Biological degradation of methyl chloride in coastal seawater

Ryszard Tokarczyk,Shari A Yvon‐Lewis,Robert M Moore,Eric S Saltzman

doi:10.1029/2002gb001949

Abstract

Methyl chloride (CH3Cl) is the most abundant halocarbon in the atmosphere, and constitutes a significant fraction of the total atmospheric halogen burden. Chemical reactions of CH3Cl in seawater are slow, and it has been believed that the oceans are not an important sink for this compound. However, direct measurements of CH3Cl degradation rates in coastal seawater (Bedford Basin, Nova Scotia), using a stable isotope incubation technique, indicate rapid loss attributed to microbial activity. A series of weekly measurements from March 2000 to May 2001 yielded degradation rates ranging from 0–30% d−1, with an annual mean of 7.4% d−1. If biological uptake of CH3Cl occurs throughout the oceans at similar rates, the mean partial atmospheric lifetime of CH3Cl with respect to oceanic removal could be a few years, rather than several decades as previously thought. This rapid removal would make the oceans a major sink for CH3Cl and lower the overall atmospheric lifetime of CH3Cl from the current estimate of 1.3 to about 1.0 years. Measurements of the degradation rate of CH3Cl in open ocean waters are needed in order to quantify the oceanic uptake rate.

Highlights

[2] Methyl chloride (CH3Cl) is a major source of atmospheric chlorine [Kurylo and Rodriguez, 1999], and a contributor to stratospheric ozone depletion
Oceanic emissions, which were previously thought to be the main source of atmospheric methyl chloride, have more recently been estimated to account for less than 12% of the global flux [Moore et al, 1Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada
Oceanic measurements of methyl chloride show that the low latitude ocean is supersaturated with respect to the overlying atmosphere, while the high latitude ocean is undersaturated [Moore et al, 1996]

Summary

Introduction

[2] Methyl chloride (CH3Cl) is a major source of atmospheric chlorine [Kurylo and Rodriguez, 1999], and a contributor to stratospheric ozone depletion. [4] In this study, we utilize a stable isotope tracer technique to directly measure the loss rate of methyl chloride in Nova Scotia coastal waters between March 2000 and May 2001. We attribute the observed CH3Cl degradation to microbial processes These observations suggest that the loss rate for CH3Cl from the surface ocean due to biological uptake may be similar in magnitude to the loss by air-sea exchange, and can explain the methyl chloride undersaturation observed in high latitudes waters. These data indicate that oceans can be a significant global sink for atmospheric. CH3Cl, and that the lifetime of this gas in the atmosphere may be shorter than previously believed

Sampling Location

Experimental Techniques

Results

Discussion

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Conclusions