Towards near-real-time air pollutant and greenhouse gas emissions: lessons learned from multiple estimates during the COVID-19 pandemic
Authors: Guevara Marc , Petetin Hervé , Jorba Oriol , Denier van der Gon Hugo , Kuenen Jeroen , Super Ingrid , Granier Claire , Doumbia Thierno , Ciais Philippe , Liu Zhu , Lamboll Robin D. , Schindlbacher Sabine , Matthews Bradley , Pérez García-Pando Carlos 
Published: July 2023

Abstract. The 2020 COVID-19 crisis caused an unprecedented drop in
anthropogenic emissions of air pollutants and greenhouse gases. Given that
emissions estimates from official national inventories for the year 2020
were not reported until 2 years later, new and non-traditional datasets to
estimate near-real-time emissions became particularly relevant and widely
used in international monitoring and modelling activities during the
pandemic. This study investigates the impact of the COVID-19 pandemic on
2020 European (the 27 EU member states and the UK) emissions by comparing a
selection of such near-real-time emission estimates, with the official
inventories that were subsequently reported in 2022 under the Convention on
Long-Range Transboundary Air Pollution (CLRTAP) and the United Nations
Framework Convention on Climate Change (UNFCCC). Results indicate that
annual changes in total 2020 emissions reported by official and near-real-time estimates are fairly in line for most of the chemical species, with
NOx and fossil fuel CO2 being reported as the ones that
experienced the largest reduction in Europe in all cases. However, large
discrepancies arise between the official and non-official datasets when
comparing annual results at the sector and country level, indicating that
caution should be exercised when estimating changes in emissions using
specific near-real-time activity datasets, such as time mobility data
derived from smartphones. The main examples of these differences are observed
for the manufacturing industry NOx (relative changes ranging between
−21.4 % and −5.4 %) and road transport CO2 (relative changes
ranging between −29.3 % and −5.6 %) total European emissions.
Additionally, significant discrepancies are observed between the quarterly
and monthly distribution of emissions drops reported by the various
near-real-time inventories, with differences of up to a factor of 1.5 for total
NOx during April 2020, when restrictions were at their maximum. For
residential combustion, shipping and the public energy industry, results
indicate that changes in emissions that occurred between 2019 and 2020 were
mainly dominated by non-COVID-19 factors, including meteorology, the
implementation of the Global Sulphur Cap and the shutdown of coal-fired
power plants as part of national decarbonization efforts, respectively. The
potential increase in NMVOC emissions from the intensive use of personal
protective equipment such as hand sanitizer gels is considered in a
heterogeneous way across countries in officially reported inventories,
indicating the need for some countries to base their calculations on more
advanced methods. The findings of this study can be used to better
understand the uncertainties in near-real-time emissions and how such
emissions could be used in the future to provide timely updates to emission
datasets that are critical for modelling and monitoring applications.