Biomass burning contributions to Mexico City's atmospheric CO₂ estimated using a multi-isotope approach

Fossil fuel combustion dominates anthropogenic emissions worldwide; however, special attention should be addressed to biomass burning, since it is an increasingly important contributor under warmer, drier fire-weather conditions exacerbated by climate change. These emissions could impact the atmospheric composition of heavily urbanized environments. To assess the influence of biomass burning, along with fossil fuels combustion and soil and plant respiration on Mexico City's atmospheric composition, we conducted a year-long (December 2023-December 2024) isotopic monitoring of atmospheric CO₂, combining radiocarbon (Δ¹⁴C), CO₂ stable isotopes (δ¹³C, δ¹⁸O), and CO/CO₂ ratios. Once defined the isotopic signatures and the characteristic CO/CO₂ that describe the sources, we performed Monte Carlo simulations under two tracer modalities (Δ¹⁴C + CO/CO₂ and Δ¹⁴C + CO₂ stable isotopes) for source apportionment. Results show that during the dry season, particularly in March-April, atmospheric Δ¹⁴C values approached and overlapped background levels despite Mexico City's fossil-fuel dominance, indicating enhanced non-fossil inputs. Moreover, backward HYSPLIT trajectories supported that these elevated Δ¹⁴C values coincided with regional wildfire activity. Monte Carlo results attributed up to ∼34 %-60 % of local CO₂ to biomass burning in April, followed by March with contributions of ∼31 %-49 %. Since these biomass burning figures contrast with the official emissions inventory of Mexico City metropolitan area, which assigns <1 % of total CO₂ emissions to biomass burning, this study could exhibit the emissions inventory underestimation of this source, and thus, the need to reassess and incorporate top-down isotopic constraints in fire-affected urban regions.