Besides the many tragedies caused by the global Covid-19 pandemic since 2020, the various lockdowns have also introduced a unique atmospheric experiment, which was unthinkable before: due to the restrictions and many people staying home, traffic originated air pollution was essentially turned off. Researchers from the Institute of Atmospheric and Earth System Research of the University of Helsinki have investigated how the Covid-19 lockdown related dramatic reductions in traffic emissions have influenced the air quality in Beijing, one of the cities, which experienced very strong lockdown measures in February 2020. The international group of researchers including many Chinese collaborators compared the observations at the Beijing University of Chemical Technology Aerosol and Haze Laboratory from the pre-lockdown and lockdown periods in 2020 with previous years. The atmospheric scientists focused on the drivers of new particle formation. During that process gas molecules cluster together and form new solid or liquid particles in the air. New particle formation can be a large source of aerosol particles in the urban atmosphere and contribute to air pollution. Surprisingly, the strong reductions in traffic emissions, especially NOx and directly emitted aerosol particles, did not result in a decrease of particles produced via new particle formation. In contrast, the researchers observed an even enhanced importance of these newly formed particles as they were growing faster to sizes where they contributed to the formation of strong pollution episodes, which were still occurring during the lockdown period. The recently published pre-print therefore shows that the strict lockdowns enabled unique atmospheric research, which could finally investigate the role of traffic in the formation of air pollution. It demonstrates that reductions in traffic emissions alone cannot improve air quality in megacities directly, but that the atmospheric chemical cocktail is much more complex and depends on the interplay of various pollution sources.