Critical assessment of the current state of scientific knowledge, terminology, and research needs concerning the role of organic aerosols in the atmosphere, climate, and global change.

In spite of impressive advances in recent years, our present understanding of organic aerosol (OA) composition, physical and chemical properties, sources and transformation characteristics is still rather limited, and their environmental effects remain highly uncertain. Therefore, the three atmosphere-related projects of the International Geosphere Biosphere Programme (IGBP) – IGAC (International Global Atmospheric Chemistry Project), iLEAPS (Integrated Land Ecosystem Atmosphere Process Study) and SOLAS (Surface Ocean-Lower Atmosphere Study) – organised a workshop with the specific goal of discussing and prioritizing issues related to organic aerosol and their effects on atmospheric processes and climate, providing a basis for future collaborative activities at the international level. Four main topical areas were addressed: (a) sources of OA; (b) formation and transformation of OA; (c) physical and chemical state of OA; (d) atmospheric modelling of OA. Key questions and research priorities regarding these four areas have been synthesized in this paper, and outstanding issues for future research are presented for each topical area. In addition, an effort is made to formulate a basic set of consistent and universally applicable terms and definitions for coherent description of atmospheric OA across different scientific scales and disciplines. In fact, the terminologies used in the past and present scientific literature are not always consistent, and this may lead to misunderstandings and confusion in the communication between specialists from different disciplines and potentially inhibit or retard scientific progress.