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

Abstract

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 (NO 3 ), which both act as key initiators of the degradation of alkenes such as biogenic monoterpenes. These reactions lead to the formation of peroxy radicals (HO 2 and RO 2 ) and hydroxyl radicals at night. We present recent observations of nighttime concentrations of NO 3 , RO 2 , HO 2 , 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 RO 2 mixing ratios reached 40 ppt while the measured RO x level went up to 22 ppt at the same time. Modeled and measured HO 2 mixing ratios were up to 6 and 4 ppt, respectively. In the case of OH, a nocturnal concentration of (1.85 ± 0.82) × 10 5 cm −3 was measured during one night. At this time, the model yielded an OH level of (4.1 ± 0.7) × 10 5 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 RO 2 radicals, 53% of the HO 2 , and 36% of the OH radicals during night. Nighttime ozonolysis formed 12% of the RO 2 , 47% of the HO 2 , and 64% of the OH radicals. Another 11% of the RO 2 radicals were formed by OH–volatile organic compound (VOC) reactions. A positive linear correlation of RO 2 and NO 3 was observed and could be reproduced in model calculations originating from the loss of both radicals by reaction with NO and the NO 3 ‐initiated RO 2 production. The contribution of nighttime OH to the atmosphere's oxidation capacity (oxidation rate of VOCs, CO, and CH 4 ) was found negligible (<0.5%).