Atmospheric acetylene and its relationship with CO as an indicator of air mass age

Abstract

Acetylene (C 2 H 2) and CO originating from combustion are strongly correlated in atmospheric observations, offering constraints on atmospheric dilution and chemical aging. We examine here the C 2 H 2 -CO relationships in aircraft observations worldwide, and interpret them with simple models as well as with a global chemical transport model (GEOS-Chem). A C 2 H 2 global source of 6.6 Tg yr -1 in GEOS-Chem simulates the ensemble of global C 2 H 2 observations without systematic bias, and captures most seasonal and regional features. C 2 H 2 /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 2 H 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 β = dlog[C 2 H 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 β relative to the square root of the ratio of C 2 H 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. Copyright 2007 by the American Geophysical Union.