Poster, Workshop on Atmospheric Chemistry: Kinetics and Spectroscopy, Bayreuth: 2010-02-24 - 2010-02-26
Formation pathways of Nitrous acid (HONO) are still controversial especially for daytime formation. Night time formation is believed to occur via heterogeneous hydrolysis of NO2 forming HONO and HNO3. Thus wetted ground surfaces should contribute a lot to HONO formation, but also act as sinks at high relative humidity when liquid films are formed and HONO is dissolved. HONO is photolyzed according to the reaction HONO + hn → NO + OH. Thus being a source of OH radicals. We have performed simultaneous HONO measurements in and above a tall spruce forest canopy using two long path absorption photometers (LOPAPs) at a field site located in the Fichtelgebirge mountains in north-eastern Bavaria, Germany (50°09’N, 11°52’E, 775m above sea level) during intensive operating periods of the EGER- project. Coupling regimes according to Thomas and Foken  were calculated from measurements of sonic anemometers. Diurnal cycles of HONO and concentration differences from above canopy (24.5m) and close to the forest floor (0.5m) are discussed in view of coupling (of the forest to the atmosphere) and the interdependency of HONO and humidity. Due to the shading of the crown the photolysis is reduced to about 10% of the above canopy level. In contrary to an expected concentration difference, due to the significantly different lifetimes, the differences of the measured HONO mixing ratios are close to zero around noon. The coupling regimes provide a useful tool for interpreting the diurnal cycle of these concentration differences.