Comparison of measured and modeled surface ozone concentrations at two different sites in Europe during the solar eclipse on August 11, 1999

The effects of the solar eclipse on 11 August 1999 on surface ozone at two sites, Thessaloniki, Greece (urban site) and Hohenpeissenberg, Germany (ele vated rural site) are investigated in this study and compared with model re sults. The eclipse offered a unique opportunity to test our understanding o f tropospheric ozone chemistry and to investigate with a simple photochemic al box model the response of surface ozone to changes of solar radiation du ring a photolytical perturbation such as the solar eclipse. The surface ozo ne measurements following the eclipse display a decrease of around 10-15 pp bv at the urban station of Eptapyrgio at Thessaloniki while at Hohenpeissen berg, the actual ozone data do not show any clear effect of eclipse on surf ace ozone. For Thessaloniki, the model results suggest that solely photoche mistry can account for a significant amount of the observed surface ozone d ecrease during the eclipse but transport effects mask part of the photochem ical effect of eclipse on surface ozone. For Hohenpeissenberg, the box mode l predicted an ozone decrease, due to the eclipse, of about 2 ppbv in relat ive agreement with the magnitude of the observed ozone decrease from the 2 h moving average while at the same time it inhibits the foreseen diurnal oz one increase. However, this modeled ozone decrease during the eclipse is sm all compared to the diurnal ozone variability due to transport effects, and hence. transport really masks such relative small changes. The different m agnitude of the surface ozone decrease between the two sites indicates main ly the role of the NOx levels. Measured and modeled NO and NO2 concentratio ns at Hohenpeissenberg during the eclipse are also compared and indicate th at the partitioning of NO and NO2 in NOx is influenced clearly from the ecl ipse. This is not observed at Thessaloniki due to local NOx sources. (C) 20 01 Elsevier Science Ltd. All rights reserved.