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, CH4,, 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. . 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. . 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,
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
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. .
In October 1994 measurements began at the WLEF TV transmitter tower in northern Wisconsin (45.95oN, 90.28oW, 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 ), 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
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,
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
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
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. .
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,
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
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
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
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
Fig. 2.22. Statistics of vertical gradients (30 m-396 m) for CO,
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,
and CO, binned by hour, for December 1995. Symbols are the same as for Figure
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
In contrast, our results for the North Carolina tower imply that the main regional
are associated with anthropogenic activity [Bakwin et al., 1995].