Experimental chemical budgets of OH, HO2, and RO2 radicals in rural air in western Germany during the JULIAC campaign 2019
Authors: Cho Changmin , Fuchs Hendrik , Hofzumahaus Andreas , Holland Frank , Bloss William J. , Bohn Birger , Dorn Hans-Peter , Glowania Marvin , Hohaus Thorsten , Liu Lu , Monks Paul S. , Niether Doreen , Rohrer Franz , Sommariva Roberto , Tan Zhaofeng , Tillmann Ralf , Kiendler-Scharr Astrid , Wahner Andreas , Novelli Anna 
Published: February 2023

Abstract. 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 (kOH, 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 (Ox= 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 ROX (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 ROX 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 ROX 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.