Atmos. Chem. Phys., 4, 1265-1277, 2004
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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 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 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 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.