Nitrous and Nitric Acid Measurements Inside and Outside of Clouds at Mt. Brocken

A measurement method for the continuous and in situ analysis of atmospheric concentration of HNO₂ and HNO₃ inside and outside of clouds has successfully been established. The approach for a wet effluent diffusion denuder (parallel plate design) was adopted and performed quasi on-line with an ion chromatography unit. The air flow through the denuder was 10 1/min and at 15 minutes of preconcentration we are able to record atmospheric concentrations of 15 ng/m³ of HNO₂ and HNO₃, respectively. Field measurements using the denuder system were done at the Mt. Brocken summit (1142 m a.s.l., Harz Mountains, Germany) in 1998 and 1999. During day-time and under clear conditions maximum values of HNO₃ up to 7000 ng/m³ were observed. Measurements performed during cloud events assess first time our expectation that the comparatively very great specific droplet surface in clouds is a place for a heterogeneous HNO₂ formation, after nocturnal accumulation up to 320 ng/m³ were found. Strong positive correlation between HNO₂ concentrations and those of NO₂ were obtained during and after cloud events to assess the origin of the observed nitrous acid. The HNO₂/NO₂ ratio reached 10% under humid conditions. The observed surface to volume ratios in investigated clouds were between 0.4 and 0.7. In dependence on the cloud water pH nitrite concentrations between 5–300 µg/l (1–30 ng/m³ air) were observed.