Tracing CO₂ sources during biomass burning seasons (2020-2023) using δ¹³C-CO₂ and air pollutants in São Paulo, Brazil

The stable isotopic composition of carbon, such as δ¹³C-CO₂, is a valuable tool for tracing the sources of atmospheric CO₂. Within this framework, the METROCLIMA project (www.metroclima.iag.usp.br) aims to develop and evaluate methodologies for quantifying urban greenhouse gas emissions and distinguishing between biogenic and anthropogenic sources. We analyzed data from a METROCLIMA station equipped with a cavity ring-down spectroscopy instrument (Picarro), which continuously measures δ¹³C-CO₂ and CO₂ in real time. The station is located in a partially vegetated urban area at the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG), University of São Paulo (USP) (-23.559478, -46.733533). This study focuses on characterizing total urban CO₂ and identifying potential CO₂ sources during the wildfire seasons (August and September) across four years (2020–2023). Additionally, we analyzed PM₁₀, PM₂.₅, O₃, and NOₓ data from the CETESB monitoring station in Pinheiros (São Paulo’s Environmental Protection Agency).

The isotopic signatures observed during the spring seasons indicate a predominant influence of biomass burning (δ¹³C-CO₂ = −22‰ to -25‰) and fossil fuel combustion (δ¹³C-CO₂ = −30‰ to -37‰), as revealed by Keeling plot analyses (based on intercepts from δ¹³C-CO₂ versus 1/CO₂). HYSPLIT backward trajectory analyses suggest that some of the air masses were likely influenced by plumes originating from the Pantanal and the Atlantic Forest (Mata Atlântica) regions. Other identified source was local wood burning, contributing to a lesser extent each year. Moreover, the temporal behavior of isotopic signals was consistent with CO₂ concentrations and CETESB pollutant data, indicating a strong influence of wildfire plumes on CO₂ levels and air pollutants such as PM₁₀, PM₂.₅, O₃, and CO. Notably, the 2022 wildfire season presented the highest CO₂ concentrations and pollutant levels among the years analyzed.

Here, we provide an initial assessment of CO₂ sources during biomass burning seasons in São Paulo, offering valuable insights that could support future greenhouse gas mitigation strategies.