GENERAL REMARKS

The manuscript presents an exhaustive description of a novel four-wavelength photoacoustic aerosol absorption spectrometer, together with the results of carefully conducted assessment studies of instrument performance. The instrument evaluation is completed by an instrument intercomparison study with filter-based light absorption measurement instruments at a rural background station in Finland. The assessment reports method-characteristic parameters like limit of detection, precision, and accuracy. The study is carefully designed and performed. The presentation of the results is well structured and clear. The manuscript fits well into the scope of the journal and can be accepted for publication, after few technical corrections have been implemented.

SPECIFIC COMMENTS

The only point which may call for clarification is the use of the term “ultimate detection limit”. In the manuscript this term is used together with “detection limit”. The authors may want to clarify whether the two terms are used synonymously or have a different meaning.

The following minor issues are mostly suggestion for rephrasing for the sake of more clarity or readability.

MINOR ISSUES

Line 15: The expression “measured across the filter thickness” may be replaced by “measured by light transmission through the filter.

Line 29: Replace “for a several years data set” by “based on a multi-year data set”.

Line 43: References might be added for the use of the AAE to separate biomass burning aerosol (Sandradewi et al., 2008; Kirchstetter et al., 2004) and dust (Petzold et al., 2009) from fossil fuel combustion aerosol.

Line 75: The description of the photoacoustic process reads like it works only for externally mixed particles. Suggestion for rephrasing: “in an aerosol containing light-absorbing particulate matter compounds”.

Line 93: Suggestion “are presented in more detail”.

Line 110: Beam ellipticity might be reported as digits with similar decimal places, i.e., 1.1:1.0.

Line 220: Rephrase: “measured prior to …”

Line 299: Shouldn’t it read. “… the same procedure is applied to …”?

Line 323: Suggestion: “… sample from a main inlet …”

Line 340: Please correct: “…long data gaps … rather result from …”.

Figure 10: Plotting the y-axis with a log scale might better show the good data agreement at the low b_abs values.

Section 5: This section presents more a summary than conclusions. It should be modified or the section header  should be adjusted.

Reference list: Most of the journals are given with their full names, but some are referred to with abbreviations. Then, some of the journal names start with capital letters, others don’t. This should be made consistent.

References

Kirchstetter, T. W., Novakov, T., and Hobbs, P. V.: Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon, Journal of Geophysical Research, 109, D21208, 10.1029/2004JD004999, 2004.

Petzold, A., Rasp, K., Weinzierl, B., Esselborn, M., Hamburger, T., Dörnbrack, A., Kandler, K., Schütz, L., Knippertz, P., Fiebig, M., and Virkkula, A.: Saharan dust refractive index and optical properties from aircraft-based observations during SAMUM 2006, Tellus Series B-Chemical and Physical Meteorology, 61, 118–130, 10.1111/j.1600-0889.2008.00383.x, 2009.

Sandradewi, J., Prevot, A. S. H., Weingartner, E., Schmidhauser, R., Gysel, M., and Baltensperger, U.: A study of wood burning and traffic aerosols in an Alpine valley using a multi-wavelength Aethalometer, Atmospheric Environment, 42, 101-112, 10.1016/j.atmosenv.2007.09.034, 2008.