This manuscript has already been reviewed by two other reviewers, who have summarized the main points. The manuscript should be published in ACP following attention to the following minor comments and English grammar corrections.
Line 43: “In NOx rich plumes”
Line 68: “of N2O5”
Line 76: “depends on” rather than “is subject to”
Line 81: “photolyzes” rather than “photolysis”
Line 89: “The N2O5 concentration”
Lines 102-13: “report” rather than “reported”, and “determine” rather than “determined”
Line 105: “ON” rather than “the ONs”
Line 106: “nighttime NOx removal via NO3 and N2O5 chemistry”
Line 115: “there is no major industry in the surroundings”
Line 141: “thermograms” is one word
Line 175: “The correction factor (0.6) is the average”
Line 176: Replace comma with period and new sentence. “during nighttime. The standard deviation”
Line 177: Replace comma with period and new sentence. “0.27 (1). If this uncertainty is extended to the nighttime correction factor, the resulting uncertainty of the nighttime correction is 45%.”
Line 178: Replace “about 50%” with a more precise statement of uncertainty.
Line 248: “for the BAM period while they reached”
Line 251: “affects” rather than “affected”
Line 252: “leads to” rather than “led to”, and “promotes” rather than “promoted”
Line 261: “the real time Cl-“
Line 263, equation 3: The equation does not seem to be actually required. This is a mass balance argument mainly, that there is enough chloride to produce the observed ClNO2. Thus, the lower limit to the needed amount of chloride is equivalent to the amount of ClNO2 observed. This simpler statement should either replace the equation or appear alongside it.
Line 265: “ranged” rather than “was range”
Line 266: There is no detail given to support how the ISORROPIA model was used. What were the inputs? Is this based on gas phase acids and bases, or on salts such as NaCl? Can this model be run in the absence of gas phase HNO3 measurements, which were not available? Furthermore, the calculated HCl mixing ratios in figure S3 are unreasonably large. Rather than showing this calculated HCl, which would be by far the largest ever reported, and is shown without error limits, the authors should make a simpler statement that states what the required ratio of HCl to particulate chloride would need to be to support the observed ClNO2. Then the only statement required is that this ratio is consistent with known aerosol thermodynamics in this region or others. Figure S3, as a measure of gas phase HCl, should not be shown.
If the authors replace the argument with a simple mass balance one, they would also not need to use equation 3 or reference the gamma x f method prior to its definition in section 4.1
Line 293: eliminate the word “were”
Line 229-302: It is also possible that N2O5 is lost to dry deposition in near surface measurements, which would drive up the ClNO2:N2O5 ratio. For these near-ground measurements, the authors should also include a sentence allowing for this possibility.
Line 307: It is not clear what is meant by “kept increasing”. Do the authors mean “increased continuously”, and if so, for how long does such an increase need to be sustained to be included in the analysis?
Line 308: Above, an increase in ClNO2 after sunrise is inferred to come from a transport process, mixing from aloft. Here, there should be a caveat included to state that ClNO2 increases can also arise from transport, but are assumed in this case to be due to chemistry alone. Is there supporting evidence for this assumption for the cases examined, such as nearly constant levels of other stable species, such as CO or O3?
Line 323-329: Most of this discussion is speculative. Is there evidence in the data from this campaign that supports the arguments for variation in gamma x f ?
Line 330: Uptake of N2O5 does not necessarily lead to particulate nitrate as the sentence implies. This soluble nitrate formed heterogeneously rapidly re-equilibrates with gas phase HNO3.
Line 336: “present” rather than “presented”
Line 337: “does not meet”. Aside from correcting the English grammar in this sentence, it does not appear reasonable to throw out data on the basis of a lack of correlation. The authors should propose some reason why there is a correlation on some nights and not others, or else state that the choice of analysis during periods of high correlation is simply arbitrary and may lead to bias since the data are selected for the correlation.
Line 338: What is “an equivalent or increase in ammonium”. Does this mean that ammonium increased at a 1:1 or greater molar ratio to nitrate?
Line 340: “ammonia rich” rather than “rich-ammonia”
Line 341: state the levels of ammonia or other data that define the term ammonia rich from the given reference.
Line 342-345: The source of the NO3- or HNO3 from N2O5 uptake or OH + NO2 is not relevant. Both reactions are sources of total nitrate, which subsequently re-partitions between the gas and aerosol phase. The wording here implies that OH + NO2 leads to HNO3, whereas it is simply a source of total nitrate regardless of phase. There is also no justification given for the time scale of this equilibration, nor is one apparently needed. Rather than stating hundreds of seconds, the authors should simply state “rapid on the time scale of total nitrate chemical production”.
Line 346: Gas particle HNO3 repartitioning means that the gas phase HNO3 is neglected in the analysis since only particulate nitrate is available. This should be reworded as simply “lack of gas phase HNO3 for total nitrate measurement”. The OH + NO2 reaction is then not relevant, especially at night.
Line 349: “increasing pNO3-“ rather than “increasing of pNO3-“
Line 354: eliminate comma, start new sentence.
Line 359: Errors cannot be “about approximately 55%” Were they 55%? Then no need for either “about” or “approximately”.
Line 359: replace “field measurements of” with “errors in”
Line 374: “in North America” should follow the word “Pennsylvania”
Line 390: The loss term referred to here is the same as the production rate of NO3 in Figure 4. The authors should use the same terminology throughout the paper.
Line 419-420: Eliminate the phrase “so that it is really important to have accurate NO2 measurement instrument involved in the future campaigns.” This phrase is colloquial.
Lines 425-428: English grammar. Does this mean “The steady state loss rate constant in the case of May 30 was was approximately ten times larger than the overall loss rate constant, and this difference was outside of the range of uncertainty. The reason for the larger difference on this night is not understood from the available measurements.”
Line 435: “67% of the total”
Line 438-439: The two numbers are quoted to 3 significant figures without uncertainty. Are they really this precise? If they are averages, then standard deviations should be given, and the level of quoted precision should be consistent with the standard deviation.
Line 451-453: It cannot be stated that organic nitrates are definitely dominated by NO3 + BVOC without also considering the photochemical RO2 + NO reaction. Does the statement refer to nighttime only? It is not clear what is being said here.
Line 454: The first term in equation 9 is not pseudo-first order since the oxidant is not in large excess over the VOC in the case of NO3. Drop the word “pseudo”, since first-order is still appropriate.
Line 454: Here and below, use “VOC” rather than “VOCs” to describe reactivity.
Line 464: “accounted for 50% each” rather than “occupies half and half”
Line 467: How different are the two rate constants? Are they in fact a factor of 1.5 different? This difference in rate constant, not the assumption of the amount of each monoterpene, should define the uncertainty.
Line 473: “Even with the NO3 concentration in the lower range, NO3 was still”
Line 480: a-pinene and b-pinene have very different ON yields, which should be accounted for here. See Hallquist et al., ES&T 1999.
Line 485: “Estimates” rather than “estimations”
Line 508: High NOx does not favor NO3 in the thermal equilibrium.