First Evidences of Methyl Chloride (CH₃Cl) Transport from the Northern Italy Boundary Layer during Summer 2017

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. View Full-Text