|Review of Sanchez et al., Atmos. Chem. Phys. Discuss. |
This is a review of the REVISED MANUSCRIPT performed by Reviewer #3.
All line numbers given below are for the revised version of the manuscript.
The authors have provided additional information that answers most of the comments from the 3 reviewers. However, there are still a few points that need to be addressed before publication in ACP:
1/ The authors used a catalytic converter to generate zero air from ambient air. The term “VOC-free air” is more appropriate since ambient NOx are not removed. This zero air is used to perform the “C2” measurement. NOx will be present in the CRM reactor during both the “C3” measurement, when ambient air is sampled, but also during the “C2” measurement. Since OH can react with NO and NO2 during both “C3” and “C2”, the CRM should be blind to the OH reactivity generated by NOx. The reported measurements of OH reactivity may therefore be biased low. Interestingly, the authors indicated L185-186 that during the SOAS campaign measurements from the UCI CRM instrument were 16% lower on average than measurements from a LIF system. Could the authors comment on this?
2/ The authors provided more information about operating conditions for their CRM instrument and indicated that the pyrrole-to-OH ratio was kept constant at a value of 3. This ratio can depend on the amount of OH that is produced from the photolysis of ambient water-vapor in the reactor due to the leakage of 185-nm photons. As a consequence the ratio usually changes with ambient humidity. This ratio was found to vary significantly for other CRM instruments when operated continuously during field campaigns. How did the authors manage to keep the pyrrole-to-OH constant? Was the geometry of the CRM reactor optimized to avoid the photolysis of ambient water-vapor?
L192-193: “An extensive intercomparison study was conducted by Fuchs et al. (2017) with various OH reactivity measurement techniques that highlighted potential analytical artifacts in the CRM technique. These artifacts have all been examined and preventive measures have been
implemented in the UCI CIMS-CRM system deployed at TRF.” - I would add some caveat here since these artefacts have not been fully investigated for this instrument. While some testing has been performed to check whether ambient NO and O3 could lead to measurement artifacts, the authors acknowledged in their responses to the first review that additional tests are needed to well characterize these artefacts.
L200-201: “Our approach to this type of interference has been to determine the maximum NO level, noticeably interfering with the calibration regression line shown in Sanchez et al. (2018). Laboratory tests indicate that the statistical agreement started to veer off when the NO level is 5 ppb in 1 𝜎 of the linear regression” – These tests are of interest for the CRM community and the reviewer recommends to show them in the supplementary material. This will also provide additional confidence in the dataset.
L214-216: “In the 2015 field campaigns conducted in Seoul South Korea (Kim et al., 2016), we conducted a standard addition experiment for the propene standard for additional ~ 30 s-1 in two different ozone environment 65 ppb and 123 ppb. The outcome illustrates an agreement between two additions within the analytical uncertainty.” – While a standard addition test could highlight an artefact impacting the linear response of CRM to OH reactivity, it cannot rule out an artefact leading to a positive or negative offset that would only depends on O3. For the later, a standard addition of 30s-1 of OH reactivity would always lead to the right change in the measured total OH reactivity. However, the total OH reactivity with (ambient reactivity + standard addition reactivity) or without (ambient reactivity) standard addition would be biased low by the same amount. This should be acknowledge in the manuscript.
L206-211: “Therefore, we performed multi-point calibrations with a propene mixture using a NIST traceable gas standard (AirLiquide LLC, 0.847 ppm) during the field campaign to avoid any circumstances where the pseudo first-order reaction regime is not established.” – Please indicate the range of OH reactivity generated during the multipoint calibration.
L244-245: The writing of Eq. 1 is still confusing (mixing of parameters and units). Please only use parameters that are defined in the main text. For instance: MRvoc=Svoc*kbenzene/kvoc*1/Rbenzene.
MR:Mixing Ratio, Svoc: normalized voc signal, kbenzene and kvoc: proton transfer rate constants, Rbenzene: benzene sensitivity