EU H2020 Research and Innovation Action

Intermediate Review Meeting

13 July 2021

Virtual meeting

The ACACIA Project has reached the end of the first period of the project duration. Therefore, we had the Intermediate Review Meeting on 13th July 2021. An overview of the project was given to our EU-officer, and the progress and achievements were discussed within the project schedule and the overall objectives. This meeting is one of the regularly scheduled project meetings. Partipants were EU-representatives, members of the ACACIA Advisory Board and the ACACIA Steering Committee members.

Two papers published on Covid-19

Mertens et al. 2021 "COVID-19 induced lower-tropospheric ozone changes":
Decrease of ozone in the planetary boundary during the Covid-19 pandemic

Quaas et al. 2021 "Climate impact of aircraft-induced cirrus assessed from satellite observations before and during COVID-19":
Fewer cirrus clouds in regions of large air traffic reduction following COVID-19

The Covid-19 pandemic gave the unique opportunity to study the effect of emission reductions on various trace gases. For example, emissions of reactive species comprising nitrogen oxide (NOx), non-methane hydrocarbons and carbon monoxide (CO), lead to the formation of ozone (O3), a pollutant and a greenhouse gas. The recent study by Mertens et al. (2021) improved the understanding of key processes concerning the role of anthropogenic emissions in the atmosphere by means of numerical simulations of the global atmosphere. The idealized study shows a decrease of lower tropospheric ozone of 8% over Europe in May 2020 due to the (idealized) emission reductions. Interestingly, the results also show that the ozone reduction is comparably lower than the reduction of the total NOx emissions (around 20% compared to about 8%). This is a result of a compensating mechanism. The study further highlights that due to the rather long lifetime of ozone, the emissions in other parts of the world strongly influence European ozone levels. Therefore, reducing emissions only in Europe will most likely not lead to envisaged ozone decreases in Europe.

The study by Quaas et al. (2021) investigated the amount of thin, high-altitude clouds (cirrus) in regions with large air traffic reduction in spring 2020 compared to previous years. Johannes Quaas, lead author of the study, says “The COVID-19 pandemic provided an unprecedented opportunity to contrast cloudiness in air corridors under very different levels of air traffic. We found a 9% reduction in cirrus cover and a 2% reduction in cirrus opacity in those regions.” The study provides clear evidence that contrails formed by aircraft leads to additional cirrus and could quantify the radiative impact of contrail-induced cirrus from observations.

Citation:

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• Mertens, M., P. Jöckel, S. Matthes, M. Nuetzel, V. Grewe, and R. Sausen (2021): COVID-19 induced lower-tropospheric ozone changes. Environ. Res. Lett. 16(6), 064005,. https://doi.org/10.1088/1748-9326/abf191.
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• Quaas, J., E. Gryspeerdt, R. Vautard, and O. Boucher (2021), Climate impact of aircraft-induced cirrus assessed from satellite observations before and during COVID-19, Environ. Res. Lett. 16, 064051, https://doi.org/10.1088/1748-9326/abf686.
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