rapidly near the ground due to surface sources. Surface fluxes of CO calculated from data obtained for 76 and 396 m are generally in good agreement. The eddy fluxes show net uptake of CO by the forest in the afternoon of up to around 0.4 ppm m s or 7 kg (C) ha h. At night the forest releases CO as is also indicated by the vertical profiles. In the future, plans are to measure CO2 fluxes continuously and to be able to determine the annual net CO balance of the forest. Some additional results from the flux measurements are presented by Davis et al. [1996].

Fig. 2.21. Time series of mixing ratios and surface fluxes of CO and mixing ratios of CO and CH measured on the WLEF tower in Wisconsin on September 1­2, 1995. Surface fluxes are calculated from eddy correlation measurements at 76 and 396 m above the ground. Mixing ratio measurements made below those levels are used to account for divergence of the fluxes in the vertical.

Figures 2.22 and 2.23 show statistics for mixing ratio gradients between 30 m and 396 m, binned by hour of the day, for CO, CH, and CO at the Wisconsin tower during August and December 1995, respectively. The gradients typically increase throughout the night as emissions accumulate in the shallow nocturnal PBL (below 396 m). In August the maximum mean gradients for CO, CH, and CO are 37 ppm, 95 ppb, and 18 ppb, respectively. If the nocturnal increase in CO in summer is attributed solely to fossil fuel combustion emissions with a CO/CO of 0.02 (mol/mol), then less than 1 ppm of the nocturnal accumulation of CO can be attributed to fossil fuel combustion. The CO/CO for emissions from the burning of forest biomass in North America is typically larger (0.15­0.25 [Hegg et al., 1990]). Hence, >95% of the CO that accumulates in the nocturnal stable layer in summer is biogenic (respiration). In December the maximum gradients for CO and CH are much smaller, only about 2 ppm and 17 ppb, but the maximum gradient for CO (28 ppb) is larger than in August. These observations likely reflect the nearly complete shutdown of biogenic sources of CO and CH in winter. Mixing ratio gradients for CO may be higher in winter due to increased combustion activity and shallower mixing depths for the nocturnal PBL than in summer.

Fig. 2.22. Statistics of vertical gradients (30 m-396 m) for CO, CH, and CO, binned by hour, for August 1995. Crosses indicate means (horizontal bars) ± the 95% confidence interval (vertical bars), circles indicate medians, and asterisks indicate upper and lower quartiles. The leftmost panel on each plot gives statistics for the mean daily vertical gradients.

Fig. 2.23. Statistics of vertical gradients (30 m-396 m) for CO, CH, and CO, binned by hour, for December 1995. Symbols are the same as for Figure 2.22.

The winter­summer comparison of vertical gradients indicates that the main source of CH in the region surrounding the Wisconsin tower is biogenic (Figure 2.21). Future plans are to determine CH fluxes at the Wisconsin tower using measurements of CO fluxes (by eddy correlation) and vertical profiles of CO and CH. In contrast, our results for the North Carolina tower imply that the main regional CH sources are associated with anthropogenic activity [Bakwin et al., 1995].

[back] [contents] [next]