Please find my comments in the attachment. 

The authors applied PMF to elucidate how particle number concentrations evolve during spring 2020 lockdown period relative to the reference years of 2014-2019. While the topic is overall interesting, I feel the novelty is very weak in the current version, and a substantial revision is needed to satisfy the standard of the journal.

1. This study heavily relies on PMF method. Is there any uncertainty? The authors seem to consider the source apportionment based on PMF as a fact, while I think there might be large uncertainties associated with this method.
2. The overall comparison between lockdown and reference period seems to be very general and not surprising. It looks like many statements has been known already.
3. In many sections, there are only a couple of sentences, briefly describing the difference between lockdown and reference period. These short paragraphs are not acceptable in my opinion, and they do not raise any scientific finding in depth.
4. There are no mechanisms at all besides of the general descriptions. I feel the authors should at least try to add some mechanism discussion, not just describe things based on PMF, and cite a few papers without any deeper understanding of the changes in PNCs.
5. Figure quality is very poor. Most figures are not easy to read and should be redrawn to be legible.

Minor comments

1. Line 417

The insolation was markedly higher in London during the lockdown period than in previous years, but the other cities show only a small increase.

Any reasons why London has much higher downward solar radiation over this period? Why not in other cities?

1. O3–associated SA and SIA

What is ozone associated SIA? It is very vague, and the readers would not know what it is. Is it secondary organic or inorganic aerosols? Is SIA also O3 associated?