Optimizing a twin-chamber system for direct ozone production rate measurement

Abstract. High Ozone Production Rate (OPR) leads to O₃ pollution episodes and adverse human health outcomes. Discrepancies between OPR observation (Obs-OPR) and OPR modeling (Mod-OPR) as calculated from observed and modeled peroxy radical and nitrogen oxides reminds of a yet-perfect understanding of O₃ photochemistry. Direct measurement of OPR (Mea-OPR) by a twin-chamber system emerges with the optimization required for suppressing the wall effect. Herein, we minimized the chamber surface area to volume ratio (S/V) to 9.8 m⁻¹ and the dark uptake coefficient of O₃ to the order of 10⁻⁹. Condition experiments further revealed a photo-enhanced O₃ uptake and recommended an essential correction. We finally characterized a measurement uncertainty of ±27 % and a detection limit of 2.8 ppbv h⁻¹ (3SD), which suggests that Mea-OPR is sensitive enough to measure OPR in urban or suburban environments. Application of this system in urban Beijing during the Winter Olympic Games recorded a noontime OPR of 7.4 (±3.8, 1SD) ppbv h⁻¹, which indicates fairly active O₃ photochemistry despite the pollution control policy implemented. Mea-OPR versus j(O¹D) slope of 6.1 × 10⁵ ppbv h⁻¹ s⁻¹ confirmed fairly active O₃ photochemistry, which was assisted by a high abundance of VOCs and NO_x, atypically high Mea-OPR even under high-NO_x conditions, but mediated by relatively weak ultraviolet (UV) radiation.