Measuring and modelling investigation of the Net Photochemical Ozone Production Rate via an improved dual-channel reaction chamber technique
Abstract. Current process-based research mainly used the box model to evaluate the photochemical ozone production and destruction rates, it is not clear to which extend the photochemical reaction mechanisms were understood. Here, we modified and improved a net photochemical ozone production rate (NPOPR, P(O3)net) detection system based on current dual-channel reaction chamber technique, which make the instrument appliable to different ambient environment, and its various operating indicators were characterized, i.e., the airtightness, light transmittance, wall losses of the reaction and reference chambers, conversion rate of O3 to NO2, the air residence time, and the performance of the reaction and reference chambers, etc. The limit of detection of NPOPR detection system were determined as 0.07, 1.4, and 2.3 ppbv h−1, at the sampling flow rates of 1.3, 3, and 5 L min−1, respectively. We further applied NPOPR detection system in the field observation at an urban site at Pearl River Delta (China). During the observation period, the maximum value of P(O3)net was 34.1 ppbv h−1, which was ~ 0 ppbv h−1 at night within the system detection error and peaks at around noon local time, the daytime (from 6:00–18:00) average value of P(O3)net was 12.8 (± 5.5) ppbv h−1. We investigated the detailed photochemical O3 formation mechanism in the reaction and reference chambers of NPOPR detection system using a zero-dimensional box model. We found that the photochemical reactions in the reaction chamber were very close to that in the ambient air, but it was not zero-chemistry in the reference chamber, on the contrary, the reaction related to the production and destruction of RO2 (= HO2 + RO2) continues in the reference chamber, which led to small amount of P(O3)net. Therefore, the P(O3)net measured here can be regarded as the lower limit of the real P(O3)net in the atmosphere, however, the measured P(O3)net were still ~ 7.5 ppbv h−1 to 9.3 ppbv h−1 higher than the modeled P(O3)net value depending on different modeling methods, this may be due to the inaccurate estimation of HO2/RO2 radicals in the modeling study. Short-lived intermediates measurements coupling with direct P(O3)net measurements are needed in future in order to understand the O3 photochemistry better. Our results show that the NPOPR detection system can achieve high time resolution and continuous field observation, which helps us to understand photochemical O3 formation better and provides a key scientific basis for the continuous improvement of air quality in China.
Yixin Hao et al.
Status: final response (author comments only)
- RC1: 'Comment on acp-2022-823', Anonymous Referee #1, 10 Mar 2023
- RC2: 'Comment on acp-2022-823', Anonymous Referee #2, 10 Mar 2023
Yixin Hao et al.
Yixin Hao et al.
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This paper describes an improved direct measurement system of the photochemical ozone production rate (P(O3)). The authors also performed a field observation of P(O3) and evaluated the instrument and observation results using a detailed box model simulation.
O3 pollution is a crucial problem for atmospheric environment, and a behavior of O3 is very difficult, so direct measurements of P(O3) such as this research are very important and valuable. However, this paper has major concerns about the evaluation by the box model simulation. So I cannot recommend this paper to be published in Atmospheric Chemistry and Physics in the present form. The authors should resolve the concerns by re-evaluation of the box model simulation or other ways.
I suspect such high concentrations of NO3 in the reference chamber. First, the value of JNO3 in the reference chamber is about 90% of that in the reaction chamber, which is sufficiently high. Second, the rate constant of the reaction of NO3 with NO is sufficiently large and the lifetime of NO3 (at 298 K) is 1.6 s and very short in the presence of NO of 1 ppbv. There might be high concentrations of NO3 if there are very large sources of NO3, but in that case, the authors should mention the evidence. I think the NO2 + O3 reaction cannot be a large source of NO3. I’m not sure about N2O5, but it is unlikely that there could be high concentrations of N2O5 under the temperature during the observation period.
On the other hand, I also suspect the reaction of NO3 with VOCs as a major source of RO2 in the reference chamber. The rate constants of the reactions of NO3 with VOCs are not so large, and it is questionable that NO3 and RO2 concentrations change in minutes by the reactions of NO3 with VOCs unless there are extremely high concentrations of VOCs.
For Figs. 9(b) and S10(b), photodissociation of OVOCs is a significant part of ROx sources in the reference chamber. Why such a large fraction in the absence of UV? The authors should mention the evidence.
I entertain doubts about the results of the present box model simulation. I think the authors should revalidate the appropriateness of the model thoroughly.
Did the authors use a commercially available chemiluminescence NOx monitor without further modification? If so, accuracy of NO2 and NOx concentrations would be low because NOz such as HNO3 and PANs interfere observed values of NO2 and NOx. I think this interference could affect discussion in this paper, so the authors should evaluate the interference quantitatively. In the fresh air masses, the interference by descendent pollutants of NOx such as HNO3 and PANs might be small, but that by HONO could be large instead.
Other minor comments:
Some mathematical formulae: The authors should use italic and roman letters correctly.
L65: at 424 nm → at < 424 nm (at less than 424 nm)
L72: are proportional to → affect ?
Fig. 1: Why do the authors use critical orifices instead of mass flow controllers? Is the temperature of orifices controlled to keep a constant flow rate?
Fig. 1: Are inner walls of the reaction and reference chambers coated with Teflon? If so, please indicate the kind of the Teflon coat.
Table S1: The authors should add standard deviation to average residence times.
Fig. S5: The regression lines have non-zero intercepts. These are significant? If so, why? The regression lines for ozone have negative intercepts. In this case, there are large losses of ozone in the high concentration of ozone? For NO2, how about relative humidity in the experiment? Is there no loss of NO2 at high relative humidity?
L218: low → high?
L226: Why the transmittivity of HONO in the reference chamber is lower than that of O3? How about accuracy and precision of the actinic flux spectrometer?
L228: agree → agreement
Table 1: What is Ultem? There are no definitions in the text.
Table 1: The values of 0.019±0.011 should be shaded.
L272-278 (The airtightness of the reaction and reference chambers): It is hard to follow this section. I think the authors should explain using a schematic diagram for the experiment in the supplement.
L297-L300: For calibration of NO2, it is not appropriate to perform calibration of NO2 using a NO2 standard gas because of low reliability. Calibration should be performed using a gas-phase titration method using NO and O3.
Reference: The authors should put the list into alphabetical order.