Atmospheric acetylene and its relationship with CO as an indicator of air mass age
Acetylene (C2H2) and CO originating from combustion are strongly correlated in atmospheric observations, offering constraints on atmospheric dilution and chemical aging. We examine 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). The slope beta=dlog[C2H2]/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 beta relative to the square root of the ratio of C2H2 and CO chemical lifetimes (1.7-1.9) measures the relative importance of dilution and chemistry.
To test the actual relative importance of chemical aging vs. dilution in GEOS-Chem, we conducted model simulations with OH increased or decreased uniformly by 50% and examined the sensitivity of the dlog[C2H2]/dlog[CO] slopes. As shown in above figure, the simulated slopes show least sensitivity to OH in the Asian and African fresh outflow (regions 1 and 7), highest sensitivity over the South Pacific (region 5 and 6) where the model slopes are highest, and moderate sensitivity over the northwestern Pacific (region 4) and eastern North America (region 8) in summer. We thus find that aging is dominated by dilution in fresh outflow but by chemical loss in remote air. The sensitivity of beta to OH concentrations over the South Pacific, combined with the model overestimate of beta suggests a model overestimate of OH in that region. Matching the observed beta would require decreasing model OH by more than a factor of 2. Further investigation is needed of a possible error in the OH simulation in the southern tropics. A full description of this work is given in Xiao et al. [2007a] .