Measurements of OH and HO₂ radical concentrations and photolysis frequencies during BERLIOZ

Abstract

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 × 10 6 and 8 × 10 8 cm −3 , respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 × 10 5 cm −3 ), HO 2 did show significant concentrations throughout most of the nights (on average 3 × 10 7 cm −3 ). 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 × 10 7 cm −3 ) but started to rise rapidly as soon as NO started to decrease.