Methyl chloride and other chlorocarbons in polluted air during INDOEX

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Abstract
Methyl chloride (CH3Cl) is the most abundant, natural, chlorine‐containing gas in the atmosphere, with oceans and biomass burning as major identified sources. Estimates of global emissions suffer from large uncertainties, mostly for the tropics, partly due to a lack of measurements. We present analyses of whole‐air canister samples for selected nonmethane hydrocarbons and chlorocarbons. The samples were collected from an aircraft during the Indian Ocean Experiment (INDOEX) campaign over the northern Indian Ocean in February and March 1999. The CH3Cl results are correlated to selected nonmethane hydrocarbons and in situ measurements of carbon monoxide (CO) and acetonitrile (CH3CN). We relate high mixing ratios of ∼750 pmol mol−1of CH3Cl to biomass burning, as observed in polluted air masses from India and Southeast Asia. We infer a relatively high enhancement ratio relative to CO, ΔCH3Cl/ΔCO ≈1.74 ± 0.21 × 10−3mol mol−1. The CH3Cl levels relate to the extensive biofuel use in India and Southeast Asia, notably the burning of agricultural waste and dung with a comparatively high chlorine content. It appears that CH3Cl emissions from biofuel consumption in India and Southeast Asia have been underestimated in the past. Furthermore, we observed enhanced dichloromethane (CH2Cl2) and trichloromethane (CHCl3) levels, correlating with high CO, acetylene (C2H2) and CH3Cl, indicating that biomass burning is a small but significant source of these species.

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