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Abstract. During the COVID-19 lockdown, the dramatic reduction of anthropogenicemissions provided a unique opportunity to investigate the effects ofreduced anthropogenic activity and primary emissions on atmospheric chemicalprocesses and the consequent formation of secondary pollutants. Here, weutilize comprehensive observations to examine the response of atmosphericnew particle formation (NPF) to the changes in the atmospheric chemicalcocktail. We find that the main clustering process was unaffected by thedrastically reduced traffic emissions, and the formation rate of 1.5 nmparticles remained unaltered. However, particle survival probability wasenhanced due to an increased particle growth rate (GR) during the lockdownperiod, explaining the enhanced NPF activity in earlier studies. For GR at1.5–3 nm, sulfuric acid (SA) was the main contributor at high temperatures,whilst there were unaccounted contributing vapors at low temperatures. ForGR at 3–7 and 7–15 nm, oxygenated organic molecules (OOMs) played amajor role. Surprisingly, OOM composition and volatility were insensitive tothe large change of atmospheric NOx concentration; instead theassociated high particle growth rates and high OOM concentration during thelockdown period were mostly caused by the enhanced atmospheric oxidativecapacity. Overall, our findings suggest a limited role of traffic emissionsin NPF.