Nighttime formation of peroxy and hydroxyl radicals during the BERLIOZ campaign: Observations and modeling studies

[1] Traditionally, tropospheric radical chemistry is discussed in terms of the daytime photochemically produced hydroxyl radical (OH). Radicals, however, are also important during nighttime: this is especially true for ozone and the nitrate radical (NO3), which both act as key initiators of the degradation of alkenes such as biogenic monoterpenes. These reactions lead to the formation of peroxy radicals (HO2 and RO2) and hydroxyl radicals at night. We present recent observations of nighttime concentrations of NO3, RO2, HO2, and OH by differential optical absorption spectroscopy (DOAS), matrix isolation electron spin resonance (MIESR), laser-induced fluorescence (LIF), and a chemical amplifier (CA) in the framework of the Berliner Ozonexperiment (BERLIOZ) campaign at Pabstthum, Germany, together with modeling studies of nocturnal radical chemistry. Modeled RO2 mixing ratios reached 40 ppt while the measured ROx level went up to 22 ppt at the same time. Modeled and measured HO2 mixing ratios were up to 6 and 4 ppt, respectively. In the case of OH, a nocturnal concentration of (1.85 ± 0.82) × 105 cm−3 was measured during one night. At this time, the model yielded an OH level of (4.1 ± 0.7) × 105 cm−3. This overestimation by the model could point to a missing nocturnal sink of OH. Nitrate radical reactions with terpenes were found responsible for producing 77% of the RO2 radicals, 53% of the HO2, and 36% of the OH radicals during night. Nighttime ozonolysis formed 12% of the RO2, 47% of the HO2, and 64% of the OH radicals. Another 11% of the RO2 radicals were formed by OH–volatile organic compound (VOC) reactions. A positive linear correlation of RO2 and NO3 was observed and could be reproduced in model calculations originating from the loss of both radicals by reaction with NO and the NO3-initiated RO2 production. The contribution of nighttime OH to the atmosphere's oxidation capacity (oxidation rate of VOCs, CO, and CH4) was found negligible (<0.5%).