Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 2 2004 10.5194/acpd-4-2003-2004 http://www.atmos-chem-phys-discuss.net/4/2003/2004/ http://www.atmos-chem-phys-discuss.net/4/2003/2004/acpd-4-2003-2004.html http://www.atmos-chem-phys-discuss.net/4/2003/2004/acpd-4-2003-2004.pdf 2003 2036 2004-04-07 Assessment of the NO-NO2-O3 photostationary state applicability on long-term measurements at the GAW global station Hohenpeissenberg, Germany K. Mannschreck k.mannschreck@schneefernerhaus.de S. Gilge C. Plass-Duelmer W. Fricke H. Berresheim Umweltforschungsstation Schneefernerhaus, Zugspitze, Germany Deutscher Wetterdienst, Meteorologisches Observatorium Hohenpeissenberg, Germany Continuous measurements of reactive gases, radiation, and meteorological parameters are carried out at the Meteorological Observatory Hohenpeissenberg (MOHp) as part of the Global Atmosphere Watch (GAW) Program. NO, NO2, O3 and JNO2 data from a four year period (March 1999–December 2002) are evaluated for consistency with photochemical steady state (PSS, Φ=1) conditions. In average PSS was reached in 17%, 13%, 22% and 32% of all cases for the year 1999, 2000, 2001 and 2002, respectively. The extent of deviation from PSS reveals a strong dependence on wind direction at the station. Median values of Φ in the south sector are in the range of 2.5–5.7 and show a high variability. In contrast, values for the other directions show a relatively low variability around a median level of 2. These differences can be explained by local effects. It is shown that the height of the sample inlet line, its distance to the forest and the surrounding topography has a strong impact on both the absolute and relative deviations from PSS. Global irradiance and thus, photolysis of NO2 is reduced within the dense forest. Since the reaction of NO with O3 is still proceeding under these conditions, increased NO2/NO ratios are produced locally in air which is transported through the forest and advected to the MOHp site. Estimates of the peroxy radical concentration (RO2) inferred from PSS are compared with peroxy radical measurements made at the site in June 2000 in a three week campaign. The PSS derived RO2 levels were higher than corresponding measured levels by at least a factor of 2–3. This analysis was made for a wind sector with minimal local effects on PSS. Thus the corresponding Φ median of 2 can be regarded as an upper limit for a deviation from PSS due to chemical reactions, i.e. by peroxy radicals and possible other oxidants converting additional NO to NO2.