Urban CO2 and CH4 pilot atmospheric measurements in the Milan city area (Northern Italy)

Urban regions are significant contributors to emissions of greenhouse gases into the atmosphere. The objective of this study, which utilizes a pilot dataset of CO2 and CH4 atmospheric measurements from July 2023 to March 2024, is to characterize their temporal variability and to test the sensitivity and applicability of an observation based CH4 emission approach under urban conditions in Milan (45°28′01″N, 9°11′24″E), the second largest mega-city in Italy. To this end, a "top-down" methodology based on the observed CH4/CO2 emission ratios has been applied to calculate total CH4 emissions, which have been preliminary compared with two “bottom-up” inventories (INEMAR and EDGAR). The seasonal variability of CH4 and CO2 was characterized by minima in summer and maxima in winter, while the diel variabilities were characterized by peaks in the morning and in the evening/night. These results are indicative of a complex interplay of temporal variability in emissions, biogenic activity and mixing layer height (MLH). Anthropogenic emissions related to combustion activities, vegetation and MLH appear to drive the CO2 variability. Usage/distribution of fossil fuels, biogenic sources (surface water or sewage system), biomass burning and MLH affected CH4. This study suggests the existence of CH4 sources in the Milan suburbs not fully accounted for in the regional "bottom-up" inventory INEMAR. However, definitive estimates cannot be provided in this pilot framework and further studies based on extended monitoring activity in the Milan urban area are required to obtain solid CH4 emission estimates to the entire urban area.