Measurements of peroxy radicals in a forested area of Portugal

As part of the FIELDVOC'94 study, ambient concentrations of peroxy radicals (RO₂, R=H, CH₃, CH₃CO, etc.) were continuously measured at a forested site in Portugal, using the chemical amplification technique. The amount of RO₂ varied between 0 and 260 pptv with maximum values being obtained under clear sky conditions. The RO₂ mixing ratio correlates with that of isoprene. The rate of production of ozone, O₃, indicates that the noon-time maximum values lay between 5 and 17 ppbv h⁻¹. On several days, the maximum O₃ mixing ratio at this rural site was similar to that found in urban areas, which experience smog episodes (O₃ mixing ratio > 90 ppbv). The RO₂ estimated assuming a photostationary state RO₂,(PSS) of NO₂, NO and O₃, was not found to have a significant statistical correlation with the measured RO₂. The latter is best explained by the errors associated with the calculation of RO₂,(PSS) at low NO_x (NO + NO₂) mixing ratios. The diurnal behavior of O₃ in the boundary layer was estimated using a relatively simple source/sink parameterization. On days having high O₃ mixing ratios, both predicted and observed O₃ behavior agree well. For days having O₃ mixing ratios < 75 ppbv, it was found that the simple parameterization needed to be extended to include entrainment of O₃ after dawn from above the night-time inversion layer.