Regional Sources of Methyl Chloride, Chloroform and Dichloromethane Identified from AGAGE Observations at Cape Grim, Tasmania, 1998–2000

Authors: Cox M.L.¹; Sturrock G.A.^{2, 3}; Fraser P.J.^{2, 3}; Siems S.T.⁴; Krummel P.B.^{2, 3}; O'Doherty S.⁵

Source: Journal of Atmospheric Chemistry, Volume 45, Number 1, May 2003 , pp. 79-99(21)

Publisher: Springer

Abstract:

There are large uncertainties in identifying and quantifying the natural and anthropogenic sources of chloromethanes – methyl chloride (CH₃Cl), chloroform (CHCl₃) and dichloromethane (CH₂Cl₂), which are responsible for about 15% of the total chlorine in the stratosphere. We report two years of in situ observations of these species from the AGAGE (Advanced Global Atmospheric Gas Experiment) program at Cape Grim, Tasmania (41° S, 145° E). The average background levels of CH₃Cl, CHCl₃ and CH₂Cl₂ during 1998–2000 were 551± 8, 6.3± 0.2 and 8.9± 0.2 ppt (dry air mole fractions expressed in parts per 10¹²) respectively, with a two-year average amplitude of the seasonal cycles in background air of 25, 1.1 and 1.5 ppt respectively. The CH₃Cl and CHCl₃ records at Cape Grim show clear episodes of elevated mixing ratios up to 1300 ppt and 55 ppt respectively, which are highly correlated, suggesting common source(s). Trajectory analyses show that the sources of CH₃Cl and CHCl₃ that are responsible for these elevated observations are located in coastal-terrestrial and/or coastal-seawater regions in Tasmania and the south-eastern Australian mainland. Elevated levels of CH₂Cl₂ (up to 70 ppt above background) are associated mainly with emissions from the Melbourne/Port Phillip region, a large urban/industrial complex (population 3.5 million) 300 km north of Cape Grim.

Keywords: chloromethanes; methyl chloride; chloroform; dichloromethane; Cape Grim Baseline Air Pollution Station; baseline data; annual cycle; trajectory analysis; pollution events; coastal source regions

Language: English

Document Type: Research article

Affiliations: 1: Department of Mathematics and Statistics, Monash University, Clayton, Victoria, Australia; Now at the Centre for Atmospheric Chemistry, York University, North York, Ontario, Canada 2: Atmospheric Research, CSIRO, Asp 3: 4: Department of Mathematics and Statistics, Monash University, Clayton, Victoria, Australia 5: School of Chemistry, University of Bristol, Bristol, U.K.

Publication date: 2003-05-01

• In this: publication
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• In this Subject: Meteorology & Climatology
• By this author: Cox M.L. ;  Sturrock G.A. ;  Fraser P.J. ;  Siems S.T. ;  Krummel P.B. ;  O'Doherty S.

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