Seasonal ozone production rate measurements by use of SAPHIR as a large continuous flow reactor during the JULIAC campaign

For the Jülich Atmospheric Chemistry Project campaign (JULIAC) at Forschungszentrum Jülich (FZJ), Germany, the atmospheric simulation chamber SAPHIR was used as a large photochemical flow reactor to study tropospheric chemistry in a rural environment. From an inlet at 50 m height above ground, ambient air was continuously fed through the chamber and exposed to natural solar radiation. A large set of instrumentation allowed for the measurement of NO, NO₂, NO₃, N₂O₅, ClNO₂, HCHO, HONO, RO₂, HO₂, OH, k_OH, CO, CO₂, CH₄, H2O, VOCs, aerosols, and O₃ in the sampled air. Intensive measurement phases were performed for one month in each season of 2019. One goal of the JULIAC project was to test our understanding of the chemistry of tropospheric ozone formation.

To determine the photochemical net ozone production rate in atmospheric air, O_X (O₃ + NO₂) was measured by commercial instruments at the inlet and inside the well mixed chamber. Through careful characterization of the flow reactor it is possible to predict a reference concentration of O_X from the inflow measurements which excludes photochemistry. The measured O_X concentration in the chamber was compared with the reference. At night, both concentrations agreed, but during daytime the chamber concentration was enhanced due to photochemical O_X production. The difference was used to determine diurnal profiles of the net ozone production with 1 hour time resolution. Production rates up to 15 ppbv/h were observed with an accuracy of 1 ppb_V/h. Uncertainties in the offsets of the instruments measuring at the inlet and inside the chamber were identified as large contributors (~0.5 ppb_V/h) to the overall error. The measured net ozone production rates are compared to production rates that are expected from the reactions of peroxy radicals (HO₂, RO₂) with NO, all of which were concurrently measured. The analysis includes other chemical reactions that may produce or destroy ozone or NO₂ in the lower troposphere.

Good agreement (within 10%) between measured and calculated ozone production rates during the spring and summer campaigns confirms that the main contributions to daytime O_X production and destruction in the troposphere are overall governed by the reactions of HO₂ and RO₂ with NO and the reaction of OH radicals with NO₂ in the rural environment studied in this project. The presentation will include a discussion of the role of the OH reactivity from VOCs for the local photochemical ozone production.