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Abstract

Atmospheric oxygenated hydrocarbons (OHCs), including acetone, were measured at altitudes between 8.5 and 12.5 km using an aircraft-based chemical ionization mass spectrometer (CIMS) instrument equipped with an ion trap (IT) mass spectrometer. The IT-CIMS instrument offers a high mass resolution. It also provided a unique opportunity to carry out fragment ion investigations of mass selected ions, which greatly improved our ability to identify trace gases. Furthermore, in the IT, water molecules are "boiled off" from hydrated ions which simplifies core ion identification and greatly reduces the risk of masking of product ions by reagent ions. Acetone could be unambiguously identified in the upper troposphere (UT)/lower stratosphere (LS) by a combination of isotope and fragment ion investigations. A protonated molecule B with mass m = 73 could for the first time be detected in the UT/LS by CIMS due to the absence of the otherwise masking reagent ion H+(H2O)(4) (m = 73). The atmospheric volume mixing ratio (VMR) of the molecule B (in = 72) is about 150 pptv in the UT and decreases very steeply above the local tropopause indicating that B has a shorter lifetime than acetone. In the UT, the abundance ratio of B and acetone was found to be around 0.3. Additional observed molecules have mass 86 and 100. (C) 2002 Elsevier Science B.V. All rights reserved.