Experimental chemical budgets of OH, HO₂, and RO₂ radicals in rural air in western Germany during the JULIAC campaign 2019

Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a 50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO₂), and organic peroxy (RO₂) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (k_OH, the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 × 10⁶ cm^-3 at noon, and median concentrations of both HO₂ and RO₂ radicals were 3 × 10⁸ cm^-3. The measured OH reactivity was between 4 and 18 s^-1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (O_x = NO₂ + O₃) determined from NO₂ and O₃ concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO₂ and RO₂ radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h^-1 for OH, HO₂, and RO₂. Total RO_X (OH, HO₂, and RO₂) initiation and termination rates were below 3 ppbv h^-1. The highest OH radical turnover rate of 13 ppbv h^-1 was observed during a high-temperature (max. 40 °C) period in August. In this period, the highest HO₂, RO₂, and RO_X turnover rates were around 11, 10, and 4 ppbv h^-1, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO₂ production and destruction rates were balanced, but unexplained RO₂ and RO_X production reactions with median rates of 2 and 0.4 ppbv h^-1, respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO₂ and RO₂ measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO₂ were found with maximum rates of 3.0 and 2.0 ppbv h^-1, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO₂ to OH conversion reaction (up to 2 ppbv h^-1) and a missing primary radical source (0.5-1.4 ppbv h^-1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO₂ isomerization reactions from isoprene, HO₂ uptake on aerosol, and RO₂ production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.