Surface ozone trends and precursor attribution in South America

Several Working Groups have been established within the frame of the second phase of the Tropospheric Ozone Assessment Report (TOAR-II). The Tropospheric Ozone Precursors focus Working Group (TOP WG) aims to examine the current regional and global distribution, variability and trends of ozone precursors. Part of our strategy has been to analyze in greater detail different regions of the globe. In particular, this work addresses South America, a region characterized by densely populated urban areas with high air pollution levels.

We use data from air quality monitoring networks that measure surface-level ozone, nitrogen oxides, carbon monoxide and meteorological variables. To date, we have validated and incorporated monitoring stations from Brazil (51), Chile (18), Colombia (13) and Ecuador (6) into our central database. To evaluate short- and long-term ozone exposure, we use the maximum daily 8-hour average (MDA8) and the peak season guideline proposed by the World Health Organization (WHO) set at 51 and 31 ppbv, respectively. We applied the Quantile Regression (QR) method to analyze ozone and precursor network trends. We also identified points in time series that mark changes in trends through a piecewise function.

The highest MDA8 ozone for 2015-2021 was found in São Paulo (52 ppbv) and Santiago (51 ppbv). In São Paulo, the short-term ozone exposure decreased by 7% compared to the average of the years prior to 2015 (period analyzed in TOAR phase I), while in Santiago, it increased by 10%. In Bogota and Quito, the MDA8 complied with the WHO guidelines (33 and 32 ppbv, respectively). Similarly, the long-term ozone exposure guideline was exceeded in São Paulo (39 ppbv) and Santiago (40 ppbv), while Bogota (25 ppbv) and Quito (26 ppbv) complied. The trend analysis showed that Quito was the only city with a negative ozone trend of -0.10 ppb/year (50th percentile) for the analyzed period. In turn, the São Paulo trend increased after 2008 (0.43 ppbv/year), while Santiago and Bogota have increased since 2017 (0.93 ppbv/year and 1.3 ppbv/year, respectively). We highlight that the positive trend in Santiago is driven mainly by the high percentiles (>70th). Underlying processes that explain trends involve more efficient photochemical ozone formation (e.g., NO₂/NO_x trend) and meteorological factors.

This ongoing work aims to include more South American cities and background stations already available in the new TOAR database. Finally, we will project ozone trends for the next decade using machine learning techniques (random forest) under precursor emission scenarios and temperature projections.