This manuscript has undergone thorough scrutiny by three reviewers at the discussion stage. Subsequently, the authors submitted a substantially revised version, which has been reviewed again by two reviewers.

Both reviewers now agree that the paper is publishable even though some methodological questions remain on the separation of particulate and gaseous nitrate by the filter technique used by the authors. While important, the best available scientific and technical knowledge on theses methods has been applied in the present study. However, further studies may be required to resolve remaining doubts. The authors consider any potential artefacts to be minimal.

The findings themselves are novel and provide a strong stimulus to further research on nitrate formation and loss processes in the atmosphere.

I am therefore happy to accept the manuscript for publication, subject to a number of minor corrections (outlined below), which I will oversee myself.

I would like to thank the reviewers for their highly valuable contributions and insights, and their support of the Atmospheric Chemistry and Physics refereeing process.

Jan Kaiser
Editor Atmospheric Chemistry and Physics

Minor corrections

1) Please adhere to the International System of Units (SI), which has not always been applied in the present version of the manuscript. Please nsult the SI brochure published by BIPM (http://www.bipm.org/en/publications/si-brochure/) and chapter 1 of the IUPAC Green Book (http://media.iupac.org/publications/books/gbook/IUPAC-GB3-2ndPrinting-Online-22apr2011.pdf)

Specifically, chemical symbols and units should not be mixed, i.e. it is not permissible to write "10 mg N m-3". To correct this, the chemical species must be identified by the quantity symbol (not the unit), e.g. "c(NO3--N) = 10 mg m-3", or, in the text, "10 mg m-3 of NO3--N", or "10 mg m-3 as N equivalents", or "a nitrate-N concentration of 10 mg m-3"

This also applies to the tables in the supplementary material.

2) Table 3, Figs. 2, 3, 4, 6 & Figs. SM-3 & 5: Axis labels should be δ/‰ or 10^3 δ (ln(1+δ)/‰ in case of Fig. 4). See BIPM brochure and IUPAC Green Book for correct examples.


1/13: Replace "ratios" with "delta values".
3/10: Replace "/filter" with "per filter".
6/4: The unit "ppb N-NO3/L" does not make sense. Also, please see comment 1 above and remove "N-NO3" here and from "0.03 N-NO3 umol/L".
6/5: Please convert 0.016 µg to µmol and remove "N".
6/25: Replace "ratios" with "values."
6/32: Delete extraneous "/1000" divisors.
7/20: Replace "ratios" with "delta values" or "deltas".
7/21 & 7/23 & 10/6 "Isotopic difference" is meaningless as a quantitative term on its own. Please rephrase so as that it is clear which delta values are meant. Also, it would be clearer to say explicitly whether particulate nitrate or HNO3 have higher or lower delta values.
8/13: Change to "not the highest delta values for w-NO3-".
8/22: Subscript "x" in "NOx". "x" should be in italics.
8/28: "NO3-"
9/4: Add space between 0 and ºC.
9/19 & R12 Table 5: "NO3" (neutral radical)
12/22: The correct unit symbol for "hour" is "h".
12/25: Please replace "ppb" and "ppt" with the SI units "nmol mol-1" and "pmol mol-1".
14/2: "The negative isotopic differences between p-NO3- and HNO3 ..." is unclear. Please rephrase so that it is clear which species has lower delta values.

Many thanks for the corrected version. There are still a few minor issues, which need to be rectified.

Axis labels must be δ/‰ or 10^3 δ (ln(1+δ)/‰ to be numerically correct, e.g. δ18O(VSMOW)/‰ or 10^3 δ18O(VSMOW). Please also see the examples in the BIPM brochure and IUPAC Green Book. The unit must not be included in the label in parentheses because it is not an argument of the quantity symbol.

Tables 2 to 3: Column headings need to be written as δ18O/‰ or 10^3 δ18O etc. to be numerically correct.

Table 6: Replace "isotopic results (‰)" with "isotope delta" (without ‰ symbol). Also, it is not clear what "r" stands for - the slope of a linear regression fit? If so, it must have units, e.g. ‰/% in the case of regressing δ18O vs. relative humidity, or ‰/ºC in case of regressing δ18O vs. temperature, "‰/(nmol mol-1)" in the case of δ18O vs. O3 mole fraction, etc.

State the quantities referred to by the symbols "PM", "SO2" and "O3", i.e. PM mass concentration, SO2 mole fraction and O3 mole fraction.

6/4: What do you mean by 2 ppb? This may relate to the mass fraction of nitrate (M = 62 g mol-1) in solution, but this is confusing, since this quantity is not used anywhere else in the paper. It should therefore best be deleted.

6/5: This value should be rounded appropriately, e.g. 1.1 µmol/L

8/4-6: Please convert to µmol/L for consistency with page 6.

30/5: The definition of D(17O) in Miller (2002) is mathematically wrong. As an example, a δ value of 1 ‰ is numerically identical to the number 0.001. It must not be divided by 1000 because this would alter it to a value of 0.000001.
Please change this equation to ln(1+δ17O) − 0.516 ln(1+δ18O).

Table SM-3: Please replace "ppb" with the SI unit "nmol mol-1"