Review of “Snowpack Nitrate Photolysis drives the summertime atmospheric nitrous acid (HONO) budget in coastal Antarctica” by Bond et al.

This paper reports the results of a field measurement study assessing the sources and contribution of HONO to oxidation capacity of the Antarctic boundary layer at a coastal (ie, sea level altitude) location.  This remains a persistent challenge and key issue in understanding within- and above-snowpack atmospheric chemical processing.  The paper combines field measurement results from a challenging environment with derivation of vertical flux using the gradient flux approach, and simple calculations to assess the contribution of HONO to OH formation.

The results are similar to those reported from other comparable locations, and advance quantitative understanding of the importance of HONO at this location; they go some way to unpicking conflicting results (from HONO measurements likely over-estimated previously) at this location.

The measurements appear to have been carefully performed with appropriate corrections and blanks etc, and are described at an appropriate level of detail.  The analysis presented is carefully considered.  Use of species measurements from previous campaigns (i.e. different years) is made, but this is unavoidable and appropriately noted / caveated.  The paper is well written and clearly presented.

I recommend the paper is accepted for publication, subject to the authors considering the corrections / suggestions outlined below.

A style point: personally I find the terminology “amount fraction” somewhat jarring – “mixing ratio” preferable.  We need to make our manuscripts accessible as well as precise !  But this is up to the Editor and journal...

R4 is termolecular and should include the third body M – line 31 and subsequently in the manuscript

L51 are reactions “accelerated” by sunlight – consider phrasing

L81 / L232 it would be possible to estimate the magnitude of potential PNA interference – using the previous data for HO2 and NO2 to estimate [HO2NO2]ss and hence the interference contribution.  This might usefully be added to the discussion

L113 the key assumption of LOPAP is that HONO is effectively entirely removed in coil 1, but that the abundance of interferents is effectively unchanged – so that subtraction of coil 2 signal from coil 1 signal results in just HONO

L139 How did the 15 min height change compare with the (liquid) residence time of the LOPAP – were the data used for flux calculations adjusted / truncated for the delay from instrument residence time between gas intake and absorption signal response

L198 what albedo assumed for the TUV calculations

L291 worth commenting on the snow surface age vs porosity (ie fresh snow or subject to many weeks freezing or…)

Fig 7 – captoin - not sure that Pss(HONO) makes any assumption about the nature of the source

Table 3 – what is y ?

L371 the comparison of HONO and NOx lifetimes is useful – previous Halley work has shown the NOx lifetime is significantly reduced form halogen nitrate photolysis (Bauguitte et al 2011), this will be quite different from that estimated due to NO2 + OH alone – and may improve agreement with the observed HONO:NOx ratio ?

L385 – HOI and HOBr are not primary sources of OH – they reflect HOx cycling, as they form from HO2+XO, so in terms of OH sources they are really similar to HO2 + NO.  Suggest the table compares either primary OH sources (HONO, O1D) or all OH sources (including HO2 + NO etc) or (best) all HOx sources – HONO, O1D, HCHO, but excluding recycling such as HO2+NO, HO2+XO.