Investigation of extreme CO2 concentration events at an Alpine station with Lagrangian Particle Dispersion Models

Emissions of greenhouse gases, such as CO 2 and CH 4 , constitute one of the primary causes of global climate warming. Thus, monitoring their concentrations in the atmosphere has gained increasing importance over the past few decades. One of the observatories at high altitude providing continuous atmospheric CO 2 concentration measurements is the Plateau Rosa Station in the Western Italian Alps, a background monitoring station. This work aims to investigate the nature and origin of events of extremely high CO 2 concentration recorded at this remote site, with reference to the atmospheric circulation transporting pollutant masses towards it, from the synoptic scale and mesoscale to the local scale. Two methods are applied to analyse the available 31-year concentration dataset, first to select the background data out of the series, then to identify the events characterised by extreme CO 2 values. About 80% of the observations are found to be representative of background concentrations, twelve episodes of extreme CO 2 values are detected, and an uncommonly low value is also considered. Then, two different Lagrangian Particle Dispersion Models, MILORD and FLEXPART-WRF, are used to investigate the areas of provenience of the tracers during such extreme events. The MILORD model is employed for long-range simulations for all the extreme episodes, while the FLEXPART-WRF model enables investigating the mesoscale and regional transport. Results show that the air masses during the extremes originate over the main European industrialised plains, and that all episodes present a common synoptic cyclonic circulation, which is not detectable in the case of the minimum. The adopted approaches are qualified to trace the source of greenhouse gas concentrations measured at remote sites, and are exportable to any other background monitoring station.