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

Continuous measurements of concentrations of re- active gases, radiation, and meteorological parameters are carried out at the Meteorological Observatory Hohenpeis- senberg (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 evalu- ated for consistency with photochemical steady state (PSS, 8=1) conditions. The extent of deviation from PSS reveals a strong dependence on wind direction at the station. Median values of 8 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 concen- trations (8ext =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 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 trans- ported through the forest and advected to the MOHp site. Estimates of the peroxy radical concentration (RO2) in- ferred from PSS are compared with peroxy radical measure- ments made at the site in June 2000 during a three-week cam- paign. The PSS derived RO2 levels were higher than corre- sponding measured levels by at least a factor of 2-3. This analysis was made for a wind sector with minimal local ef- fects on PSS. Thus the corresponding 8 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.