OH measurements in the coastal atmosphere of South China: missing OH sinks in aged air masses
Abstract. The hydroxyl radical (OH) is the main oxidant responsible for the removal of many reduced trace gases and the formation of secondary air pollutants. However, due to technical difficulties in measuring OH, the existing measurements of atmospheric OH concentrations are limited, and its sources and sinks are not well understood under low NOx conditions. In this study, we observed the OH concentrations using chemical ionization mass spectrometry at a coastal site in Hong Kong from October to November 2020. The average noontime OH concentration over the study period was measured at 4.9 ± 2.1 × 106 cm-3. We found that a box model with comprehensive observational constraints reproduced the observed daytime OH concentrations when air parcels originated from the continental regions. However, this model overpredicted the observed daytime OH concentrations for coastal air parcels by 73 % on average. Missing OH reactivity is proposed to be the cause of this overprediction. High missing OH reactivity was found in the case of low concentrations of nitrogen oxides (NOx) and volatile organic compounds, as well as in aged air, suggesting that there could be unmeasured chemical species that cause the model to overestimate OH in aged coastal air parcels. Further studies are needed to identify these unmeasured chemical species and their contributions to the OH budget, in order to better quantify the formation of secondary air pollutants.
Zhouxing Zou et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2022-854', Anonymous Referee #1, 09 Nov 2022
- AC1: 'Reply on RC1', Tao Wang, 08 Feb 2023
RC2: 'Comment on egusphere-2022-854', Anonymous Referee #2, 18 Dec 2022
- AC2: 'Reply on RC2', Tao Wang, 08 Feb 2023
Zhouxing Zou et al.
Zhouxing Zou et al.
Viewed (geographical distribution)
The paper reports ground-based measurements of OH concentrations that were obtained in a coastal area in southern China in fall 2020. Additional measurements of trace gases (O3, NO, NO2, HONO, CO, VOCs, OVOCs) and meteorological parameters were used as input to a zero-dimensional box model to simulate the OH concentrations which are then compared to the observations. Modeled and measured OH concentrations agreed during the day when continental air arrived at the measurement site, but the model overestimates the observed OH concentrations in coastal air by a factor of 1.7. The disagreement is attributed to unmeasured atmospheric components which are supposed to be missing in the model as OH sinks. Atmospheric OH measurements are generally difficult and rare. New observations in previously understudied regions, as in this work, are therefore of potential interest. However, the paper in its current form has major shortcomings. In particular, since important quantities such as jO1D, HO2, and OH reactivity were not measured during the field campaign, few new insights can be gained from the reported OH observations that would expand our understanding of atmospheric OH chemistry. The manuscript would potentially qualify as a Measurement Report if (a) the documentation of the measurement instrument is improved, (b) the analysis of measured OH diurnal cycles is extended to include nocturnal values, and (c) the interpretation of the comparison between model and measurement is more balanced. The title of the paper would need to be adjusted accordingly.
1. Instrumental description
One problem is that the applied OH instrument is newly developed. The applied CIMS concept is well known in the literature, but the specific characterization, calibration and treatment of potential interferences of the present instrument is not well documented. The authors refer to a preprint that was submitted to the journal AMT (Pu et al., Development of a chemical ionization mass spectrometry system for measurement of atmospheric OH radical, amt-2020-252), but was not accepted for final publication. Reference to that manuscript is problematic because it is not clear to the readers which of the statements made there are valid for the current work or led to a rejection. Without further explanation, the cited work is insufficient to support the quality of the measurements here. The present paper should stand on its own independently of the preprint in AMT.
The current manuscript provides sufficient references for the measurement principle of the instrument, but is lacking explanations how the specific calibration error, detection limits and overall accuracy of the new instrument were determined. Which factors contributed how much to the total accuracy of 44 %? How was the detection limit calculated and why was it larger in the field than in the laboratory? How large was the background compared to the OH signal? Were interference tests performed and what was the result? Were any corrections made for chemical interferences in the inlet as discussed by Berresheim et al. (2000)? These questions should be answered and supporting material could be presented in the Supplement. I also suggest to move Table 3 to the Supplement and give there some explanations of its contents.
2. Measured diurnal OH profiles
The measured diurnal OH profiles in Figure 7 show plausible variations during daytime as can be seen from the correlation with solar UV and the OH model simulation. However, the considerable nocturnal OH concentrations between 0.5 x 10^6 cm-3 and 1 x 10^6 cm-3 (Figure 5, 6) are an order of magnitude larger than the simulation shown in Figure 7. The unexpectedly high nighttime values are not commented or discussed. They could be due to a systematic instrumental offset or indicate real atmospheric OH at night. This needs to be discussed. For example, is there an instrumental baseline problem that cannot be eliminated by the chemical modulation in the CIMS inlet? Mauldin et al. (2012) reported a non-OH source of sulfuric acid in a Boreal forest (probably not applicable here) and Berresheim et al. (2014) found evidence for an unknown oxidant in coastal air that converts SO2 to sulfuric acid in their CIMS inlet. Could these unknown oxidants play a role in the measurements reported here? What would happen if the unknown oxidant chemistry in the instrument inlet would be influenced by the OH scavenger? Have you tried a different scavenger other than C3F6? If there is a problem with the baseline, it could potentially affect the daytime OH measurements as well. If the nocturnal OH levels observed by CIMS indicated true OH levels, this would be of considerable atmospheric relevance. How do the values compare to previous observations of nighttime OH in PRD (Lu et al., Atmos. Chem. Phys., 14, 4979–4999, 2014 ; Tan et al., 2019)?
3. Comparison of modeled and observed OH concentrations
The authors report agreement of the modeled and measured OH in continentally influenced air and find that the model overestimates the observed OH in coastal air. What can be learned from this result? Unfortunately, measurements of HO2 concentrations and OH reactivity were not performed in this campaign. Measurements of these quantities have become standard in most field campaigns over the past decade and are absolutely essential if new insights into atmospheric OH chemistry are to be gained. For example, field studies have shown that the agreement between modeled and measured OH can be misleadingly good. Kanaya et al. (Atmos. Chem. Phys., 12, 2567–2585, 2012) and Whalley et al. (2018) reported missing OH production in their MCM models that was coincidentally compensated for by the model's overprediction of HO2, resulting in good agreement between modeled and observed OH concentrations.These model deficiencies were only detected because HO2 and OH reactivity measurements were available as additional constraints. These two parameters are of paramount importance for the understanding of OH since they dominate the chemical OH budget in most cases. For the same reasons, it is not clear if missing OH reactivity is the major reason for the overestimated modeled OH in coastal air. Without knowing how well the model reproduces HO2, it is difficult to quantify the amount of missing OH reactivity. The authors assume that unknown atmospheric trace gases react with OH and form products that do not undergo further reactions (page 13, line 23-24). The assumption that the products are inert is not very likely. Missing OH reactivity is most probably caused by unmeasured VOCs or OVOCs, which produce RO2 and HO2 when they react with OH. The additional peroxy radicals recycle some OH and thereby increase its total production rate (called P_constrain in Eq 9). It means that the required amount of missing OH reactivity is probably higher than the authors' estimate that is based on a fixed OH production rate. Here, additional HO2 measurements are missing to determine the total OH production independent of model assumptions.
4. Quantification of VOCs
The total amount of VOCs is expressed in many places in the paper as the sum of the VOC mixing ratios (ppb). While the total mixing ratio is a useful quantity to indicate the amount of measured organic carbon, it tells us little about its relevance for the OH chemistry. Since the rate constants for different VOC species may differ by orders of magnitude, it is better to report the total organic OH reactivities of the measured VOCs and their subgroups (AVOC, BVOC, etc.) to characterize the chemical conditions.
5. Ozone photolysis frequency
The photoysis of ozone forming O(1D) is one of the major processes that produce HOx. The corresponding j-value is calculated in the present work by using a clear-sky parametrization from Saunders et al. (2003). The values are then scaled with the ratio of modeled-to-measured jNO2 (to correct for cloud effects?). The whole approach has a considerable error that is not discussed in the paper. jO1D depends on the total atmospheric ozone column and air temperature, which are both not considered in the parametrization. The parametrization is useful for pure modeling studies, but not a good choice for the description of real ozone photolysis frequencies in a field campaign. Ideally, jO1D is measured as is done by many groups. The next best approach would be to simulate the clear-sky values by a radiative transfer model (for example by the freely available Tropospheric Ultraviolet and Visible (TUV) Radiation Model from NCAR) taking total ozone and temperature into account. Also note that jO1D responds differently to cloudiness compared to jNO2 (see for example, Walker et al., Environ. Sci.: Atmos., DOI: 10.1039/d2ea00072e). The authors should attempt to estimate a more realistic jO1D by means of the TUV or a similar radiative transfer model.
6. Literature review
In the introduction of the paper, the authors present Table 1 for an overview of previously published comparisons between modeled and measured atmospheric OH. The table takes up a large part of the paper, but is not very informative due to the lack of its discussion. In order to judge the listed comparison results, detailed explanations would be needed on how the past measurement techniques and chemical models have improved over the last 2-3 decades. To keep the paper focussed, I suggest to remove Table1 and Figure 1. It is sufficient to refer to corresponding review articles (e.g., Heard and Pilling, Chem. Rev. 2003, 103, 5163-5198; Stone et al., 2012; Rohrer et al., 2014; Lu et al., National Science Review 6: 579–594, 2019).