Measuring and modeling investigation of the net photochemical ozone production rate via an improved dual-channel reaction chamber technique

Current process-based research mainly uses box models to evaluate photochemical ozone production and destruction rates, and it is unclear to what extent the photochemical reaction mechanisms are elucidated. Here, we modified and improved a net photochemical ozone production rate (NPOPR, inline-formulaP(Oinline-formula₃)inline-formula_net) detection system based on the current dual-channel reaction chamber technique, which makes the instrument applicable to different ambient environments, and its various operating indicators were characterized, i.e., “airtightness”, light transmittance, wall losses of the reaction and reference chambers, conversion rate of Oinline-formula₃ to NOinline-formula₂, air residence time, and performance of the reaction and reference chambers. The limits of detection of the NPOPR detection system were determined to be 0.07, 1.4, and 2.3 ppbv hinline-formula⁻¹ at sampling flow rates of 1.3, 3, and 5 L mininline-formula⁻¹, respectively. We further applied the NPOPR detection system to field observations at an urban site in the Pearl River Delta (China). During the observation period, the maximum value of inline-formulaP(Oinline-formula₃)inline-formula_net was 34.1 ppbv hinline-formula⁻¹, which was inline-formula∼ 0 ppbv hinline-formula⁻¹ at night within the system detection error and peaked at approximately noon local time. The daytime (from 06:00–18:00 LT) average value of inline-formulaP(Oinline-formula₃)inline-formula_net was 12.8 (inline-formula± 5.5) ppbv hinline-formula⁻¹. We investigated the detailed photochemical Oinline-formula₃ formation mechanism in the reaction and reference chambers of the NPOPR detection system using a zero-dimensional box model. We found that the photochemical reactions in the reaction chamber were very close to those in ambient air, but there was not zero chemistry in the reference chamber because the reaction related to the production and destruction of ROinline-formula₂ (inline-formula= HOinline-formula₂ inline-formula+ ROinline-formula₂) continued in the reference chamber, which led to a small amount of inline-formulaP(Oinline-formula₃)inline-formula_net. Therefore, the inline-formulaP(Oinline-formula₃)inline-formula_net measured here can be regarded as the lower limit of the real inline-formulaP(Oinline-formula₃)inline-formula_net in the atmosphere; however, the measured inline-formulaP(Oinline-formula₃)inline-formula_net was still inline-formula∼ 7.5 to 9.3 ppbv hinline-formula⁻¹ higher than the modeled inline-formulaP(Oinline-formula₃)inline-formula_net value depending on different modeling methods, which may be due to the inaccurate estimation of HOinline-formula₂ inline-formula $M43inlinescrollmathml/$ 8pt14ptsvg-formulamathimg38128a0aedc95cb5017766f863958541 acp-23-9891-2023-ie00001.svg8pt14ptacp-23-9891-2023-ie00001.png  ROinline-formula₂ radicals in the modeling study. Short-lived intermediate measurements coupled with direct inline-formulaP(Oinline-formula₃)inline-formula_net measurements are needed in the future to better understand Oinline-formula₃ photochemistry. Our results show that the NPOPR detection system can achieve high temporal resolution and continuous field observations, which helps us to better understand photochemical Oinline-formula₃ formation and provides a key scientific basis for continuous improvement of air quality in China.