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Data Set ID:
NSIDC-0313

Antarctic Ice Cores: Methyl Chloride and Methyl Bromide, Version 1

This data set is an analysis of methyl chloride (CH3Cl) and methyl bromide (CH3Br) in Antarctic ice core samples. Investigators reported mixing ratios of methyl chloride gas extracted from samples taken from the South Pole Remote Earth Science and Seismological Observatory (SPRESSO) core, drilled as part of the International Trans Antarctic Science Expedition (ITASE). This data covers an age range of 2159 - 140 years before present (Y.B.P.) where the year 2000 was used as present. Investigators analyzed trace gases in ice core samples from Siple Dome, West Antarctica (dry-drilled C core and deep, fluid-drilled A core) and from South Pole, Antarctica (300 m dry drilled SPRESSO core). Data are available in Microsoft Excel format and are available via FTP.

Geographic Coverage

Parameter(s):
  • Air Quality > Sulfur Oxides > Carbonyl Sulfide
  • Atmospheric Chemistry > Sulfur Compounds > Carbonyl Sulfide
  • Glaciers/Ice Sheets
  • Ice Core Records
  • Glaciers/Ice Sheets > Ice Sheets
  • Atmospheric Chemistry > Halocarbons and Halogens > Halocarbons > Methyl Chloride
  • Methyl Bromide
  • Snow/Ice > Snow/Ice Temperature
Spatial Coverage:
  • N: -89.93, S: -89.93, E: -144.39, W: -144.39

Spatial Resolution: Not Specified
Temporal Coverage: Not specified
Temporal Resolution: Not specified
Data Format(s):
  • Microsoft Excel
Platform(s) Not specified
Sensor(s): CORING DEVICES, GAS CHROMATOGRAPHS, MASS SPECTROMETERS
Version: V1
Data Contributor(s): Eric Saltzman, Murat Aydin, Margaret Williams, Cheryl Tatum, NSIDC User Services

Data Citation

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Saltzman, E., M. Aydin, M. Williams, and C. Tatum. 2007. Antarctic Ice Cores: Methyl Chloride and Methyl Bromide, Version 1. [Indicate subset used]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: http://dx.doi.org/10.7265/N5DN430Q. [Date Accessed].

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Detailed Data Description

Format

Data files are in Microsoft Excel format viewable with spreadsheet software and an Adobe Acrobat PDF.

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File Naming Convention

Files are named according to the following convention.

File name File Size Description
SPO_MeCl_NSIDC.xls 24 KB Methyl chloride (CH3Cl) gas ratios Microsoft Excel File
SPO_MeBr_NSIDC1007.xls 48 KB Methyl bromideide (CH3Br) gas ratios Microsoft Excel File
SPO_MeCl_fig1.pdf 44 KB Methyl chloride (CH3Cl) gas mixing Adobe Acrobat figure
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Volume

The entire data set is approximately 100 KB.

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Spatial Coverage

Southernmost Latitude: 89.93° S
Northernmost Latitude: 89.93° S
Westernmost Longitude: 144.39° W 
Easternmost Longitude: 144.39° W

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Temporal Coverage

2159 Y.B.P. to 140 Y.B.P.

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Parameter or Variable

Parameter Description

Methyl chloride (CH3Cl), which is produced primarily by natural sources, is the most abundant halocarbon in the atmosphere. It currently has an average mixing ratio of approximately 530 (parts per trillion by volume) pptv over Antarctica and accounts for more than 10% of the ozone-depleting halogen delivered to the stratosphere. The atmospheric lifetime of CH3Cl is estimated to be 1.3 years.

Methyl bromide (CH3Br) is a stratospheric halogen and a contributor to stratospheric ozone depletion whose industrial production has been phased out by the Montreal Protocol. It has a complex biogeochemical cycle with natural sources and sinks both terrestrial and oceanic. However, the CH3Br budget is not well characterized and there appears to exist a naturally occurring "unknown source." The atmospheric lifetime of methyl bromide ranges from 0.7 years to 0.9 years. It currently has an average mixing ratio of 9.5 pptv, but its preindustrial mixing ratios have proven quite difficult to model as is shown by Saltzman et al. (2004

Parameter Range

Methyl chloride (CH3Cl): 461.7 pptv - 579.5 pptv
Methyl bromide (CH3Br): 3.2 pptv - 9.5 pptv

Sample Data Record

The data below are the first 10 data samples showing methyl chloride (CH3Cl) mixing ratios at various depths

depth

gas age

mixing ratio

error

(m)

years BP

ppt

ppt

124.05

140.4

500.2

20.7

131.08

225.4

432.5

7.7

132.02

236.8

481.6

19.7

134.99

272.7

455.8

14.2

136.64

292.7

459.7

23.6

137.58

304.1

470.7

9.3

137.985

309.0

450.9

15.3

139.985

333.2

494.2

22.5

141.03

345.8

494.1

22.2

141.03

345.8

453.4

11.1

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Software and Tools

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Data Acquisition and Processing

Sensor or Instrument Description

Micromass Autospec: High Resolution Mass Spectrometer for Trace Gas Analysis

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Sample collection and gas dating

The South Pole SPRESSO (South Pole Remote Earth Science and Seismological Observatory) ice core was drilled during 2002, at 89.93°S, 144.39°W, to a depth of 295 meters. Detailed firn air studies have not been carried out at this location, so we assumed the firn properties to be the same as those at other South Pole sites (Aydin et al., 2004; Battle et al., 1996; Butler et al., 1999). The age of CH3Cl at pore close-off was assumed to be 112.8 years, based on a CO2 age at close-off of 92 years, adjusted for the diffusivity of CH3Cl (0.12 cm2 at 224 K, (Wilke and Lee, 1955).

The width of the age distribution at half height was estimated at 30 years, based on a numerical 1-D firn air model using a diffusivity profile inferred from observed South Pole profiles of carbon dioxide(CO2) and chlorofluorocarbon (CFC-11) (Aydin et al., 2004). The ice age-depth relationship was determined from visual layer counting in the SPRESSO ice core (Gow, 2006). The difference in mean age between the air bubbles and the surrounding ice was estimated to be 1029 years, based on the gas age at pore close-off and the ice age-depth relationship. The age/depth/density relationship gives an average accumulation rate at the SPRESSO site of 8.3 cm weq/year.

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Gas extraction

The apparatus and procedures used to extract gases from the ice core samples were similar to those used in this laboratory for earlier ice core trace gas studies (Aydin et al., 2002; Aydin et al., 2004; Saltzman et al., 2004). Samples weighted 500-800 g, and the outer 2-3mm were scraped away by hand with a scalpel. The sample was then loaded into a pre-cooled (-50°C) stainless steel vacuum chamber (14.5cm diameter x 20cm height, sealed with copper-gasket ConFlat (CF) flanges). The chamber is repeatedly flushed with nitrogen (N2) and evacuated to 0.1 torr. The ice core sample was shredded under vacuum by 1 Hz linear motion across a flat stainless steel grater mounted inside the chamber. After shredding, the liberated air was cryo-focused into a stainless steel collection tube (1/4” O.D., 70 cm in length) immersed in liquid helium (He).

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Gas analysis

Samples were analyzed by gas chromatography with isotope dilution mass spectrometry, using a dual sector mass spectrometer (Micromass Autospec, Waters Inc.), operated at a mass resolution of about 6,000 (m/m at 5% peak height). Trace gases were cryogenically focused onto a glass bead trap in liquid nitrogen, allowing non-condensable components of air to pass into a calibrated volume. An internal standard containing CH3Cl was added to each sample, and quantification was carried out using the ratio of peak areas of the labeled and unlabeled parent ions. Calibration was based on high pressure, ppm-level aluminum cylinders prepared in this laboratory and periodically inter-compared with the NOAA-GMD calibration scale. Part per trillion level working standards were prepared by diluting the ppm standards with moist N2 (75% relative humidity) in low pressure, electropolished, stainless steel flasks. New working standards were made every 2-3 weeks.

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Sample blanks

Extraction of each ice core sample was preceded and followed by “blanks”, where N2 gas was introduced into the stainless steel vessel and then extracted and analyzed as a sample. These blanks reflect CH3Cl contamination associated with the extraction line, sample vessel, valves and ice surface. The pre- and post-shred blanks for each sample were averaged and subtracted from the ice core CH3Cl signal. The variance of the blanks for each sample was treated as part of the analytical uncertainty. For the entire SPRESO dataset, the ratios of the signals from ice core samples to those from blanks were greater than 50:1. At 3 depths in the core, individual samples were cut in half and analyzed as “replicates”. These pairs differed in CH3Cl mixing ratio by 0.8%, 2.1% and 7.9% respectively. Contamination of samples with modern air either during analysis or via trapping in micro fractures in the ice sample was detected by measuring CFC-12.

All samples in this dataset had CFC-12 levels below 2 ppt.

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References and Related Publications

Contacts and Acknowledgments

Eric Saltzman
3325 Croul Hall
Department of Earth System Science
University of California, Irvine 
Irvine, California 92697 USA 

Murat Aydin
3325 Croul Hall
Department of Earth System Science
University of California, Irvine 
Irvine, California 92697 USA

Margaret Williams
3325 Croul Hall
Department of Earth System Science
University of California, Irvine 
Irvine, California 92697 USA 

Cheryl Tatum
3325 Croul Hall
Department of Earth System Science
University of California, Irvine 
Irvine, California 92697 USA 

Acknowledgments: 

This research was supported by National Science Foundation (NSF) Office of Polar Programs (OPP) grant 0338359 awarded to E. Saltzman and M. Aydin.

Document Information

Document Creation Date

November 2007

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