enhancement of greenhouse gas mole fractions is not far from, but on the high side of, observed values for the lowest 1 km, consistent with the fact that over long fetches the emissions tend not to remain confined to the boundary layer. For CH4, the observed annual average enhancement in the boundary layer of the United States, relative to background values, is 20-60 parts per billion (ppb), with large standard deviation of midday means of 20-30 ppb. For N2O, these numbers are 0.2-0.4 ppb, and daily standard deviation of 0.3-0.7 ppb; for SF6, the average enhancement is 0.05-0.2 parts per trillion (ppt), with daily variability of 0.07 to 0.13 ppt. For N2O, the highest observed enhancement is 0.7 ppb over Iowa, which is indicative of intense regional emissions. The World Meteorological Organization-recommended accuracies for in situ measurements are 2 ppb for CH4, 0.1 ppb for N2O, and 0.02 ppt for SF6.

Table B.3 explores expected enhancements of the CO2 mole fraction over metropolitan areas. The signal expected to be produced over a single large city relative to its surroundings is comparable to, and in many cases larger than, the average produced by an entire country. Note that the observed average fossil-fuel CO2 enhancement at the surface in Los Angeles based on 14C measurements of plants (Figure 4.5) is about five times larger over part of the basin than the number estimated in Table B.3.

The latter is derived with a standard assumption of a steady 5 m s–1 average wind vector, which would imply that the residence time of air over the metropolitan area would be ~4 hours. Because Los Angeles is surrounded by mountains on three sides, the residence time over the city is much longer.

SIGNAL FROM A 1 GW(E) COAL FIRED POWER PLANT

One gigawatt (GW) corresponds to 8.76 109 kWh yr–1. Applying the U.S. average 25 mol C kWh–1, the plant would produce 2.19 1011 mol yr–1, or 6,900 mol s–1. If the perpendicular distance across the plume is 1.7 km at some distance downwind, and the wind speed is 5 m s–1, then 1 second of CO2 emissions is diluted into 1,700 × 5 m2 s–1 × 3.56 105 mol of air per square meter (full atmospheric column). The number 1.7 km is chosen to correspond to the 3 km2 footprint of a single Orbiting Carbon Observatory (OCO) sounding. The CO2 increase in the total column is then 2.3 ppm.

SIGNAL FROM A GEOLOGICAL SEQUESTRATION LEAK

Significant quantities of CO2 may one day be captured at large point sources in the utility and industrial sectors and injected into storage sites in the Earth, rather than released to the atmosphere. If this occurs, it will be important to monitor both the quantity of CO2 that is injected into storage sites and the amount of any CO2 that leaks from these sites (IPCC, 2005). Leaks from geological sequestration are relatively easy to detect. The emissions are at the surface and are not buoyant like those from a power plant. At night they

TABLE B.3 Expected CO2 Signals for Selected Metropolitan Areas

City

Area (km2)a

Emissions (Mton CO2yr–1)

Emissions (μmol m–2s–1)

Total Column (ppm)

Boundary Layer 1 km (ppm)

Los Angeles

3,700

73.2

14.2

0.49

4.3

Chicago

2,800

79.1

20.3

0.60

5.4

Houston

3,300

101.8

22.2

0.72

6.4

Indianapolis

900

20.1

16.1

0.27

2.4

Tokyo

1,700

64

27

0.63

5.6

Seoul

600

43

52

0.71

6.3

Beijing

800

74

67

1.1

9.4

Shanghai

700

112

116

1.8

15

NOTES: Mton CO2 is million metric tons of CO2.

aArea represents the contiguous area of intense and activity and was estimated using Google maps in “satellite” mode, which shows built up areas by color and road density.

SOURCES: Emissions in 1998 for four east Asian cities from Dhakal et al. (2003). U.S. estimates are from the VULCAN emissions inventory for 2002 (<www.purdue.edu/eas/carbon/vulcan>).



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement