1Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Unité Mixte CEA-CNRS-UVSQ, UMR8212, (Commissariat à l'Energie Atomique, Centre National de la Recherche Scientifique, Université de Versailles Saint-Quentin-en-Yvelines), 91198 Gif-sur-Yv
2Indian Institute of Science Education and Research (IISER) Mohali, Sector 81 SAS Nagar, Manauli PO Punjab 140306, India
3Max Planck Institute for Chemistry, Air Chemistry Department, 55128 Mainz, Germany
4Aix Marseille Université, Laboratoire Chimie Environnement, 3 place Victor, 13331 Marseille, France
5Université de Lille Nord de France, 59000 Lille, France
6Ecole de Mines Douai, Departement Chimie environnement, 59508 Douai, France
7Leibniz – Institut für Troposphärenforschung (IFT), Leipzig, Germany
Abstract. Hydroxyl radicals play a central role in the troposphere as they control the lifetime of many trace gases. Measurement of OH reactivity (OH loss rate) is important to better constrain the OH budget and also to evaluate the completeness of measured VOC budget. Total atmospheric OH reactivity was measured for the first time in an European Megacity: Paris and its surrounding areas with 12 million inhabitants, during the MEGAPOLI winter campaign 2010. The method deployed was the Comparative Reactivity Method (CRM). The measured dataset contains both measured and calculated OH reactivity from CO, NOx and VOCs measured via PTR-MS, GC-FID and GC-MS instruments. The reactivities observed in Paris covered a range from 10 s−1 to 130 s−1, indicating a large loading of chemical reactants. The present study showed that, when clean marine air masses influenced Paris, the purely local OH reactivity (20 s−1) is well explained by the measured species. Nevertheless, when there is a continental import of air masses, high levels of OH reactivity were obtained (120–130 s−1) and the missing OH reactivity measured in this case jumped to 75%. Using covariations of the missing OH reactivity to secondary inorganic species in fine aerosols, we suggest that the missing OH reactants were most likely highly oxidized compounds issued from photochemically processed air masses of anthropogenic origin.