the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Nov 2024
Development of a portable laser-flash photolysis Faraday rotation spectrometer for measuring atmospheric total OH reactivity
Abstract. Quantitative measurements of atmospheric total OH reactivity (kOH') provide crucial insights into atmospheric photochemistry. However, widespread application of total OH reactivity measurements is challenging due to insufficient equipment and the complexity of existing instrumentation. In this work, we report the development of a portable laser-flash photolysis Faraday rotation spectroscopy (LP-FRS) instrument for real-time and in-situ measurement of kOH'. To achieve efficient overlapping between the pump and probe laser and realize a long effective absorption path length, thus enabling high sensitivity measurement, a specific Herriott-type pump-probe optical multi-pass cell was designed with an overlapping factor of up to 75.4 %. The instrument’s optical box dimensions were 130 cm × 40 cm × 35 cm. The obtained efficient absorption path was ~ 28.5 m in a base length of 77.2 cm. The kOH' detection precisions of the LP-FRS instrument were 2.3 s-1 and 1.0 s-1 with averaging times of 60 s and 300 s, respectively. The kOH' measurement uncertainty was evaluated to be within 2 s-1. Field measurement was performed, and the difference between the measured kOH' and the model simulated from the measured reactive species was analysed. The developed portable LP-FRS instrument extends the measurement methods of atmospheric total OH reactivity, and has certain advantages in cost, operation, and transportation, which will play an increasingly important role in future atmospheric chemistry research.
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RC1: 'Comment on amt-2024-184', Anonymous Referee #1, 05 Dec 2024
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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 180th 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?
Citation: https://doi.org/10.5194/amt-2024-184-RC1
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