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.