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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D12305, doi:10.1029/2006JD008268, 2007

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

Yaping Xiao

Department of Earth and Planetary Sciences and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA


Daniel J. Jacob

Department of Earth and Planetary Sciences and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA


Solene Turquety

Department of Earth and Planetary Sciences and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA


Abstract

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 C2H2 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 dC2H2/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[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 β 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. 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.

Received 20 November 2006; accepted 5 March 2007; published 21 June 2007.

Keywords: Acetylene; nonmethane hydrocarbons; emission inventory; C2H2-CO correlation; air mass age.

Index Terms: 0365 Atmospheric Composition and Structure: Troposphere: composition and chemistry; 0466 Biogeosciences: Modeling; 0490 Biogeosciences: Trace gases; 3399 Atmospheric Processes: General or miscellaneous; 4808 Oceanography: Biological and Chemical: Chemical tracers.

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Citation: Xiao, Y., D. J. Jacob, and S. Turquety (2007), Atmospheric acetylene and its relationship with CO as an indicator of air mass age, J. Geophys. Res., 112, D12305, doi:10.1029/2006JD008268.