Volatile Organic Compounds (VOCs) play a central role in the chemistry of the atmosphere, as they are involved in many processes that affect regional air quality and global climate change. However, a growing body of evidence suggests an incomplete understanding of the VOC budget, bringing into question the reliability of atmospheric models to address current environmental issues. To address this concern, we propose to develop an original analytical tool and its deployment in the field to provide new information on the sources and chemistry of atmospheric VOCs.This new tool relies on coupling the Comparative Reactivity Method with a Proton Transfer Reaction Time-Of-Flight Mass Spectrometer (CRM-PTR-TOFMS). The CRM-PTR-TOFMS will be tested for fast measurements of total OH reactivity and a full suite of poorly characterized oxygenated VOCs, including carbonyls, dicarbonyls, carboxylic acids and peroxyacyl nitrates. In addition, a new methodology will be developed to detect and identify potentially important unknown VOCs. Measurements provided by the field deployment of this instrument will be used to perform a detailed characterization of the VOC budget. These results will help to assess whether there is a gap in our understanding of the VOC budget, its potential impact on strategies of pollution control, and whether it is important to fill this gap.The outcomes of the project will benefit the atmospheric science community by advancing our knowledge of the VOC sources and chemistry. It will promote excellence in European research through the transfer of knowledge from the applicant to the host institution in the field of OH reactivity measurements. This project will also provide excellent research and educational opportunities for the applicant to further his career goals and to secure a research position in his home country.