[1] 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 (HO 2 and RO 2 ) 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 Â 10 6 cm À3 for OH and 20-30 ppt for RO 2 and HO 2 . Fairly high OH concentrations were observed during the morning hours in the presence of high-NO x mixing ratios (>20ppb). The ''master chemical mechanism'' (MCM) was used to calculate OH, HO 2 , and RO 2 concentrations from the simultaneously measured data comprising a comprehensive set of speciated hydrocarbons and carbonyl compounds, O 3 , CO, NO, NO 2 , HONO, PAN, J(NO 2 ), J(O 1 D), and meteorological parameters. The calculated OH concentrations are in excellent agreement with the measurements during the morning hours at high-NO x (>10 ppb). However, at low NO x conditions the model overestimates OH by a factor 1.6. The modeled concentrations of HO 2 and RO 2 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 HO 2 and RO 2 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 RO 2 production upon reaction with OH. The measured RO 2 /HO 2 ratio of about one is well reproduced by the MCM, whereas a simple model without recycling of RO 2 from decomposition and isomerisation of alkoxy radicals underpredicts the measured ratio by about a factor of two. This finding highlights the importance of RO 2 recycling in the chemical mechanism. The ozone production rate P(O 3 ), 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.

No abstract

[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 (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) Â 10 5 cm À3 was measured during one night. At this time, the model yielded an OH level of (4.1 ± 0.7) Â 10 5 cm À3. 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%).

[1] The Photochemistry Experiment in BERLIOZ (PHOEBE) was conducted in July/August 1998 at a rural site located near the small village Pabstthum, about 50 km NW of downtown Berlin. More than 60 nonmethane hydrocarbons (NMHC) in the range of C 2 -C 10 were measured using two in situ gas chromatography (GC) systems. The first (GC1) was capable of measuring C 2 -C 10 hydrocarbons with a relatively high separation efficiency but low time resolution (80-90 min), while GC2 provided quasi-continuous measurements of C 5 -C 10 hydrocarbons with a time resolution of 20 min but with a poorer separation efficiency than GC1. The advantages of both systems were joined by interpolation between two data points of GC1 with the pattern given by GC2. For compounds that could not be reliably measured with GC2, patterns of compounds with similar reactivity were used. Air masses with the lowest photochemical age as estimated from the toluene/benzene ratio and the highest hydrocarbon mixing ratios were observed on 20 and 21 July when air was advected from the direction of Berlin. Alkanes were the most abundant hydrocarbons ($60%) on a molecular basis, followed by alkenes and aromatics. The reactivity of the hydrocarbons toward OH was dominated by the alkenes (>60%), with isoprene and a-pinene constituting the major part. The hydrocarbon data were used together with the other trace gases measured at Pabstthum to simulate OH, HO 2 , and RO 2 concentrations with the condensed chemical box model RACM. Relatively good agreement of the simulated radical concentrations with the spectroscopic measurements made at Pabstthum is observed for NO x mixing ratios >5 ppb, whereas the model overestimates OH and HO 2 by 100% and 40%, respectively, at low NO x . The discrepancy between measured and modeled OH does not correlate with the concentration of particles. The RO 2 concentrations are in good agreement with the measurements over the entire range of NO x . Sensitivity studies show that peroxyacetyl nitrate (PAN) is an important radical source and that missing volatile organic compound (VOC) reactivity is an unlikely explanation for the overestimation of HO x : By doubling of the VOC reactivity, OH and HO 2 can be brought into agreement. However, the model then overestimates the organic RO 2 concentrations by almost a factor of 2. Another important finding is that RACM overestimates the measured NO/NO 2 ratio by 25%. This and the overestimation of HO 2 lead to an overprediction of the local ozone formation rate by about 40% at low NO x mixing ratios.

No abstract