Measuring and simulating particulate organics in the atmosphere: problems and prospects

Although organics constitute approximately 10–70% of the total dry fine particle mass in the atmosphere, their concentrations and formation mechanisms are less well understood than those of other components such as sulfate and nitrate. This is because particulate organic matter is an aggregate of hundreds of individual compounds whose concentrations cannot be characterized by a single analytical technique; more than half of the organic carbon mass has not yet been identified as individual compounds. Moreover, the collection process itself can alter the gas–particle equilibrium of a number of condensable organics resulting in both positive and negative sampling biases. The incomplete characterization of particulate organics coupled with the complexity of the photochemical reactions that produce particulate matter from volatile organic emissions has prevented the development of a first principle simulation approach. These limitations are providing an impetus for numerous scientific studies, proving organics to be the next frontier for particle characterization and simulation. This paper reviews the current state of organic aerosol sampling, analysis, and simulation, examines the limitations of the current technology, and presents prospects for the future. The emphasis is on distilling findings from recent atmospheric, smog chamber, and theoretical studies to provide a coherent picture of what has been accomplished, especially during the last five years, and what problems are ripe for further investigation.