The International Global Atmospheric Chemistry Program (IGAC) has conducted a series of Aerosol Characterization Experiments (ACE) that integrate in situ measurements, satellite observations, and models to reduce the uncertainty in calculations of the climate forcing due to aerosol particles. ACE‐Asia, the fourth in this series of experiments, consisted of two focused components: (1) An intensive field study that sought to quantify the spatial and vertical distribution of aerosol concentrations and properties, the processes controlling their formation, evolution, and fate, and the column‐integrated radiative effect of the aerosol (late March through May 2001). (2) A longer‐term network of ground stations that used in situ and column‐integrated measurements to quantify the chemical, physical, and optical properties of aerosols in the ACE‐Asia study area and to assess their spatial and temporal (seasonal and interannual) variability (2000–2003). The approach of the ACE‐Asia science team was to make simultaneous measurements of aerosol chemical, physical, and optical properties and their radiative impacts in a variety of air masses, often coordinated with satellite overpasses. Three aircraft, two research ships, a network of lidars, and many surface sites gathered data on Asian aerosols. Chemical transport models (CTMs) were integrated into the program from the start, being used in a forecast mode during the intensive observation period to identify promising areas for airborne and ship observations and then later as tools for integrating observations. The testing and improvement of a wide range of aerosol models (including microphysical, radiative transfer, CTM, and global climate models) was one important way in which we assessed our understanding of the properties and controlling processes of Asian aerosols. We describe here the scientific goals and objectives of the ACE‐Asia experiment, its observational strategies, the types of observations made by the mobile platforms and stationary sites, the models that will integrate our understanding of the climatic effect of aerosol particles, and the types of data that have been generated. Eight scientific questions focus the discussion. The intensive observations took place during a season of unusually heavy dust, so we have a large suite of observations of dust and its interaction with air pollutants. Further information about ACE‐Asia can be found on the project Web site at http://saga.pmel.noaa.gov/aceasia/.

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. This paper discusses and prioritizes issues related to organic aerosols and their effects on atmospheric processes and climate, providing a basis for future activities in the field. Four main topical areas are addressed: i) sources of OA; ii) formation transformation and removal of OA; iii) physical, chemical and mixing state of OA; iv) atmospheric modelling of OA. Key questions and research priorities regarding these four areas are 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 scales and disciplines.