A three-dimensional model study of the effect of new temperature-dependent quantum yields for acetone photolysis

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Abstract

[1] We have used the TOMCAT three-dimensional chemical transport model (CTM) to investigate the impact of recent laboratory measurements of the temperature dependence of the acetone photolysis quantum yield on global tropospheric chemistry. The new acetone quantum yields cause a significant decrease in the calculated acetone loss rate in troposphere. The annual global mean photolysis loss of acetone is reduced by a factor of ∼2, making OH oxidation the dominant acetone sink. Photolysis rates decrease by between ∼80 and 90% in the cold upper troposphere (UT). The atmospheric lifetime of acetone increases from 22 to 35 days, with an increase in the global burden from 2.6 to 4.1 Tg. This is maintained through a global source strength of 42.5 Tg/yr, which is approximately half of that inferred by some previous model studies. Comparisons of modeled and observed acetone profiles from the remote tropical Pacific demonstrate much improved agreement with the new quantum yields, with a reduction in the model bias relative to aircraft observations from −50 to −17%. With the new quantum yields, modeled peroxyacetylnitrate (PAN) decreases in the UT and throughout the Northern Hemisphere. PAN increases are modeled in Southern Hemisphere, as the increases in acetone outweigh the slower rate of peroxyacetyl production. The new quantum yields reduce the model HOx(=OH + HO2) throughout the troposphere. The locations of largest changes to HOx and the OH:HO2 ratio, caused by changes in NOx, mean the impact on model global OH is small (−0.5%). The net effect of using the new quantum yields on tropospheric ozone is also small; the model predicts a maximum 1% decrease in the Northern Hemisphere lower troposphere.

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