The authors describe the development of a novel instrument to measure total OH reactivity based on laser flash photolysis of O₃ in the presence of water vapour to generate OH radicals with detection of OH using Faraday rotation spectroscopy used to determine total OH loss rates in ambient air, and consequently the OH reactivity. OH reactivity is the inverse of the chemical lifetime of OH, and can be used to assess the presence and impact of unmeasured or unquantified species in ambient air, providing information regarding the production regime for secondary pollutants such as ozone. While several techniques for measuring OH reactivity have been described in the literature, measurements remain sparse and there is a need for development of alternative methods for long-term measurements.

The manuscript details the operating principles of the technique, and the development and characterisation of the instrument, as well as examples of initial results obtained from measurements of ambient air. The manuscript is well-written and will be of interest to the atmospheric science community. I have only minor comments, detailed below, which should be addressed prior to final publication.

Line 19: The details of the ‘over-lapping factor’ are probably best left to the section of the manuscript where the term is defined, the effective path length is the more significant parameter to consider (and should be ‘effective’ rather than ‘efficient’ in line 20).

Line 25: The abstract mentions ‘advantages in cost, operation, and transportation’ but these are not really discussed in the manuscript, and there is no real comparison to other methods available. The costs are not mentioned at all.

Line 61: The statement ‘without needing to determine the reaction time’ is a little confusing, knowledge of the reaction time is essential.

Lines 64-65: It would be helpful to provide a brief explanation of the problems at high NO concentrations in instruments using photolysis of water vapour to produce OH, and how measurements of H2SO4 provide information on OH.

Line 68: A brief summary of the main conclusions of the work by Fuchs et al. would be helpful.

Line 103: What does the line strength equate to in terms of a cross-section under the operating conditions of the experiment?

Line 154: ‘angel’ to ‘angle’.

Table 1 and discussion lines 168-173: The comparison of overlapping factors seems a little unnecessary, and perhaps misleading. The papers described measure in different regions of the spectrum, where absorption cross-sections are likely higher, and so have less need for the longer effective path lengths developed for the measurements described in the manuscript. It would be more beneficial to provide a comparison of limits of detection.

Line 190 (and elsewhere): It would be better to be consistent throughout with use of µV and nV.

Line 246: It would be better to give the equation in terms of the concentration (or signal) of OH.

Line 249: It would be better to give the time since photolysis in place of ‘the 180^th data point’.

Figure 5: Time zero is more commonly described as the point at which photolysis occurs.

Line 262: Please reformat the equations to express the uncertainties more clearly.

Figure 6: Please give the equation for the line in terms of physical parameters (x is also missing in panel a), and add lines representing the literature values for the rate coefficients to the plots for comparison.

Line 279: How does the correction factor impact the uncertainties of the reactivity measurements?

Line 282 (and elsewhere): Please provide the uncertainties for measured rate coefficients.

Line 326: A table summarising the species measured and mean/median concentrations would be helpful.

Figure 9: There are large changes in JNO2 throughout the measurement period, including one day when it is near-zero at midday. Is there an explanation for this variation?

The present manuscript describes in detail a new experimental technique developed for measuring total OH reactivity. The instrument is compact and can easily be deployed in field campaigns. The compact size has benefits compared to currently used instruments and will possibly become a standard instrument, also for long-term measurements.

The paper is very well written, with many details on the instrumental design and the validation. I have no major remarks, only a few minor comments that might improve the manuscript:

Line 67 : There has been another intercomparison campaign between CRM and LP-LIF technique, which you might want to cite for completeness: Hansen et al., Intercomparison of the Comparative Reactivity Method (CRM) and Pump-Probe technique for measuring total OH reactivity in an urban environment, AMT 8, 4243 (2015)

Line 113 : demodulated

Line 123 : spots ARE arrangeD

Line 135: consistsING

Line 145: undergoesING

Line 156: below than 79.2

Line 169: several

Line 199: affecting

Line 205: correspondingS

Line 222: producingED

Line 241: were was

Line 245: flowing following

Figure 5: It would be more convenient to define as time 0 the time when the photolysis laser is triggered.

Line 290: You consider that the remaining around 80% of the k_zero is due to reaction with impurities?

Figure 7: How k_OH can be negative? I guess you removed the overall average value (5.2s-1?) from each individual value? Please specify.

Line 299: with A total a

Line 300: Can you confirm that the measurement precision is already converted to atmospheric conditions, ie. that the precision of the measurement itself is 5 times better?

Line 302: improved

Figure 8: same question: how can k_OH be negative? And what is the mean value of 0.2 s-1?

Line 329: (RH) concentration

Line 376: missing reactivity of

Finally, some more information on energy consumption, total weight, and also an indication on the cost of the instrument could be interesting to the reader.

Quantitative measurements of total OH reactivity (k_OH’) provide important insights into atmospheric photochemistry. This paper reports the development of a portable LP-FRS instrument for real-time and in-situ measurement of k_OH’. The size of the instrument is reduced to 130×40×35 cm. Its advantages in cost, operation, and transportation make it of great value in field observation and laboratory research. The optical and mechanical structure, the key MPC subsystem, parameter optimization, laboratory performance analysis, and field operation demonstration of the instrument are introduced in detail. I recommend acceptance after considering the following minor comments:

1 Attention should be paid to textual details, such as the missing “x” in Fig.6(a), line 159 (missing r_c unit), and the inconsistencies of time resolution value between line 299 and caption of Fig.7(a).

2 Table 1 does not seem to show the performance, but rather the compactness of pump-probe multi pass cells. It would be better to make a modification.

3 What is model of the UV lamp used to generate O₃?

4 Adding some explanations to the beginning of line 67 of the introduction section will be more helpful in explaining the problems of the current methods and instruments.

5 The role of r_c is clear because it directly affects the overlap. However, the effect of g value is not obvious and needs to be briefly explained.

6 Why is the error bar of OH+NO in Fig.6 larger than the other two experiments?

7 Line 261. The expression of “the slope errors of the fittings were less than 0.1” is not clear.

8 Line 312. The residual reactants in water are also one of the sources of uncertainty affecting the zero determination of the instrument.