On the variability of ozone in the equatorial eastern Pacific boundary layer

Observations of surface ozone (O-3) mixing ratios carried out during two ground-based field campaigns in the Galapagos Islands are reported. The first campaign, Primera Investigacion sobre la Quimica, Evolucion y Reparto de Ozono, was carried out from September 2000 to July 2002. The second study, Climate and HAlogen Reactivity tropicaL EXperiment, was conducted from September 2010 to March 2012. These measurements complement the Southern Hemisphere ADditional OZonesonde observations made with weekly to monthly frequency at Galapagos. In this work, the daily, intraseasonal, seasonal and interannual variability of O-3 in the marine boundary layer are described and compared to those observed in other tropical locations. The O-3 diurnal cycle shows two regimes: (i) photochemical destruction followed by nighttime recovery in the cold season (July to November) and (ii) daytime advection and photochemical loss followed by nighttime depositional loss associated to windless conditions in the warm season (February to April). Wavelet spectral analysis of the intraseasonal variability of O-3 reveals components with periods characteristic of tropical instability waves. The O-3 seasonal variation in Galapagos is typical of the Southern Hemisphere, with a maximum in August and a minimum in February-March. Comparison with other measurements in remote tropical ocean locations shows that the change of the surface O-3 seasonal cycle across the equator is explained by the position of the Intertropical Convergence Zone and the O-3 levels upwind.