Heatwave Driven Urban Ozone Extremes Modulated by Circulation and Pollution Regimes in Brazilian Metropolitan Areas

Under continued climate change, urban ozone pollution emerges as a multistressor whose processes governing its interactions with extreme events remain poorly characterized, particularly the enhancement of concentrations during heatwaves and the occurrence of compound heat and ozone episodes. In Brazil, this issue is further shaped by a large vehicular fleet operating on biofuels, emitting substantial amounts of volatile organic compounds (VOCs), key precursors of tropospheric ozone. This study presents a quasi continental, long term assessment (2010–2025) of ozone trends during heatwaves across distinct pollution and meteorological regimes. We integrate in situ measurements of ozone, meteorological variables and co occurring pollutants such as PM_2.5, NOx, and VOCs in 8 of the larger metropolitan areas in southern, southeastern and northeastern Brazil. Highest MDA8hO₃ levels were observed in the metropolitan regions of Rio de Janeiro and São Paulo, notably in spring and summer, with frequent exceedances of the air quality standard (100 µg m⁻³). In the southern and northeastern regions, concentrations are lower, with median values of 25-50 µg m⁻³. Recirculation and stagnation regimes are associated with higher pollution levels in most cities, although Salvador, Belo Horizonte and Rio de Janeiro exhibit an increase in ozone under ventilated conditions. Atmospheric circulation also modulates the temperature ozone relationship, with clear differences among climatic regions. Precursor rich areas (high NOx and more reactive VOCs) show a stronger association between MDA8hO₃ and daily maximum temperature. Ozone enhancement during heatwaves varies regionally with pollution levels and prevailing circulation. In São Paulo, at stations directly influenced by vehicular emissions, ozone enhancement reaches 52% under heatwave conditions combined with stagnation. Ozone waves occurred in all southeastern cities within high emission zones. Consequently, combined heat and ozone waves were also detected, especially in Belo Horizonte, where 61% of ozone wave events occurred during heatwaves in spring and summer. These findings indicate that ongoing climate change, through the intensification of heatwaves, is likely to increase the frequency of high ozone episodes in densely populated areas, posing escalating public health challenges. The results underscore the need for integrated mitigation and adaptation strategies, including strengthened control of ozone precursor emissions, particularly within the Brazilian context of biofuel driven vehicular emissions.

This work was developed under the scope of Project OVERHEAT.SA - COllaboratiVE Research on Compound Drought and HEATWave events in South America financed by CNPQ grant number 443285/2023-3.