Krüger et al. present observations of the effect of the COVID-19 lockdowns on black carbon emissions using a combination of airborne in situ observations and models. The impact of lockdowns on emissions from transport is by now well established, however the majority of studies to date have focused on NO2 as this is a regulated pollutant and high resolution observations are available in new satellite data products. This work instead uses a state-of-the-art airborne facility (DLR HALO) to report data on black carbon, which has established impacts both as a airborne pollutant damaging to human health, and as a highly potent short-lived climate forcing agent. Data from during the European lockdown period is compared with the prior EMeRGe campaign (https://acp.copernicus.org/articles/special_issue1074.html). The authors found a decrease in BC concentrations of 48% over Western and Southern Europe, of which 7% was likely due to meteorological differences during the two campaigns and 3-9% was due to a long-term downward trend, leading them to conclude that the lockdowns were overall responsible for a reduction in ambient concentrations of 32-38%. By seizing this unique opportunity, this study offers a new and unparalleled insight into the current nature of black carbon emissions from Europe. It also demonstrates how airborne in situ measurements and models can be used in combination to study the emission and transport of pollutants in the troposphere.

Tropical convective clouds distribute water vapour across the tropical tropopause layer (TTL), which affects the radiative budget in both the troposphere below and the stratosphere above. Water isotopes can be used as tracers to conclude on the history of water vapour in these altitudes, i.e. whether it went through phase change processes and was part of ice or liquid clouds before. de Vries and coworkers used an innovative modelling scheme capable of simulating convection together with the phase-change processes producing the isotopic signatures to simulate tropical ice clouds and compare their simulations to respective observations. By this, they were able to disentangle processes of convective updraft versus cirrus cloud formation in tropical cloud systems and demonstrate how the isotopic fractionation of water vapour does help to distinguish respective processes. Over all, this paper is innovative and a considerable step forward in studies of convection and transport of water vapour into the upper tropospher/lower startosphere (UTLS).