Ground-based PTR-MS measurements of reactive organic compounds during the MINOS campaign in Crete, July-August 2001

G. Salisbury1, J. Williams1, R. Holzinger1, L. Lange1, X. Xu1, N. Mihalopoulos2, M. Vrekoussis2, J. Sciare3, H. Berresheim4, M. Lawrence1, R. von Kuhlmann1 and J. Lelieveld1

1 Department of Air Chemistry, Max-Planck-Institute for Chemistry, Mainz, Germany
2 Department of Chemistry, University of Crete, Heraklion, Crete, Greece
3 Laboratoire des Sciences du Climat et de l'Environnement, Gif/Yvette, France
4 German Weather Service, Hohenpeissenburg, Germany


This study presents measurements of acetonitrile, benzene, toluene, methanol and acetone made using the proton-transfer-reaction mass spectrometry (PTR-MS) technique at the Finokalia ground station in Crete during the Mediterranean Intensive Oxidant Study (MINOS) in July-August 2001. Five-day back-trajectory analysis based on ECMWF data shows that most of the air sampled during the campaign arrived at Finokalia from the north, having passed over the Aegean Sea; three periods with broadly consistent back trajectories are distinguished. In the first (29th July-3rd August), air advected from Eastern Europe without significant biomass burning influence (mean acetonitrile mixing ratio 173 pmol/mol) was observed. In the second period (4th-7th August), the sampled air masses originated in Western Europe, and were advected approximately east-south-east, before turning south-west over the Black Sea and north-western Turkey. The third well-defined period (8th-12th August) included air masses advected from Eastern Europe passing east and south of/over the Sea of Azov, and showed significant influence by biomass burning (mean acetonitrile mixing ratio 434 pmol/mol). The mean toluene/benzene ratios observed in the three campaign periods described were 0.37, 0.37 and 0.22, respectively; the use of this quantity to determine air mass age is discussed. Methanol and acetone were generally well-correlated both with each other and with carbon monoxide throughout the campaign; the mean acetone/CO, methanol/CO and acetonitrile/CO ratios during the first period of the campaign, where air of Eastern European origin was observed and biomass burning influence was at its minimum, were 0.020, 0.022 and 0.001, respectively. The relative importance to the methanol and acetone budgets of both direct emission from anthropogenic and biogenic sources and secondary photochemical production is discussed in the light of a modeling study covering the campaign period.