[1] The photolysis of nitrous acid (HONO) in the early morning hours is an important source of OH radicals, the most important daytime oxidizing species. Although the importance of this mechanism has been recognized for many years, no accurate quantification of this OH source is available, and the role of HONO photolysis is often underestimated. We present measurements of HONO and its precursor NO 2 by Differential Optical Absorption Spectroscopy (DOAS) during the Berliner Ozonexperiment (BERLIOZ) field campaign in July/August 1998 at Pabstthum near Berlin, Germany. HONO concentrations, formation rates, and simultaneously measured HONO photolysis frequencies are used to calculate the total amount of OH formed by HONO photolysis during a full diurnal cycle. A comparison with the OH formation by photolysis of O 3 and HCHO and by the reaction of alkenes with ozone shows that HONO photolysis contributed up to 20% of the total OH formed in a 24 hour period during this campaign. In the morning hours, HONO photolysis was by far the most important OH source during BERLIOZ.

[1] This paper presents the measurements of OH and HO 2 radical concentrations as well as photolysis frequencies of different molecules during the Berliner Ozone (BERLIOZ) field experiment in July/August 1998 at the rural site Pabstthum about 50 km NW of Berlin. Radical concentrations were measured using laser-induced fluorescence (LIF) spectroscopy, while filter radiometers and a scanning spectroradiometer were used to obtain photolysis frequencies. The radical data set covers the time period from 20 July to 6 August and consists of more than 6000 simultaneous measurements of OH and HO 2 with a typical time resolution of about 90 s. The maximum OH and HO 2 daytime concentrations were 8 Â 10 6 and 8 Â 10 8 cm À3 , respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 Â 10 5 cm À3 ), HO 2 did show significant concentrations throughout most of the nights (on average 3 Â 10 7 cm À3 ). The OH concentration was mainly controlled by solar UV radiation and showed a high linear correlation with J(O 1 D). A deviation from this general behavior was observed around dawn and dusk, when OH concentrations well above the detection limit were observed, although J(O 1 D) was essentially zero. A comparison with data sets from previous campaigns revealed that even though the linear correlation is found in other environments as well the slope [OH]/J(O 1 D) differs significantly. The diurnal cycles of HO 2 were less dependent on the solar actinic flux but were predominantly influenced by NO. During episodes of high NO, HO 2 remained below the detection limit (1 Â 10 7 cm À3 ) but started to rise rapidly as soon as NO started to decrease.

Formaldehyde mixing ratios are reported for rural areas around Jülich and for maritime air at the north coast of Germany. The measurements were made using a ground‐based UV‐optical absorption technique which allowed the simultaneous determination of CH2O, NO2, and O3 with detection limits of 0.1, 0.1, and 1 ppb, respectively. In Jülich, which may be regarded as typical for central European background atmosphere, mixing ratios varied from 0.1 to 6.5 ppb from May through October 1978. In maritime air under conditions when photochemical equilibrium was expected, formaldehyde concentrations of 0.2 ppb were observed, which can be accounted for by photochemical oxidation of methane alone. However, the high concentrations of formaldehyde found at Jülich indicate other sources of formaldehyde.

[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.

[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%).