2.3. METHANE

2.3.1. IN SITU METHANE MEASUREMENTS

Quasi-continuous in situ measurements of atmospheric CH₄ continued at MLO and BRW. Details of the measurement techniques and analysis of the in situ data through early 1994 were published in late 1995 [Dlugokencky et al., 1995]. Daily averaged CH₄ mole fractions (in nanomol/mol or 10^-9 mole/mole; abbreviated ppb) are plotted in Figure 2.9 for BRW (a) and MLO (b). The data have been edited for instrument malfunction using a rule-based expert system [Masarie et al., 1991], but were not selected for meteorological conditions. High CH₄ values at BRW are due to emissions from local sources. Limitations of the unselected data sets have been discussed previously [Dlugokencky et al., 1995].

Fig. 2.9. Daily mean CH₄ mixing ratios in 10^-9 mole/mole (abbreviated ppb) for (a) BRW and (b) MLO for 1994 and 1995. The data have not been selected for meteorological conditions, but have undergone a quality control step to ensure that the analytical instrument was working optimally when they were obtained [Masarie et al., 1991].

Previously it was reported that the precision of the measurements (~0.2%) was limited in part by variations in laboratory temperature which affects the flow rate of H₂ to the FID [Peterson and Rosson, 1993]. On December 1, 1995, a new analytical system was installed at MLO ushering in a new era of high precision in situ CH₄ measurements at MLO. Main components of the system are an HP 6890 GC with FID, an HP 35900E analog-to-digital converter (A/D), a temperature controlled box for the sample valve, and a HP UNIX workstation. A similar system will be installed at Barrow during Spring 1996.

The gas chromatography has not changed significantly from what was used previously, except that the carrier gas was switched to N₂ to improve sensitivity. Two columns are used to separate CH₄ from air, and flame ionization is used for detection. Column head pressure and flow rates for the FID gases are controlled electronically by the GC. The signal from the FID is amplified by an electrometer and sent to the A/D. Previously the A/D was included in a stand-alone integrator. The new HP 35900E A/D is 24-bit (versus 16 bit for the integrator), so it does not limit the measurement precision.

Integration is now done on the UNIX workstation using an algorithm developed by SIO and incorporated into a program developed by the Carbon Cycle Group called GCPLOT. This program allows integration and display of chromatograms, and it is a powerful diagnostic that can be used to troubleshoot problems with the CH₄ chromatography and GC system. This integration system is also paperless; about 1-month's worth of chromatograms are stored on the workstation hard disk, and these can be displayed on the computer monitor with GCPLOT. A typical chromatogram is shown in Figure 2.10. The peak at about 78 seconds is the air disturbance. The CH₄ peak response (at retention time = 124 seconds), from the baseline to the top of the peak, is ~16.8 mV. Peak-to-peak noise is ~8mV. Previously, peak height was used to calculate CH₄ mole fractions since height resulted in about a factor of 3 better precision than peak area response. With the new integration algorithm, peak area yields slightly better precision than height; therefore, peak area is now being used as the quantitative measure of CH₄ peak response. Using area is preferable to height because it gives a linear response over a larger range of mixing ratios. The mole fractions are calculated as before; the peak area from the sample is ratioed to the average peak area of the bracketing standard gas injections. This ratio is then multiplied by the assigned value for the standard gas cylinder. By only using the injections of standard gas, this calculation can be used to assess the instrument precision as described in the following paragraph.

Fig. 2.10. Typical CH₄ chromatogram from MLO obtained with the new analysis system. A/D sampling rate was 10 Hz. On the y-axis, there are 10⁵ counts mV^-1. Signal-to-noise is ~2000, based on a peak height of 16.8 mV and peak-to-peak noise of ~8mV. The retention time for CH₄ is 124.3 seconds, and the full-width-at-half-max is 8 seconds.

The preceding improvements have lead to an overall improvement in precision at MLO of a factor of 4. Typical relative precision is 0.04 to 0.07% (or <1 ppb CH₄ for ambient levels of about 1700 ppb). In Figure 2.11, relative differences between measurements of standard gas and the assigned value for the standard gas cylinder ("Relative Precision") are plotted for a 24-hour period. The relative precision for this day (assessed as 1s) was 0.03% (0.6 ppb).

Fig. 2.11. Relative measurement precision, assessed as the difference between measurements of standard gas and the assigned value of the standard gas cylinder, plotted for a 24-hour period. The relative instrument precision on this day, based on 1s for each measurement of reference gas, was 0.03% (plotted as the dashed lines). This corresponds to a precision, in mole fraction, of ±0.6 ppb for each measurement.

The new system is controlled by a program run on the UNIX workstation. This program chooses between ambient and standard gas flows from the stream selection valve, switches the gas sample valve to start the run, and records the digitized chromatogram. A VXI bus acts as the interface between the UNIX workstation and other system components. Various other programs that can be used to look at the results are also available at the workstation.

2.3.2. DISCRETE SAMPLE MEASUREMENTS OF METHANE

During 1994-1995, the determination of the global distribution of atmospheric CH₄ continued from 46 sampling sites of the Carbon Cycle Group's cooperative air sampling network. Provisional annual mean values for 1994-1995 are given in Table 2.7.

TABLE 2.7. Provisional 1994 and 1995 Annual Mean CH₄ Mixing Ratios From the Air Sampling Network

		1994	1995
Code	Station	CH4 (ppb)	CH4 (ppb)
ALT	Alert, N.W.T., Canada	1809.8	1811.3
ASC	Ascension Island	1684.0	1690.4
AZR	Terceira Island, Azores	[ ]	1783.2
BAL	Baltic Sea	1828.6	1853.7
BME	Bermuda (east coast)	1773.1	1780.5
BMW	Bermuda (west coast)	1765.3	1771.0
BRW	Barrow, Alaska	1821.4	1822.3
CBA	Cold Bay, Alaska	1801.9	1804.2
CGO	Cape Grim, Tasmania	1671.8	1679.8
CMO	Cape Meares, Oregon	1788.5	[ ]
CRZ	Crozet Island	[ ]	1679.3
GMI	Guam, Mariana Islands	1731.3	1741.2
GOZ	Dwejra Point, Gozo, Malta	1798.2	1804.2
HUN	Hegyhatsal, Hungary	1853.3	1870.7
ICE	Heimaey, Iceland	1799.1	1806.8
ITN	WITN, Grifton, N. Carolina	1817.1	1817.0
IZO	Izaña Observatory, Tenerife	1754.0	1757.2
KEY	Key Biscayne, Florida	1751.1	1765.1
KUM	Cape Kumukahi, Hawaii	1753.7	1756.8
LEF	WLEF, Park Falls, Wisconsin	[ ]	1825.4
MBC	Mould Bay, Canada	1812.0	1816.8
MHT	Mace Head, Ireland	1793.2	1792.3
MID	Midway Island	1763.5	1772.8
MLO	Mauna Loa, Hawaii	1736.7	1739.7
NWR	Niwot Ridge, Colorado	1764.9	1774.0
PSA	Palmer Station, Antarctica	1672.4	1679.1
QPC	Qinghai Province, China	1777.8	1782.2
RPB	Ragged Point, Barbados	1740.5	1740.2
SEY	Mahé Island, Seychelles	1696.3	1700.7
SHM	Shemya Island, Alaska	1801.4	1804.8
SMO	American Samoa	1679.3	1684.8
SPO	South Pole, Antarctica	1671.2	1678.1
STM	Ocean Station M	1803.2	1807.0
SYO	Syowa Station, Antarctica	1671.5	1678.9
TAP	Tae-ahn Peninsula, S. Korea	1830.5	1821.3
UTA	Wendover, Utah	1779.1	1783.7
UUM	Ulaan Uul, Mongolia	1802.1	1803.0
ZEP	Ny-Alesund, Svalbard	1806.1	1815.2

Square brackets indicate insufficient data to calculate annual mean.

The effects of the eruption of Mt. Pinatubo on the growth rates of trace species such as CH₄, CO₂, CO, and N₂O still remain an area of great interest to our group. Studies of perturbations in growth rate that are associated with a specific event such as the eruption can be a useful tool in understanding the trace gas budgets. The eruption of Mt. Pinatubo on June 15, 1991, injected 20 Mt SO₂ and 3-5 km³ of ash into the upper troposphere and lower stratosphere, and CH₄ and CO mixing ratios in the tropics immediately increased. The increased growth rates were short-lived as CH₄ [Dlugokencky et al., 1994a] and CO [Novelli et al., 1994] growth rates showed dramatic decreases later during 1992 and 1993.

In Figure 2.12a, CH₄ zonal means for the latitude zone 30-90°S are plotted (open triangles) along with a function fitted to the zonal means (dashed line) of the form:

Equation (1) is used to approximate (or model) the average trend and seasonal cycle for atmospheric CH₄. Starting in late 1991, there is a significant departure of the "model" from the zonal means. The solid line is the deseasonalized trend (see Dlugokencky et al., 1994b for details of the curve fitting process). Its derivative, the instantaneous CH₄ growth rate, is shown in Figure 2.12b. The largest perturbation in CH₄ growth rate observed in this time series was during late-1991 and early 1992.

Fig. 2.12. (a) Zonally-averaged CH4 mixing ratios for 30-90°S (symbols). The dashed line is a function (Eq. (1)) fitted to the zonal means to approximate the long-term trend and average seasonal cycle. The solid line is the deseasonalized trend; it is a combination of the polynomial in Eq. (1) and the result of the 650-day cutoff filter. (b) Instantaneous, smoothed growth rate for atmospheric CH₄ in the latitude zone 30-90°S. The curve is calculated as the derivative of the solid curve in (a).

The CH₄ growth rate is due to a relatively small imbalance between sources and sinks; therefore, the perturbation in 1991 could be due to either a change in one or more sources or a change in the sink. The major sink for CH₄ is reaction with hydroxyl radical

OH + CH₄ --> H₂O + CH₄ . (2)

In the clean marine troposphere, most OH formation is initiated through photolysis of O₃ to give electronically excited oxygen atoms

O₃ + hn (330    290 nm) --> O(¹D) + O₂ . (3)

Most O(1D) is quenched to ground state O atoms, but a small fraction reacts with water,

O(¹D) + H₂O -->2OH . (4)

The photolysis rate coefficient for formation of O(¹D), jO₃(O(¹D)), is a function of the actinic flux in the appropriate wavelength region, the ozone cross section, and the quantum yield (for O(¹D) formation). Anything that affects the flux of radiation in the wavelength 330  l  290 nm, also affects the CH₄ sink.

The large increase in CH₄ growth rate in 1991 is consistent with decreased actinic flux in the wavelength region 290-330 nm due to UV absorption by SO₂ and enhanced scattering by sulfate aerosols. In Figure 2.13, the change in jO₃(¹D)) calculated with a radiative transfer model is plotted. Initially, direct absorption of UV radiation by SO₂ lead to a 12% decrease in jO₃(¹D)). This effect was relatively short-lived due to the short lifetime (~30 days) for SO₂. Later, UV scattering from sulfate aerosol produced by oxidation of the SO₂ maintained lower than normal values for j for more than 1 year after the eruption. It is suggested that the decreased UV flux led to a decreased steady-state concentration of atmospheric OH in the tropics and midlatitudes of the southern hemisphere, and this led to the observed perturbation in CH₄ growth rate.

Fig. 2.13. Relative change in jO₃(O(¹D)) after the eruption of Mt. Pinatubo. The solid line includes the effects of direct absorption by SO₂ and scattering by sulfate aerosol; the dotted line includes aerosol only. The change is plotted relative to a 10-year climatological background.

Of more general interest are the far reaching effects of the eruption of Mt. Pinatubo on trace gas budgets. In the case of CH₄, it has been suggested previously that Mt. Pinatubo resulted in initially enhanced growth rates during 1991 and early 1992. Cooler temperatures resulting from the eruption [Dutton and Christy, 1992] also likely led to decreased CH₄ emissions from natural wetlands in the northern hemisphere [Hogan and Harriss, 1994], which in turn may have been largely responsible for the large observed decrease in CH₄ growth rate during late-1992 and 1993 in the high northern latitudes. This is consistent with isotopic measurements of CO₂ that suggest that the biosphere was a larger than normal sink for fossil CO₂ during 1992 and 1993 through either increased photo-synthesis or decreased respiration [Ciais et al., 1995b], either of which could result from short-term variations in temperature or precipitation as a result of the eruption.

2.3.3. MEASUREMENT OF ¹³C/¹²C OF METHANE

Although many sources of CH₄ have been identified, the uncertainty in individual source terms remains large. In order to explain trends in the CH₄ growth rate, such as the period of almost no growth in 1992 and 1993 [Dlugokencky et al., 1994a,b] a more precise understanding of the CH₄ budget is needed. The global measurement of the stable carbon isotopes of CH₄ (d¹³C) afford an excellent means of furthering our understanding of the CH₄ budget. The three primary processes that produce CH₄ (bacterial fermentation, fossil fuel extraction, and biomass burning) all have different characteristic isotopic "signatures." Thus, global measurement of d¹³C used together with a transport and chemistry model will allow for a more accurate characterization of sources than is currently possible.

A system is under development for the automated analysis of small (20 mL) air samples for d¹³C. The technique employed is gas chromatography coupled with isotope­ratio mass spectrometry. Methane is chromatographically extracted from air, cryofocused, combusted to produce CO₂, and then admitted to the mass spectrometer. The total analysis time, including reference gas analysis, is less than 30 minutes per sample. The automation, small sample size, and short analysis time are key design elements so that these isotopic measurements may be easily incorporated into our cooperative air sampling network.

For our sample, the shot­noise limited precision would be 0.01‰; therefore, the goal of a precision of 0.1‰ is attainable even with such a small sample size. To date, our best precision for five replicate samples of air from Niwot Ridge is 0.16‰ (one standard deviation) (Figure 2.14).

Fig. 2.14. Reproducibility of repeated measurements of¹³C of methane in the same air. br>

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