Antarctic Ice Cores: Methyl Chloride and Methyl Bromide

Summary

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.

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Saltzman, Eric., Murat Aydin, Margaret Williams, and Cheryl Tatum. 2007. Antarctic Ice Cores, Methyl Chloride and Methyl Bromide. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N5DN430Q.

Overview Table

Category	Description
Data format	Microsoft Excel
Spatial coverage and resolution	Southernmost Latitude: 89.93° S Northernmost Latitude: 89.93° S Westernmost Longitude: 144.39° W Easternmost Longitude: 144.39° W Maximum Depth: 295 m
Temporal coverage and resolution	2159 to 140 Y.B.P.
File naming convention	SPO_MeBr_NSDIC1007.xls SPO_MeCl_NSDIC.xls SPO_MeCl_fig1.pdf
File size	Approximately 100 KB
Parameter(s)	CH3Cl and CH3Br mixing ratios
Procedures for obtaining data	Available via FTP.

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator(s)

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 

Technical Contact

Acknowledgements

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

2. Detailed Data Description

Format

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

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_MeCl_NSIDC.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

Spatial Coverage

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

Temporal Coverage

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

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

3. Data Access and Tools

Data Access

Available via FTP.

Volume

The entire data set is approximately 100 KB.

Related Data Collections

• Analysis of Siple Dome Ice Core: Carbonyl Sulfide (COS), Methyl Chloride (CH₃Cl), and Methyl Bromide (CH3Br)
• Biogenic Sulfur in the Siple Dome Ice Core

4. Data Acquisition and Processing

Sensor or Instrument Description

Micromass Autospec: High Resolution Mass Spectrometer for Trace Gas Analysis

Data Acquisition Methods

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.

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).

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.

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.

5. References and Related Publications

Williams, M.B., M. Aydin, C. Tatum and E.S. Saltzman. 2007. A 2000 Year Atmospheric History of Methyl Chloride from a South Pole Ice Core: Evidence for Climate Controlled Variability. Geophysical Research Letters. 34, doi:10.1029/2006GL029142

Aydin, M., E. S. Saltzman, W. J. De Bruyn, S. A. Montzka, J. H. Butler, and M. Battle. 2004. Atmospheric variability of methyl chloride over the last 300 years from an Antarctic ice core and firn air. Geophysical Research Letters. Vol. 31. L02109, doi: 10.1029/2003GL018750.

Saltzman, E. S., M. Aydin, W. J. De Bruyn, D. B. King, and S. A. Yvon-Lewis. 2004. Methyl bromide in preindustrial air: measurements from an Antarctic ice core. Journal of Geophysical Research. Vol. 109. No. D5, D0530, doi: 10.1029/2003JD004157.

Aydin, M., W. J. De Bruyn, and E. S. Saltzman. 2002. Preindustrial atmospheric carbonyl sulfide (OCS) from an Antarctic ice core. Geophysical Research Letters. Vol. 29. 9. 73-1 73-4.

Battle M., M. Bender, T. Sowers, P.P. Tans, J.H. Butler, J.W. Elkins, J.T. Ellis, T. Conway, N. Zhang, P. Lang and A.D. Clarke. 1996. Atmospheric gas concentrations over the past centruy measured in air from firn at the South Pole. Nature. 383:231-235

Butler, J.H., M. Battle, M. Bender, S.A. Montzka, A.D. Clarke, E.S. Saltzman, C. Sucher, J. Severinghaus, and J.W. Elkins. 1999. A twentieth century record of atmospheric halocarbons in polar firn air. Nature. 399:749-765.

6. Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

FTP	File Transfer Protocol
ITASE	International Trans Antarctic science Expedition
NSF	National Science Foundation
NSIDC	National Snow and Ice Data Center
OPP	Office of Polar Programs
SPRESSO	South Pole Remote Earth Science and Seismological Observatory
URL	Uniform Resource Locator

Document Creation Date

November 2007

Document URL

http://nsidc.org/data/docs/agdc/nsidc0313_saltzman/index.html