Changes in South American Surface Ozone Trends: Exploring the Influences of Precursors and Extreme Events

Seguel, Rodrigo J.; Castillo, Lucas; Opazo, Charlie; Rojas, Néstor Y.; Nogueira, Thiago; Cazorla, María; Gavidia-Calderón, Mario; Gallardo, Laura; Garreaud, René; Carrasco-Escaff, Tomás; Elshorbany, Yasin

In this study, 21st-century ground-level ozone trends and its precursors in South America were examined, which is an understudied region where trend estimates have rarely been comprehensively addressed. Therefore, we provided an updated regional analysis based on validated surface observations. We tested the hypothesis that the recent increasing ozone trends, mostly in urban environments, resulted from intense wildfires driven by extreme meteorological events impacting cities where preexisting volatile organic compound (VOC)-limited regimes dominate. We applied the quantile regression method to estimate trends, quantify their uncertainties, and detect trend change points. Additionally, the maximum daily 8-hour average (MDA8) and peak-season metrics were used to assess present-day short- and long-term exposure levels (2017–2021). Our results showed lower levels in tropical cities (Bogotá and Quito), varying between 39 and 43 ppbv for short-term exposure and between 26 and 27 ppbv for long-term exposure. In contrast, ozone mixing ratios were higher in extratropical cities (Santiago and São Paulo), with a short-term exposure level of 61 ppbv and long-term exposure levels varying between 40 and 41 ppbv. Santiago (since 2017) and São Paulo (since 2008) exhibited positive trends of 0.6 and 0.3 ppbv yr-1, respectively, with very high certainty. We attributed these upward trends, or no evidence of variation, such as in Bogotá and Quito, to a well-established VOC-limited regime. However, we attributed the greater increase in the extreme percentile trends (≥ 90th) to heat waves and, in the case of southwestern South America, to wildfires associated with extreme meteorological events.

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Seguel, Rodrigo J. / Castillo, Lucas / Opazo, Charlie / et al: Changes in South American Surface Ozone Trends: Exploring the Influences of Precursors and Extreme Events. 2024. Copernicus Publications.

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