TEMPERATURE-DEPENDENT ABSORPTION CROSS-SECTIONS FOR ACETONE AND NORMAL-BUTANONE - IMPLICATIONS FOR ATMOSPHERIC LIFETIMES

Temperature dependent absorption cross-sections for acetone and n-butanone have been measured over the temperature range 260-360 K for 260 nm < lambda < 360 nm. The temperature dependence of the cross-sections was fitted using simple, two-level models to allow the measured cross-sections to be extrapolated to upper tropospheric/lower stratospheric temperatures. Atmospheric photoexcitation rates were calculated as a function of altitude using both room temperature and temperature dependent cross-sections Absorption in the 310-330 nm region makes the dominant contribution to solar photoexcitation in the troposphere and lower stratosphere. Neglect of the temperature dependence of the absorption cross-sections results in significant over-estimations of the solar photoexcitation rate for both acetone and n-butanone in the upper troposphere and lower stratosphere. Since the atmospheric lifetimes of these compounds are inversely proportional to the photoexcitation rate, current models may underestimate the upper atmospheric lifetimes of these compounds by almost-equal-to 50%.