The Isotopic Composition of Urban CO2 Sources

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The isotopic composition of fossil fuel emissions is of great interest in atmospheric studies of the carbon cycle as well as local studies of urban CO2 sources (Andres et al., 2000). Emissions of CO2 from fossil fuel combustion are naturally more depleted in 13C than is atmospheric co2, and cause 13C dilution of the atmosphere (Keeling et al., 1979). In order to utilize isotopic tracers to estimate the role of biological uptake of atmospheric CO2 at the global scale, the contributions of fossil fuels must be removed. Two major sources of fossil-derived CO2 are gasoline and natural gas combustion, which are generally isotopically distinct. In addition, plants and soils in cities comprise an 'urban forest,' which also contributes to local carbon cycling. Distinguishing between these CO2 sources can improve estimates of the seasonal and spatial distribution of fossil fuel isotopic composition for regional and global atmospheric studies, as well as provide insight into local urban ecosystem function.

Both coal and petroleum deposits are derived from biological sources and have <513C values of approximately —21 to —32%o (Deines, 1980). The carbon isotope composition of various fossil fuels has been estimated globally by Tans (1981) and Andres et al. (2000), who considered sources from combustion of coal, natural gas, and petroleum, as well as gas flaring by oil refineries and CO2 emissions in cement production (Table 12.1). They report that while <513C of coal is fairly constant at —24.1 %o regardless of origin, the <513C of petroleum exhibits variation depending on the location of its source. Andres et al. (2000) estimated that petroleum utilized by the world's leading oil-producing nations may be as heavy as -26.1 or as light as -30.0%o.

Approximately 80% of commercial natural gas originates from thermogenic (non-biological) processes in buried organic matter (Rice and Claypool, 1981). The largest constituent of natural gas is methane, which exhibits a wide natural range of ¿13C values, but is generally more depleted in 13C than the products of photosynthesis. Methane of thermogenic origin has 513C values ranging from about —60 to —20%o, while bacterial-derived methane may range from —100 to —40%o (Schoell, 1988). Natural gas

Table 12.1 Globally Averaged Values (%o, V-PDB) for the Carbon Isotope Composition (<513C) of Fossil Fuel

Fossil fuel emission source Tans (1981) Andres et al. (2000) Observed range

Table 12.1 Globally Averaged Values (%o, V-PDB) for the Carbon Isotope Composition (<513C) of Fossil Fuel

Fossil fuel emission source Tans (1981) Andres et al. (2000) Observed range

Coal

-24.1

-24.1

-20 to -271

Petroleum

-26.5

-26.5

— 19 to —352,4

Natural gas

-41

-44

-20 to-100s

Gas flaring

-41

-40

-20 to -603*

Cement production

0

0

-

*The range of values reported for methane of thermogenic, non-biological origin.

Observed ranges are reported from Jeffery et at. (1955)1, Yeh and Epstein (1981)2, Tans (1981), Schoell (1988)3, and Andres et at. (2000)4.

*The range of values reported for methane of thermogenic, non-biological origin.

Observed ranges are reported from Jeffery et at. (1955)1, Yeh and Epstein (1981)2, Tans (1981), Schoell (1988)3, and Andres et at. (2000)4.

also contains small quantities of ethane, butane, propane, and other gases which are isotopically heavier than methane (Deines, 1980; Tans, 1981). Tans (1981) and Andres et al. (2000) applied this range of values to obtain weighted global estimates of —41 and —44%o, respectively, for the <513C of global natural gas combustion.

Urban regions with natural gas-fired electrical power plants and/or that contain a large proportion of natural gas furnaces for residential heating may be associated with relatively 13C depleted atmospheric CO2. Oil refineries that distill petroleum into gasoline also emit 13C-depleted CO2 during gas flaring—the combustion of low molecular weight compounds (Deines, 1980; Andres et al., 2000). There is a great potential to distinguish between combustion of gasoline/coal and natural gas over urban areas using carbon isotopes, although local studies of the isotopic composition of these fossil fuels are required in each case due to the potential for geographic variability, particularly in natural gas. Data from Salt Lake City, USA indicated that emissions from local gasoline and natural gas combustion were isotopically distinct by approximately 10%o (Table 12.2). Gasoline and diesel exhaust were significantly different, but the difference was small in absolute terms (0.7%o). These results were within 2%o of measurements made in Paris, France (Widory and Javoy, 2003). In contrast, a study in Dallas, USA showed that gasoline and natural gas emissions differed by almost 15%o at that location (Clark-Thorne and Yapp, 2003; Table 12.2).

The 513C of ecosystem respiration (S I3Cr ) has been well studied in natural ecosystems, particularly in temperate forests. Pataki et al. (2003b) reviewed 137 measurements of <513C/j made in more than 40 C3 ecosystems (primarily forests) and found that values ranged from —21.4 to —28.9%o with an average of —26.2%o. Analogous results for urban plant and soil respiration are not available. Two opposing factors may influence <S13Cr in urban areas: physiological stress caused by atmospheric and soil pollutants may reduce

Table 12,2 The Carbon Isotope Composition (<513C, V-PDB) of Automobile and Residential Natural Gas Furnace Exhaust in Salt Lake City, Utah*, Paris, France1", and Dallas, TX+

Table 12,2 The Carbon Isotope Composition (<513C, V-PDB) of Automobile and Residential Natural Gas Furnace Exhaust in Salt Lake City, Utah*, Paris, France1", and Dallas, TX+

Exhaust source

Salt Lake City, USA

Paris, France

Dallas, USA

<513C (%O)

n

<51SC (%0)

n

<513C (%o) n

Gasoline, vehicle

-27.9(0.1)a

42

-28.7(0.2)

10

-27.2(0.1) 3

Diesel, vehicle

—28.6(0.l)b

39

-28.9(0.1)

7

Natural gas, furnace

—37.8(0.3)e

6

-39.2(0.5)

6

Gas, laboratory

—37.3(0.7)e

6

-38.4

1

-42.0 1

* Hush et at., unpublished observations. +Widory and Javoy (2003).

* Clark-Thome and Yapp (2003).

Standard errors are given in parenthesis. The sample size n refers to the number of vehicles or furnaces sampled, and letters indicate statistical differences by least squares difference for the Salt Lake City data (a — 0.05). Wall gas refers to combustion of the natural gas supplied to the laboratory by the local utility.

* Hush et at., unpublished observations. +Widory and Javoy (2003).

* Clark-Thome and Yapp (2003).

Standard errors are given in parenthesis. The sample size n refers to the number of vehicles or furnaces sampled, and letters indicate statistical differences by least squares difference for the Salt Lake City data (a — 0.05). Wall gas refers to combustion of the natural gas supplied to the laboratory by the local utility.

the ratio of intercellular to ambient CO2 and increase expected values of 8 |9'C/i, while 13C dilution of the urban atmosphere by fossil fuel combustion may decrease <513Cr if plants incorporate depleted source air into biomass (Kiyosu and Kidoguchi, 2000; Dongarra and Varrica, 2002; Lichtfouse et al., 2003).

The <$180 of ecosystem respiration (5lsO/f) is strongly influenced by the oxygen isotope composition of soil and leaf water, as respiratory CO2 equilibrates with water, a process which is accelerated by the presence of carbonic anhydrase in leaves and microbes in soils (Gillon and Yakir, 2001). While soil water is generally more 180-enriched at the soil surface than at depth due to evaporation (Barnes and Allison, 1983), Miller etal (1999) found that the effective depth for isotopic exchange between soil C02 and soil water was between 5 and 15 cm. Soil water at this depth is less lsO-enriched than leaf water, which is enriched relative to soil water because of the evaporative effects of transpiration (Dongmann et al., 1974; Flanagan and Ehleringer, 1991). Therefore, <518Or is affected by the relative balance of contributions from leaf and soil respiration, which can be offset by as much as 40%o (Fig. 12.1). Estimates of the proportional contributions of leaf and soil respiration to the total biological signal can be difficult to obtain, and are currently not available at all for urban ecosystems. For the example of Salt Lake City, USA, which has soil water values close to —15%o, applying an estimate of equal contributions of leaf and soil respiration to total respired CO2 results in values of ¿>l80/,> that are distinct from the expected value of combustion (Fig. 12.1).

Salt Lake City, 2002

Figure 12.1 Upper panel—The oxygen isotope ratio (<5180) of urban COg sources in Salt Lake City, USA. Plant and soil respiration were modeled hourly according to (Flanagan et al., 1997) with climate data, a source water value of —15%o, and a water vapor value of —21%o; the 10-day running average of nighttime means are shown. Also shown is the annual trend for total nighttime biological respiration if leaf and soil respiration contribute in equal proportions. The solid line shows the expected <5180 of combustion, equal to S'80 of diatomic oxygen in the atmosphere (Kroopnick and Craig, 1972). Modified from Pataki et al. (2003a). Lower panel—Average nighttime temperature and relative humidity for the same period.

0 50 100 150 200 250 300 350 Day of year

Figure 12.1 Upper panel—The oxygen isotope ratio (<5180) of urban COg sources in Salt Lake City, USA. Plant and soil respiration were modeled hourly according to (Flanagan et al., 1997) with climate data, a source water value of —15%o, and a water vapor value of —21%o; the 10-day running average of nighttime means are shown. Also shown is the annual trend for total nighttime biological respiration if leaf and soil respiration contribute in equal proportions. The solid line shows the expected <5180 of combustion, equal to S'80 of diatomic oxygen in the atmosphere (Kroopnick and Craig, 1972). Modified from Pataki et al. (2003a). Lower panel—Average nighttime temperature and relative humidity for the same period.

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