Peroxy radicals during BERLIOZ at Pabstthum: Measurements, radical budgets and ozone production

Abstract

The Photochemistry Experiment during BERLIOZ (PHOEBE) was conducted in July and August 1998 at a rural site located near the small village of Pabstthum, about 50 km northwest of downtown Berlin. In this paper, spectroscopic measurements of hydroxyl (OH) and peroxy radicals (HO2 and RO2) are discussed for two intensive days (20 and 21 July) of the campaign. On both days peak values of the radical concentrations were similar, reaching 6–8 × 106 cm−3 for OH and 20–30 ppt for RO2 and HO2. Fairly high OH concentrations were observed during the morning hours in the presence of high‐NOx mixing ratios (>20ppb). The “master chemical mechanism” (MCM) was used to calculate OH, HO2, and RO2 concentrations from the simultaneously measured data comprising a comprehensive set of speciated hydrocarbons and carbonyl compounds, O3, CO, NO, NO2, HONO, PAN, J(NO2), J(O1D), and meteorological parameters. The calculated OH concentrations are in excellent agreement with the measurements during the morning hours at high‐NOx (>10 ppb). However, at low NOx conditions the model overestimates OH by a factor 1.6. The modeled concentrations of HO2 and RO2 are in reasonable agreement with the measurements on 20 July. On the next day, when isoprene from nearby sources was the dominant VOC, the model overpredicted HO2 and RO2 in addition to OH. Radical budgets solely calculated from measured data show that a missing sink for OH must be responsible for the overestimation by MCM. Missing VOC reactivity is unlikely, unless these VOC would not lead to RO2 production upon reaction with OH. The measured RO2/HO2 ratio of about one is well reproduced by the MCM, whereas a simple model without recycling of RO2 from decomposition and isomerisation of alkoxy radicals underpredicts the measured ratio by about a factor of two. This finding highlights the importance of RO2 recycling in the chemical mechanism. The ozone production rate P(O3), calculated from the peroxy radical concentrations and NO, had a maximum of 8 ppb/hr at 0.5 ppb NO, which is in good agreement with results from previous campaigns at Tenerife and Schauinsland.