Dear colleagues,

you have clearly clarified several aspects which the reviewers have rightly questioned. However, having carefully read the paper, I am not convinced that you have fully taken into account some of the quite critical remarks eg of the third reviewer. You have attempted to pursue a challenging task, retrieve the emissions of EC&OC over almost the whole of North America from one time series at one station. This seems to me overambitious. It is also not totally clear why two models are used, where exactly the supposedly large increases in "other" emissions are coming from, how the imperfect representation of wet/dry deposition and of SOA formation impact the results, where exactly on the map the missing emissions could be found, and how model imperfections drive the inversion result.

I have thus decided to reject the paper in its current form. It would require more than a major revision to be considered for ACP.

kind regards
Michael Schulz

Dear Ben
While one of the referees is now satisfied with your paper and the responses you provided, the second referee is still quite critical and does not recommend acceptance. My view is that there is value in your paper, but you have rather skirted around the issue of how OC is modelled and how to deal with the lack of deposition in Flexpart. These are the two remaining issues that need to be addressed. Clearly you are not going to include chemistry or deposition at this stage. However, to ensure the results of your paper are not misleading, I would like to see quantitative statements about the effect of these model assumptions (omissions) on the results and conclusions. It's not sufficient to just say that this is a complex topic and lots of different studies should be published. In some environments the production of OC during transport is the _main_ source, not just a small contribution. WOrk by others like Alan Robinson has shown that OC increases downwind of cities in the US because VOCs are slowly converted to SOA. Also, some primary OC evaporates and is then recondensed after gas phase oxidation. These processes are now quite well established, and should not be ignored in your analysis.

In the conclusions I suggest a significant new paragraph in which you quantitatively estimate the effect of your assumptions on the results. This could start with a clear statement like "We make two important assumptions in our modelling: ..." You should discuss the evidence for OA production in previous studies and use it to quantitatively explain the consequences for your work. Please also ensure that you make appropriate statements in other parts of the paper.

I will review your changes and may decide to send the paper out for further review.