Continuous measurements of the CH4/CO ratio at the remote site of Mt.Cimone and their application for the estimate of regional CH4 emissions
Authors: Fratticioli Cosimo , Trisolino Pamela , Cristofanelli Paolo , Calzolari Francescopiero 
Published: March 2022

&lt;p&gt;In order to monitor the contribution of different regions to the atmospheric concentrations of climate-altering gases, the estimate of the atmospheric gas emissions is regularly performed by governmental agencies and is reported into emission inventories. These &amp;#8220;bottom-up&amp;#8221; emission inventories are generally obtained using country-specific activity data (like population density, land&amp;#160; use, fuel consumptions etc.) and source-specific emission factors. However, these emission estimates can be subjected to large uncertainties, especially for non-CO&lt;sub&gt;2&lt;/sub&gt; GHGs. This is particularly true for methane (CH&lt;sub&gt;4&lt;/sub&gt;) and some further studies based on the use of atmospheric observations are required in order to provide more precise emissions and/or to validate existing estimations.&lt;/p&gt;&lt;p&gt;Since 2018, hourly mean concentrations of CH&lt;sub&gt;4&lt;/sub&gt; and CO are produced by using CRDS technique at the Mt. Cimone WMO/GAW global station (CMN, 2165 m a.s.l., Italy) in the framework of ICOS-RI (&lt;em&gt;Integrated Carbon Observation System&lt;/em&gt;). Being overlooking the Po basin, atmospheric observations at this measurement site can be used to investigate anthropogenic emissions occurring over this densely inhabited and industrialized region.&lt;/p&gt;&lt;p&gt;In this work, we will present the atmospheric variability of CH&lt;sub&gt;4&lt;/sub&gt; and CO at CMN during the period May 2018 &amp;#8211; December 2020 as obtained from the ICOS dataset &lt;sup&gt;5&lt;/sup&gt;. Then, we investigate the possibility to use these observations for evaluating the uncertainty related with bottom-up inventories of CH&lt;sub&gt;4&lt;/sub&gt; emissions over the Po basin. In particular, as based on the approach proposed by Kuwayama et al. &lt;sup&gt;3&lt;/sup&gt; , hourly data of CH&lt;sub&gt;4&lt;/sub&gt; and CO are fitted in order to extract the CH&lt;sub&gt;4&lt;/sub&gt;/CO ratio for each month and each year. Then, the CH&lt;sub&gt;4&lt;/sub&gt; emissions in the Po basin are estimated by combining the observed CH&lt;sub&gt;4&lt;/sub&gt;/CO ratio with the CO emission extracted by the &amp;#8220;state-of art&amp;#8221; emission inventories EDGAR (&lt;em&gt;Emissions Database for Global Atmospheric Research&lt;/em&gt;) v5.0 and v6.0 &lt;sup&gt;1,2&lt;/sup&gt; and compared with the national bottom-up inventory &lt;sup&gt;4&lt;/sup&gt; produced by ISPRA &lt;em&gt;(Istituto Superiore per la Protezione e la Ricerca Ambientale&lt;/em&gt;).&lt;/p&gt;&lt;p&gt;A critical assessment of strengths and caveats of this methodology will be provided and we present the results of sensitivity tests related to the use of different sub-setting of the CMN dataset (in term of observation time and wind direction), to the different methodologies used for CH&lt;sub&gt;4&lt;/sub&gt;/CO calculation, to the ability of the measurement site to be representative for the investigated emission region.&lt;/p&gt;&lt;p&gt;The obtained results show a good agreement with the inventory based emission estimations (both EDGAR and ISPRA)&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; European Commission Joint Research Center. Global Air Pollutant Emissions v5.0. &lt;strong&gt;https://edgar.jrc.ec.europa.eu/index.php/dataset_ap50&lt;/strong&gt;, 2019. &amp;#160;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt; European Commission Joint Research Center. Global greenhouse gas emissions v6.0. &lt;strong&gt;https://edgar.jrc.ec.europa.eu/index.php/dataset_ghg60&lt;/strong&gt;, 2020.&lt;/p&gt;&lt;p&gt;&lt;sup&gt;3&lt;/sup&gt; Kuwayama T. et al. , Source Apportionment of Ambient Methane Enhancements in Los Angeles, California, To Evaluate Emission Inventory Estimates. Environmental Science &amp; Technology, 2019.&lt;/p&gt;&lt;p&gt;&lt;sup&gt;4&lt;/sup&gt; Istituto Superiore per la Protezione e la Ricerca Ambientale. Disaggregazione dell&amp;#8217;inventario nazionale. &lt;strong&gt;http://emissioni.sina.isprambiente.it/serie-storiche-emissioni/&lt;/strong&gt;, 2021.&lt;/p&gt;&lt;p&gt;&lt;sup&gt;5&lt;/sup&gt; ICOS RI. ICOS Atmosphere Release 2021-1 of Level 2 Greenhouse Gas Mole Fractions of CO2, CH4, N2O, CO, meteorology and 14CO2 (1.0). &lt;strong&gt;https://doi.org/10.18160/WJY7-5D06&lt;/strong&gt;, 2021&lt;/p&gt;