Abstract
Methyl Chloride (CH3Cl) is a chlorine-containing trace gas in the atmosphere contributing significantly to stratospheric ozone depletion. While the atmospheric CH3Cl emissions are predominantly caused by natural sources on the global budget, significant uncertainties still remain for the anthropogenic CH3Cl emission strengths. In summer 2007 an intensive field campaign within the ACTRIS-2 Project was hosted at the Mt. Cimone World Meteorological Organization/Global Atmosphere Watch global station (CMN, 44.17° N, 10.68° E, 2165 m a.s.l.). High-frequency and high precision in situ measurements of atmospheric CH3Cl revealed significant high-frequency variability superimposed on the seasonally varying regional background levels. The high-frequency CH3Cl variability was characterized by an evident cycle over 24 h with maxima during the afternoon which points towards a systematic role of thermal vertical transport of air-masses from the regional boundary layer. The temporal correlation analysis with specific tracers of anthropogenic activity (traffic, industry, petrochemical industry) together with bivariate analysis as a function of local wind regime suggested that, even if the role of natural marine emissions appears as predominant, the northern Italy boundary layer could potentially represent a non-negligible source of CH3Cl during summer. Since industrial production and use of CH3Cl have not been regulated under the Montreal Protocol (MP) or its successor amendments, continuous monitoring of CH3Cl outflow from the Po Basin is important to properly assess its anthropogenic emissions.
Highlights
Methyl chloride, CH3 Cl, is a major carrier of reactive chlorine into the stratosphere and the understanding of its natural and anthropogenic sources is important for the further development of global policies on ozone-depleting chemicals [1]
CH3 Cl has become increasingly important as a source of chlorine to the stratosphere, as the abundances of anthropogenic halogenated trace gases have been declining under the Montreal Protocol (MP) regulation [2]
For CH3 Cl, variabilities occurring at different time scales are evident: a 24 hour-scale variability is superimposing a multiday component
Summary
CH3 Cl, is a major carrier of reactive chlorine into the stratosphere and the understanding of its natural and anthropogenic sources is important for the further development of global policies on ozone-depleting chemicals [1]. While recent studies have contributed to a better understanding of emissions from natural sources (e.g., [8,9,10]), the CH3 Cl global budget still remains unbalanced [2]. Because the use as chemical feedstock is not supposed to release CH3 Cl to the atmosphere, this has not been taken into account in the national inventory reports (NIRs) submitted to the United Nation Framework Convention for Climate Change (UNFCCC)
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