This paper uses a set of synthetic black carbon-containing aerosol mixtures made from materials with known composition to assess the ability of four different techniues used to isolate the black carbon component for carbon isotope determination (13C and 14C). The advantage of this approach is that the 'true' results are known, and thereofre the reliability of th methods used can be assessed. This is an innovative approach to the long-standing issues around method specific analytical biases, and therefore I think represents a signficant step forward in achiveing more reliable source apportionment for aerosol samples. This is important because of the significant role that black carbon in aerosols plays in anthropogenic warming. The study fingd that the hypy technique out-performs the other techniques in its ability to produce reliable results across a range of BC compositions. 

I think the paper is well and clearly written and the experimental and analytical components sound. I have no substantive issues with the interpretaion or conclusions of the study, but have made a number of grammatical suggestions, with some requests for clarification on the annotated pdf attached.

This short paper by Zhang et al is a nice attempt to explore some of the difficulties of quantifying and especially isotopically characterisation the “black carbon” fraction of a number of reference materials.  Full disclosure – as one of the people responsible for developing the HyPy methodology I was delighted to see it applied here, and that it performed quite well against some of the other techniques.

That said my main comment on the paper is that the authors need to get to grips with te terminology used.  They appear to use BC and EC interchangeably throughout and also OC and TC.  I’m not sure TC is actually defined anywhere either.  In both cases it would probably be better if you were consistent with terminology used throughout so that the reader is not confused.  Generally I think I know what you intend, but it is not clear.  As an example in line 210 you use both EC continuum and BC continuum in the same sentence.  Likewise you talk about “separating” EC from OC, but they are part of the same thing so “isolating” may be better.

More minor comments.  Don’t use initials in citations in the text (e.g. line 76, 80, 97 and others).  Do use dates in the list (unless not to is a weird requirement of the journal).

Line 197 – sentence beginning “Compared with” currently does not make sense.

Line 202 – “Black carbon isotopes” specifically or black carbon content?

Line 282 – “coking” should this read “charring”?

NMR and SEM are brought into the discussion but not previously introduced into the methods section.