dear authors
Please consider the points raised by the reviewer. Please also provide a more explicit statement of your reasons why you consider this manuscript as an independent paper of its own. I suggest to express this in title, abstract and introduction of both manuscripts. If the manuscripts should be independent of each other, I would suggest to not join them as companion papers.

dear authors

I acknowledge that the reaction studied in this manuscript, NO2 + SO2, in absence of O2, is important enough to merit publication as a separate manuscript. The formation of sulfate from S(IV) in aerosol particles especially under heavy pollution conditions has been suggested to operate with NO2 being the oxidant. Even though this study has not covered the atmospherically relevant NO2 partial pressure range and also has not looked at the effect of pH or especially high pH values for which the reaction NO2 + SO2 has been suggested to contribute most to the formation of sulfate, it provides valuable insight into the mechanism of this reaction at low pH (relevant for most atmospheric conditions). It is important to document the low reaction rates resulting from this reaction.

To make sure that these aspects are well documented and explained I am asking for a few minor additional revisions in the relevant sections of the manuscript:

abstract, line 17: please also mention that it is not important also in comparison to the other aqueous phase pathways, such as reaction with O2, w/o transition metals, or H2O2.

line 38: replace 'liquid water' by aqueous solution. Strictly speaking, 'liquid water' refers to pure water only. please check the usage of the term 'liquid water' also in the remainder of the manuscript.

line 63: ionic strength (not ion strength)

line 67 and 78: mention the contribution of this reaction to sulfate formation in the Cheng /Wang / Xue studies in comparison to that involving O2 (if they looked at it). For the Cheng et al. reference: explicitly mention that they considered this reaction (not with O2) being the most important pathway.

line 114: maybe explain why the majority of experiments were not performed with Ca(NO3)2 directly. I don't think that reaction of CaCO3 with NO2 is the main source of Ca(NO3)2 in the atmosphere. I guess HNO3 is doing this mostly.

line 160: of H2O in aqueous solution.

line 364: add a caveat here that this study did not look at the pH dependence, and especially not at high pH, for which recent studies have claimed this reaction to be important. Also please mention that more likely other oxidation pathways, such as with H2O2, O3 and O2 as oxidants, and w/o transition metals, must be more important.

lines around 370: I think the statements about the mixing state of CaSO4 with nitrate are ok, but this is independent of the oxidation mechanism; but the source of nitrate is more likely HNO3 and N2O5, and rather not NO2 under atmospheric conditions.