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Measurements of HONO below and above a spruce forest canopy

Matthias Sörgel1, Ivonne Trebs2, Alexander Moravek2, Cornelius Zetzsch3
1 Atmospheric Chemistry Research Laboratory, University of Bayreuth, Germany
2 Max Planck Institute for Chemistry, Biogeochemistry Department, Mainz, Germany
3 Atmospheric Chemistry Research Laboratory, University of Bayreuth, Germany; Fraunhofer Institute for Toxicology and Experimental Medicine, Hannover, Germany

P 4.1 in Klimaforschung

Nitrous Acid (HONO) plays an important role in atmospheric chemistry because it is easily photolyzed to NO and OH, whereas OH is the most important oxidizing agent in the atmosphere. Understanding sources and sinks of HONO (especially during daytime) leads to a better estimation of OH-budget.  
We have made simultaneous HNO2 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 northeastern Bavaria, Germany (50°09’N, 11°52’E, 775m above sea level). The LOPAP is a wet chemical instrument actively correcting for interferences (Kleffmann et al. 2002). Measurements were made simultaneously with two LOPAP devices from 13-25 Sep 2007 on a tower (z = 24.25m, above canopy) and in the trunk space (z = 0.48m close to the forest floor). Several other micrometeorological and chemical quantities were measured aiming to investigate the coupling between soil, canopy and atmospheric boundary layer.
The measured HNO2 mixing ratios showed typical diel cycles with higher values during nighttime, especially during periods when no precipitation occurred. Average mixing ratios measured with the LOPAP instruments ranged from 40 ppt to 80 ppt during the day and from 80 ppt to 150 ppt during the night with a higher variability during nighttime, but were found to be independent of NO2 mixing ratios. According to the measured data HNO2 mixing ratios are related to relative humidity during nighttime and to radiation during daytime. Although, the HNO2 photolysis rate is 10 times lower in the trunk space than above the forest, daytime mixing ratios in and above the canopy were often comparable. Thus, the higher photolysis rate above the forest must be balanced by other processes delivering HNO2 (e.g., vertical mixing or photochemical formation).       

 [1] Jörg Kleffmann, Jörg Heland, Ralf Kurtenbach, Jutta Lörzer and Peter Wiesen; ESPR-Environ Sci & Pollut Res; 2002; Special Issue 4

Letzte Änderung 08.03.2009