Surface Ozone Trends in South America: Unraveling the Influence of Precursor Shifts and Extreme Events

This work analyzes ground-level ozone trends in South America, an understudied region with scarce comprehensive trend estimates. We present an updated regional analysis and test a hypothesis proposing that the recent increase in ozone levels, particularly in urban environments, may be linked to intense wildfires induced by extreme meteorological events within a preexisting volatile organic compounds (VOC)-limited regime. Utilizing the quantile regression method, we estimate trends, quantify uncertainties, and identify change points. Short- and long-term exposure is assessed using the maximum daily 8-hour average and peak season metrics. Our findings reveal lower ozone levels in tropical cities (Bogotá and Quito), ranging between 39-43 ppbv for short-term and 26-27 for long-term exposure. In contrast, extratropical cities (Santiago and São Paulo) exhibit higher ozone levels, with short-term exposure at 61 ppbv and long-term exposures between 40-41 ppbv. Santiago (since 2017) and São Paulo (since 2008) show positive trends of 0.6 ppbv yr^-1 and 0.2 ppbv yr^-1, respectively, with very high certainty. We attribute these upward trends, or the absence of evidence of variation as observed in Bogotá and Quito, to the established VOC-limited regime. However, the higher increase in extreme percentile trends (≥ 90th) is linked to the impact of wildfires and biomass burning, particularly in southwestern South America, associated with extreme meteorological configurations.