There is a current debate on the driving mechanism(s) of NPF in urban environments, more specifically, sulfuric acid-amine clustering or oxidized organics originating from traffic emissions. The lockdown during COVID-19 pandemic provided a unique chance to definitively resolve this issue. Very simply put, if the strength of NPF reduced significantly during the lockdown, the dominant role of traffic emissions can be confirmed. A recent paper by Shen et al., (2021), that is also cited in this study, showed that NPF was stronger during the lockdown, which may suggest the less important role of traffic emissions in NPF in Beijing. In this study, the authors show consistent observational results with what have been reported by Shen et al., (2021), and further extended the mechanistic understanding of such NPF enhancement by performing detailed molecule-level analyses on NPF precursors, i.e., sulfuric acid and oxygenated organics. The authors found that the enhanced NPF were an overall result of two facts: first, the sulfuric acid-amine clustering remained as the driving mechanism and led to a similar J1.5; second, the growth and survival of very small particles were enhanced by the elevated abundance of condensable oxidized organics.

Overall, I think this paper presents a significant advance in the understanding of NPF in urban environments, and thus I recommend accepting it for publication with a few minor comments/questions that I hope the authors can answer:

1. Line 216 -218 “In addition, the concentration of OOMs increased by about 50% during the lockdown. This is because the concentration of volatile organic compounds (VOCs) only declined slightly in the lockdown period (Shen et al., 2021b), but the photochemistry was much more enhanced.”

      The overall pollution level was more serve during lockdown period. So will some of the OOMs be able to transport from other region(s) to the measurement site along with PM_2.5, and thus leading to the enhancement of OOM concentration?

        2.   Line 285 “… range of our observations (Fig. S6).” Is this Fig. S6 should be Fig. S5?

        3.   Line 291 “periods (Fig. S7). This is less than …” Is this Fig. S7 should be Fig. S6?

       4.    Line 334-336 “When the NO concentration declined from the pre-lockdown period to the lockdown period, the ratio of C_6-9H_7,9,11,13O₆N concentration to C_6-9H_7,9,11,13O₅ concentration decreased as well.”Will the photolysis of nitrogen-containing aromatic OOMs influence the ratio of C_6-9H_7,9,11,13O₆N concentration to C_6-9H_7,9,11,13O₅ Concentration? And what will happen if color Fig. 5 (A) with UVB?

       5.    Line 340 – 342 “They have a double bond equivalent (DBE) of 1, suggesting that they originate from aliphatic rather than aromatic precursors”

It seems that the authors have some criteria to infer the VOC precursor of OOMs. Is this based on some published results? I would like the authors to reply with more details.

This paper presents the effect of the COVID-19 lockdown on atmospheric new particle formation. Indeed, the COVID-19 lockdown provided us a unique opportunity to investigate the effect of reduced anthropogenic emissions (probably similar to pre-industrial conditions) on a variety of atmospheric processes. Shen et al (2021) recently reported enhanced nanoparticle formation and growth during the COVID-19 lockdown in urban Beijing, but without much of the process-level explanation of nanoparticle formation and the role of key vapors. Here, the authors provide a more detailed analysis of nano particles and the role of sulfuric acid and oxygenated organic molecules in particle formation and growth. Authors report that the formation rate of 1.5 nm clusters was unchanged by drastically reduced traffic emissions. However, the cluster's survival probability was increased due to the higher formation of sulfuric acid, oxygenated organic molecules, and other vapors, indicating the enhanced atmospheric oxidative capacity.

Authors conclude that traffic emissions play a limited role in atmospheric NPF as opposed to the previous reports showing traffic as a high source of ultrafine particles such as Rönkkö et al., 2017 (https://doi.org/10.1073/pnas.1700830114), Guo et al., 2020 (https://doi.org/10.1073/pnas.1916366117). While Okuljar et al., 2021 (https://doi.org/10.5194/acp-21-9931-2021) also showed that traffic contribution to sub-3nm particles is lower during NPF events, Gani et al., 2021 (10.1039/D1EA00058F) showed NPF contributions to ultrafine particles in locations with high concentrations of precursors (e.g. traffic) are critical. Another recent study from an Indian urban location Kanawade et al., 2022 (https://doi.org/10.1029/2021JD035392), however, showed that NPF and growth events were suppressed under the reduced anthropogenic emissions during the lockdown. Kanawade et al. also reported an unaltered particle formation rate of 1.5 nm (and number concentrations of sub-3nm particles), but nanoparticle growth was limited by likely lower condensable vapors. This probably hints the role of micro-meteorology is also imperative. I suggest authors discussing all the above papers.

Overall Recommendation: The paper presents detailed analyses using new techniques that can characterize nanoparticles and provide new insights into the response of NPF to drastic changes in the atmospheric chemical cocktail. The manuscript should be published after the authors' elaborate discussion as indicated above and the following minor issues are addressed.

The pre-lockdown period falls during the peak winter season, followed by the lockdown during early spring, the temperature is expected to increase as the season progresses. The role of different micro-meteorological conditions should be highlighted between the time periods considered in this study. Or is it the critical factor for more occurrence of NPF and growth during lockdown with elevated temperature (more active photochemistry) rather than reduced anthropogenic emissions as background concentrations are on the higher side in urban areas.

Lines 85-90: there are laboratory studies showing clustering between sulfuric acid and organic acids e.g. Schobesberger et al. (https://doi.org/10.1073/pnas.130697311) or multi-component nucleation of sulfuric acid, ammonia, and organics (10.1126/sciadv.aau5363),  and traffic is not the only source of organic acids to the atmosphere. For better readability, remove “on one hand” and “on other hand".

Line 185: Fig. S4 cited for particles in the size range of 10-30 nm, but Fig. S4 in the supplementary shows diel patterns of temperature and UVB

Lines 202-203: Correct as Fig. S4

Supplementary figures are incorrectly cited in the main text at most places. Please check carefully.

Line 292: you mean to say “i.e., 1.3 pptv”?