Minor comments:
* Stating that BC represents only 10% of PM at night does not rule out primary emissions; while this would be considered a low fraction of diesel emissions, it could be considered typical for biomass burning from space heating. While the authors later state that domestic sources are unimportant, they should really state that at this point as well.
* More detail must be provided on the statistical tests. The sigmas reported on figure 6 would appear to be standard errors, not standard deviations, and the formulation (or a reference) for the t-test performed should be given, along with the numerical results.

Technical comments:
* The terms 'proximity measure' and 'proxy' are not synonymous (they are not even linguistically related, being derived from the Latin 'proximus' and 'procurare' respectively). In this instance, the authors should be using the word 'proxy'. 'Proximity measure' is a term used to describe the similarity of two vector quantities (often used in machine learning) and is not appropriate here.
* In section 4.2, it seems odd that the motivation for the use of a method should be stated late in the section. I would try to put this sentence closer to the beginning.
* The use of the word 'prototype' to describe Beijing on page 15 is a little odd.
* Generally, the standard of English still needs improvment, but I am assuming this will be rectified during copy-editing.

The caption of figure 6 still needs fixing. Currently, it reports sigma as being standard deviation but then goes on to define it as sigma divided by root(n-1), which firstly doesn't make sense (defining a quantity in terms of itself), but to me implies that it is a standard error (assuming a t distribution), not a standard deviation. This needs fixing.

Also, the inclusion of the individual t-tests in the supplement is probably not necessary. The purpose of my comment was to say that the formulation should be referenced (which has been done). This was in addition to the results presented in the previous version; I don't think it's necessary to go into such detail.