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[1] Acetylene (C2H2) and CO originating from combustion are strongly correlated in atmospheric observations, offering constraints on atmospheric dilution and chemical aging. We examine here the C2H2-CO relationships in aircraft observations worldwide, and interpret them with simple models as well as with a global chemical transport model (GEOS-Chem). A C2H2 global source of 6.6 Tg yr 1 in GEOS-Chem simulates the ensemble of global C 2H 2 observations without systematic bias, and captures most seasonal and regional features. C2H2/CO concentration ratios decrease from continental source regions to the remote atmosphere in a manner consistent between the observations and the model. However, the dC 2H 2/dCO slope from the linear regression does not show such a systematic decrease, either in the model or in the observations, reflecting variability in background air. The slope b = dlog[C 2H 2]/dlog[CO] of the linear regression of concentrations in log space offers information for separating the influences of dilution and chemical aging. We find that a linear mixing model with constant dilution rate and background is successful in fresh continental outflow but not in remote air. A diffusion model provides a better conceptual framework for interpreting the observations, where the value of b relative to the square root of the ratio of C 2H 2 and CO chemical lifetimes (1.7–1.9) measures the relative importance of dilution and chemistry. We thus find that dilution dominates in fresh outflow but chemical loss dominates in remote air. This result is supported by GEOS-Chem sensitivity simulations with modified OH concentrations, and suggests that the model overestimates OH in the southern tropics