As noted by the other reviewers of this manuscript, the paper presents new results on the formation of nitrate aerosol by N2O5 uptake at modest altitudes above surface level during winter in Beijing. The result is important since ozone titration by NOx at surface level makes the production rate at the surface zero. The paper quantifies the production rate in an altitude range up to 250 m above the surface and is thus a valuable contribution to the literature.
The authors quantify uncertainties in their analysis due to NO3 reactivity and ClNO2 production, both of which are unmeasured. They do not quantify the uncertainty due to partitioning between gas phase HNO3 and particulate nitrate. This aspect should be addressed, even if it is simply to state by way of assumption that all nitrate formed from N2O5 uptake goes to the particle phase rather than the gas phase. If the authors have other information to indicate that partitioning (other literature) they should state this explicitly.
Otherwise the manuscript should be published subject to the following minor comments and grammatical corrections.
Line 26: remove the word “easily”
Line 27: The result for gamma(N2O5) is specific to the very large aerosol surface area present in Beijing during these events. The sentence in the abstract should note this so as not to imply that a gamma value of 2x10-3 is generally the point at which other regions would become insensitive to this parameter.
Line 28: Suggest replacing “could be” with “is”. At the author’s discretion.
Line 43: replace “severely limited” with “very low in concentration”
Line 162 and equation 5: The idea behind the definition of s(t) is fairly clear, but the form of equation 5 is not. Further explanation of the form of the equation is required.
Figure 2: The scale for O3 in the upper panel goes to 100 ppbv, while the O3 itself only goes to 25 ppbv. The scale should show the actual variability in O3.
Line 235: The choice of kNO3 is arbitrary and is intended to simply represent a high value. The word arbitrary should appear in the sentence, i.e., “… kNO3 was set to an arbitrary and relatively high value of …”
Line 268: Do the authors mean to refer to December 19 rather than December 18?
Line 277: Is NO greatly diminished, or zero? Nonzero O3 at night implies zero NO if the mixing ratio of O3 is sustained for any length of time.
Line 303: Omit the word “about”
Line 306: “rapid” in place of “quick”
Line 314: What relationship between Ox and pNO3- has been used to calculate the Ox equivalence in Figure 6? Have the authors assumed a 1:1 relationship, or have they used the Ox equivalent in pNO3, which is larger than 1? See: Brown et al., Nocturnal odd-oxygen budget and its implications for ozone loss in the lower troposphere. Geophys. Res. Lett., 2006. 33: p. L08801.
Line 332: Can the authors compare the 28 micrograms m-3 figure to the day over day change in nitrate mass during haze events in Beijing? In other words, what is the daily growth in nitrate mass during either this event or during typical events, and how much is explained by this 28 microgram m-3 per night rate?
Line 334: To what feature are the authors referring in stating a morning peak of 60 micrograms m-3 on Dec 20? This feature is not apparent in Figure 1.
Line 346: Correct English grammar. Use a period and new sentence rather than a comma. The second part of the sentence should read: “Low N2O5 uptake coefficients correspond to several types of aerosols, such as …”
Line 352: Logic of sentence is incorrect. The ClNO2 yield is not the variable that maximizes the conversion capacity of N2O5, as the sentence implies. Rephrase as: “The conversion capacity of N2O5 uptake to pNO3 is maximized for a given, fixed value of the ClNO2 yield”
Line 363: It is not clear what is intended by the phrase “valid NOx loss.” The authors should clarify or search for other wording.
Line 364: “the N2O5 uptake coefficient” rather than “N2O5 uptake”
Line 369: remove the word “was”
Line 373: “become insensitive to gamma(N2O5).” Then start a new sentence “This region is defined as …”
Line 387: Sentence needs improved English grammar. The meaning of “during the heating period” is not clear. Does this refer simply to colder weather during the winter season?
Line 394: This result of 2.5 ppbv refers to a model, not a measured value. This should be made clear.
Line 397: “As the error of pNO3 formation simulation was subject to” should be replace by “Since the modeled pNO3 formation is sensitive to”
Line 399: The reference is to Figure 8, not Figure 7
Line 415: replace “evidenced” with “found evidence for”
Line 718, figure caption 7: “via N2O5 uptake” rather than “on N2O5 uptake”, “NO2 and O3 were set to”, “Sa was set”, “reaction time was set “