Coagulation of organic carbon nanoparticles in exhaust conditions

The concentration of combustion-generated Nanoparticles of Organic Carbon (NOC) in vehicle emissions and urban atmospheres was evaluated using particle measurement techniques sensitive to NOC (Atomic Force Microscopy, Differential Mobility Analysis and Far UV Absorption Spectroscopy) and a theoretical model was used to predict particle size distributions throughout the exhaust plume. A significant fraction, and in some cases, the majority of particulate formed in combustion conditions of modern engines are NOC, which are an order of magnitude smaller than graphitic soot particles. NOC are mostly smaller than the detection limit of commercial particle sizing instrument used for atmospheric measurements (d<5-10 nm). The smallest NOC have a higher affinity for water, higher mobility, higher number concentration, and longer lifetime in flames than larger soot particles. Therefore, NOC may contribute to observed human health effects associated with pollution. In previous works, these observations were explained by a theoretical model that predicts the particle coagulation rate as steeply increasing with particle size. The same theoretical model was used in this study to simulate aerosol dynamics in vehicle exhausts and dilution plumes as they mix with urban air and estimate their size distributions. The model predicts bimodal particle size distributions near roadways with a modal diameter of 2–3 nm, similar to the size of combustion-generated NOC measured in flames and engine exhausts. Predictions indicate the presence of NOC in urban atmospheres, similar to measurements by cryogenic sampling and AFM, presented in this work, and characterization studies of organic species in collected rain and fog samples.