OH, HO2, and RO2 radical chemistry in a rural forest environment: Measurements, model comparisons, and evidence of a missing radical sink
Abstract. The hydroxyl (OH), hydroperoxy (HO2), and organic peroxy (RO2) radicals play important roles in atmospheric chemistry. In the presence of nitrogen oxides (NOx), reactions between OH and volatile organic compounds (VOCs) can initiate a radical propagation cycle that leads to the production of ozone and secondary organic aerosols. Previous measurements of these radicals under low-NOx conditions in forested environments characterized by emissions of biogenic VOCs, including isoprene and monoterpenes, have shown discrepancies with modeled concentrations.
During the summer of 2016, OH, HO2 and RO2 radical concentrations were measured as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport – Atmospheric Measurements of Oxidants in Summer (PROPHET-AMOS) campaign in a mid-latitude deciduous broadleaf forest. Measurements of OH and HO2 were made by laser-induced fluorescence – fluorescence assay by gas expansion techniques (LIF-FAGE) and total peroxy radical (XO2) mixing ratios were measured by an ethane chemical amplification (ECHAMP) instrument. Supporting measurements of photolysis frequencies, VOCs, NOx, O3, and meteorological data were used to constrain a zero-dimensional box model utilizing either the Regional Atmospheric Chemical Mechanism (RACM2), or the Master Chemical Mechanism (MCM). Model simulations tested the influence of HOx regeneration reactions within the isoprene oxidation scheme from the Leuven Isoprene Mechanism (LIM1). On average, the LIM1 models overestimated daytime maximum measurements by approximately 40 % for OH, 65 % for HO2, and more than a factor of two for XO2. Modelled XO2 mixing ratios were also significantly higher than measured at night. Addition of RO2 + RO2 accretion reactions for terpene-derived RO2 radicals to the model can partially explain the discrepancy between measurements and modelled peroxy radical concentrations at night but cannot explain the daytime discrepancies when OH reactivity is dominated by isoprene. The models also overestimated measured concentrations of isoprene-derived hydroxyhydroperoxides (ISOPOOH) by a factor of ten during the daytime, consistent with the model overestimation of peroxy radical concentrations. Constraining the model to the measured concentration of peroxy radicals improves the agreement with the measured ISOPOOH concentrations, suggesting that the measured radical concentrations are more consistent with the measured ISOPOOH concentrations. These results suggest that the models may be missing an important daytime radical sink and could be overestimating the rate of ozone and secondary product formation in this forest.
Brandon Bottorff et al.
Status: open (until 05 Jun 2023)
- RC1: 'Comment on egusphere-2023-790', Anonymous Referee #1, 16 May 2023 reply
Brandon Bottorff et al.
Brandon Bottorff et al.
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This study focuses on the comparison between measured OH, HO2 and the sum of HO2 and RO2 (XO2) radicals and results from 2 different chemical mechanisms, one the RACM (lumped) and the other the MCM (semi-explicit). Measurements were conducted in an isoprene dominated forest in Michigan where a lot of ancillary species as well as ISOPOOH were detected.
This study seems to suggest that quite a large loss rate (termination reaction) for both HO2 and XO2 is needed for both chemical mechanisms investigated to agree with the measurements. Several hypotheses are made such as fast deposition of the radicals on surfaces, faster than used RO2+RO2 reactions for isoprene RO2, RO2 radicals reaction with alkenes and/or segregation could play a role. The authors suggest that most probably a combination of all the above could explain the discrepancy although a rather large loss rate of about 60% during daytime is needed.
The paper is well written and structured, and the arguments are presented in a clear manner. The study is interesting but as it is not possible to give a clear conclusion on what is causing the discrepancies, I would recommend adding a bit of analysis to try and see if something more can be understood and after that I would recommend the publication.
My first suggestion would be to try and perform an experimental budget with the available data. I understand that it might not be possible for the HO2 and RO2 but it would be possible to perform it for the OH radical. In this way it should be clear if the very low OH observed in the morning hours is an instrument artefact as it is quite dubious. The OH budget would also help (possibly) to clarify if indeed there doesn’t seem to be the need for additional sources of OH radicals as the comparison with the model seems to show. I have to say that isomerization reactions for isoprene-RO2 are more or less a given now so I am also wondering why there seems to be such a large overestimate of the OH radical when the most up to date mechs are used.
My second suggestion concerns the XRO2. I am wondering if it would not make sense to remove the HO2 fraction from it from the LIF measurement and then have a more or less RO2 measurement. I understand it does not make much of a difference since the measurement is compared with the some of HO2 and RO2 but to e able to compare with previous studies it would make it easier if it was RO2 instead of XO2.
My third suggestion is about the ISOPOOH. I am not aware of many ISOPOOH ambient measurements and although I could imagine a different publication focusing on that, it would be good to extend the discussion about it in this study. One thing that I find a bit odd is that the measured concentration of ISOPOOH is more or less zero (within the uncertainty) for the all time? I can see a bit of an increase but it is rather small. Even the model results after constraining HO2 and RO2 would expect quite a bit more. Could this be an instrument artefact? Is this consistent with previous measurements? Are there other products that show up which would compensate the production rate of RO2 that, as mentioned, was rather high, and the reacted isoprene must go somewhere.
I noticed that on few occasions the subscripts are not correct so I would recommend checking and fixing that.
I also recommend adding the work by J. Medeiros et al. (2022) which is consistent with LIM1.