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Assessment of the applicability of NO-NO<sub>2</sub>-O<sub>3</sub> photostationary state to long-term measurements at the Hohenpeissenberg GAW Station, Germany

by K. Mannschreck, S. Gilge, C. Plass-Duelmer, W. Fricke, H. Berresheim
Atmospheric Chemistry and Physics ()


Continuous measurements of concns. of reactive gases, radiation, and meteorol. parameters are carried out at the Meteorol. Observatory Hohenpeissenberg (MOHp) as part of the Global Atm. Watch (GAW) Program. NO, NO2, O3 and JNO2 data from a four-year period (Mar. 1999-Dec. 2002) are evaluated for consistency with photochem. steady state (PSS, Φ=1) conditions. 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. When taking into account peroxy radical concns. (Φext=1) PSS was reached in 13-32% of all cases for the years 1999-2002. The differences in wind sectors 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 topog. has a strong impact on both the abs. 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. Ests. of the peroxy radical concn. (RO2) inferred from PSS are compared with peroxy radical measurements made at the site in June 2000 during a three-week campaign. The PSS derived RO2 levels were higher than corresponding measured levels by at least a factor of 2-3. This anal. was made for a wind sector with minimal local effects of PSS. Thus the corresponding Φ median of 2 can be regarded as an upper limit for a deviation from PSS due to chem. reactions, i.e. by peroxy radicals and possible other oxidants converting addnl. NO to NO2. [on SciFinder(R)]

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