Observational Evidence of Large Contribution from Primary Sources for Carbon Monoxide in the South Asian Outflow

South Asian air is
among the most polluted in the world, causing
premature death of millions and asserting a strong perturbation of
the regional climate. A central component is carbon monoxide (CO),
which is a key modulator of the oxidizing capacity of the atmosphere
and a potent indirect greenhouse gas. While CO concentrations are
declining elsewhere, South Asia exhibits an increasing trend for unresolved
reasons. In this paper, we use dual-isotope (δ13C
and δ18O) fingerprinting of CO intercepted in the
South Asian outflow to constrain the relative contributions from primary
and secondary CO sources. Results show that combustion-derived primary
sources dominate the wintertime continental CO fingerprint (fprimary ∼ 79 ± 4%), significantly
higher than the global estimate (fprimary ∼ 55 ± 5%). Satellite-based inventory estimates match
isotope-constrained fprimary-CO, suggesting
observational convergence in source characterization and a prospect
for model–observation reconciliation. This “ground-truthing”
emphasizes the pressing need to mitigate incomplete combustion activities
for climate/air quality benefits in South Asia.