The impact of acetone photolysis to the production of HOx around the tropopause

Since May 2005 the CARIBIC passenger aircraft (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container - Lufthansa, Airbus 340-600) measures ∼100 trace gases and aerosol components in the UTLS (9-12 km altitude) on four consecutive long-distance flights per month. Acetone along with other VOCs like acetonitrile and methanol is measured with a PTRMS (proton-transfer-reaction mass spectrometer). The contribution of acetone photolysis to HOx formation around the tropopause (TP) is compared to primary HOx production from ozone photolysis and subsequent reaction with water vapor. The presented data analysis is in large part based on CARIBIC in-situ data of acetone, ozone and water and takes into account newer acetone photolysis rates (recommended by the JPL in Evaluation No. 17, JPL Publication 10-6) that question the former picture of acetone as being an important HOx precursor in the dryer upper TP region. Modeling parameters not easily accessible via experiment like photolysis rates were taken from the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. It can be shown that acetone photolysis contributes up to ~40 % to the total (= acetone and ozone photolysis) HOx production rate at the TP in autumn and around ~30 % in summer. Circa 2 km above the TP the contribution from acetone photolysis becomes smaller due to the quickly decreasing mixing ratio of acetone at higher altitudes ( up to ~20 % in summer and up to ~ 30% in autumn). The findings are contrasted to results based on older photolysis rates that are larger by a factor of ~3.5 in summer at cruising altitudes (~ 9-12 km). Furthermore, as a sensitivity study acetone, ozone and water data from the EMAC model were used as input parameters in order to assess the reliability of the above data analysis.