Intraseasonal variability of surface ozone in Santiago, Chile: Modulation by phase of the Madden–Julian Oscillation (MJO)

Abstract

In Santiago, Chile, summertime surface ozone (O3) concentrations regularly exceed local and international health thresholds due to high antecedent pollutants, frequent clear skies, and warm surface air temperatures. However, few (if any) studies exist that have examined the intraseasonal variability of surface O3 or its modulation by phase of the Madden–Julian Oscillation (MJO). Therefore, the main objectives of this study were to investigate the intraseasonal variability of surface O3 and the meteorological parameters known to affect O3 concentrations during summer months in Santiago, and connect any observed variability to phase of the MJO. Ozone concentrations at seven stations in the Chilean National Air Quality Information System (SINCA), along with upper-air, surface, and reanalysis data, were used to create composites for each phase of the MJO. Results confirm that for the Santiago metropolitan region, both maximum daily O3 concentrations, as well as the diurnal cycle of O3, depend on MJO Phase. Ozone concentrations were highest during Phases 5 and 6 and lowest during Phases 1 and 2. Cloud cover anomalies best agreed with this pattern of O3 variability, with low (high) cloud cover anomalies occurring during days with high (low) ozone. Surface temperature and strength and height of the lower-troposphere temperature inversion had similar, but less pronounced, connections to O3, with slightly warmer surface temperatures and stronger inversions closer to the ground occurring on days with higher O3. Wind velocity was found to vary little between days with low and high ozone.