The effects of the COVID-19 lockdowns on the composition of the troposphere as seen by In-service Aircraft for a Global Observing System (IAGOS) at Frankfurt

The European research infrastructure IAGOS (In-service Aircraft for a Global Observing System) equips commercial aircraft with a system for measuring atmospheric composition. A range of essential climate variables and air quality parameters are measured throughout the flight, from take-off to landing, giving high-resolution information in the vertical in the vicinity of international airports and in the upper troposphere-lower stratosphere during the cruise phase of the flight. Six airlines are currently involved in the programme, achieving a quasi-global coverage under normal circumstances. During the COVID-19 crisis, many airlines were forced to ground their fleets due to a fall in passenger numbers and imposed travel restrictions. Deutsche Lufthansa, a partner in IAGOS since 1994 was able to operate an IAGOS-equipped aircraft during the COVID-19 lockdown, providing regular measurements of ozone and carbon monoxide at Frankfurt Airport. The data form a snapshot of an unprecedented time in the 27-year time series. In May 2020, we see a 32 % increase in ozone near the surface with respect to a recent reference period, a magnitude similar to that of the 2003 heatwave. The anomaly in May is driven by an increase in ozone at nighttime which might be linked to the reduction in NO during the COVID-19 lockdowns. The anomaly diminishes with altitude becoming a slightly negative anomaly in the free troposphere. The ozone precursor carbon monoxide shows an 11 % reduction in MAM (March-April-May) near the surface. There is only a small reduction in CO in the free troposphere due to the impact of long-range transport on the CO from emissions in regions outside Europe. This is confirmed by data from the Infrared Atmospheric Sounding Interferometer (IASI) using retrievals performed by SOftware for a Fast Retrieval of IASI Data (SOFRID), which display a clear drop of CO at 800 hPa over Europe in March but otherwise show little change to the abundance of CO in the free troposphere.