Inter-comparison between airborne measurements of methanol, acetonitrile and acetone using two differently configured PTR-MS instruments

Proton-transfer-reaction mass spectrometry (PTR-MS) has emerged as a useful tool to study the atmospheric chemistry of volatile organic compounds (VOCs). The technique combines a fast response time with a low detection limit, and allows atmospheric measurements of many important VOCs and their oxidation products. Here, we inter-compare the results obtained with two differently configured PTR-MS instruments operated onboard a Falcon aircraft during the Mediterranean Intensive Oxidants Study (MINOS) campaign in the Mediterranean region. One PTR-MS was operated at a drift tube pressure of 2.3 mbar and an electric field divided by gas number density value (E/N) of 120 Td for the detection of VOCs and aromatic hydrocarbons. The other PTR-MS was operated at an increased pressure of 2.8 mbar and a reduced E/N of 97 Td for the detection of peroxyacetyl nitrate (PAN). As a consequence, more H3O+(H2O)(n) cluster ions were present in the drift tube, which undergo proton-transfer reactions with VOCs similar to H3O+ ions. The results for methanol (CH3OH), acetonitrile (CH3CN) and acetone (CH3COCH3) obtained with the instruments compared very well. The agreement between the two results was relatively independent of the ambient mixing ratio of water, which influences the H3O+(H2O)(n) cluster ion distribution, and of ozone, which has been implicated in the artificial formation of aldehydes and ketones. (C) 2004 Elsevier B.V. All rights reserved.