METROCLIMA-MASP: pioneering urban greenhouse gas monitoring in South America’s largest megacity

• The establishment of a GHG network in São Paulo, the largest megacity in South America, contributes to understanding the role of urban areas in global climate change and carbon footprints. • In São Paulo, major contributors to GHG include vehicular emissions, industrial activities, waste decomposition, and natural gas use . • The monitoring data reveal spatial and temporal variability in GHG mole fractions, driven by differences in land use, topography, and emission source types, and include the characterization of an urban background station. Cities account for over half of global greenhouse gas (GHG) emissions, yet monitoring is very limited, especially in the Global South. A GHG monitoring network was established in 2019 in São Paulo, the largest metropolitan area in the Southern Hemisphere. Five sites were strategically selected, including four measuring surface mole fraction of carbon monoxide (CO), carbon dioxide (CO 2 ), and methane (CH 4 ): Pico do Jaraguá (PDJ; –23.46°, −46.77°), IAG (–23.56°, −46.73°), UNICID (–23.54°, −46.56°), and ICESP (–23.56°, −46.67°). The fifth site, CIENTEC (–23.65°, −46.62°), was dedicated to measuring CO 2 fluxes within a vegetated area. At IAG, δ 13 C-CO 2 was also measured. The results revealed the influence of local sources, vegetation, and surface characteristics on GHG patterns. The elevated, forested site of PDJ provided urban background GHG levels, while urban core sites were heavily affected by traffic and biomass burning. CO 2 levels increased across the city, with annual growth rates of 4.86 ± 2.84 ppm at UNICID (from 2021 to 2024), 4.92 ± 0.56 ppm at IAG (from 2021 to 2024), and 1.67 ± 0.38 ppm at PDJ (2019–2024). CH 4 levels were more stable, exhibiting minor seasonal variability and no clear trend at PDJ. At UNICID, strong CO 2 -CO correlations (r > 0.9) during traffic peaks suggested emissions dominated by combustion, while weaker correlations during outside peak hours (r < 0.7) indicated the uptake of CO 2 by the vegetation. Diurnal CO 2 patterns across stations reflected the balance between human activity and biogenic processes, driven by photosynthesis and respiration, emphasizing the role of vegetation in modulating urban CO 2 levels. These results highlight the importance of urban GHG measurements for understanding emission dynamics, assessing the influence of urban structure and meteorology, and supporting mitigation strategies at the city scale. This work clearly highlights the need to establish similar networks across Latin America to fill this critical data gap, which is key for developing effective climate mitigation strategies.