Peking University, July 28, 2015: Recently, Prof. Duan Huiling’s group from the Department of Mechanics and Engineering Science, College of Engineering, reported new findings for the understanding of PM_2.5 airborne pollutants through nanoscale characterization. This work has been developed in collaboration with the Chinese Center For Disease Control and Prevention (CDC), as well as with the groups headed by Professor Lin Hao (Rutgers University, USA) and Mario Lanza (Soochow University). The research results have been published in Nature Scientific Reports, in an article titled Nanoscale characterization of PM_2.5 airborne pollutants reveals high adhesiveness and aggregation capability of soot particles (Sci. Rep., DOI:10.1038/srep11232) which is online available at:

http://www.nature.com/srep/2015/150716/srep11232/full/srep11232.html

Among all kinds of contamination, air pollution has become the major problem of many modern cities, not only in China but also around the globe, and it is affecting billions of persons. For example, in 2013 Beijing suffered PM_2.5 concentrations above the limit recommended by the World Health Organization for more than 219 days. Among all the noxious pollutants in air, fine particles with an aerodynamic diameter of 2.5 micrometers or less (PM_2.5) are the most harmful for human health because they are small enough to invade even the smallest airways and penetrate to the lungs.

In the past few years, many studies reported the amount, size, shape, composition and source of PM_2.5, as well as their effect in human health. Nevertheless, although chemical toxicity is the number one health concern, chemistry alone does not determine the noxiousness of the particles, and particle morphology (i.e. shape and surface properties) affect their dynamic behavior and thermodynamic properties, which have notable impact on their biological activity. This investigation reports the first nanomechanical characterization of PM_2.5 airborne pollutants, by measuring with unprecedented resolution parameters like surface roughness and adhesion force. The experiments have been developed using the Atomic Force Microscope (AFM), which has helped to detect which PM_2.5 airborne pollutant has most noxious properties.

This study identifies mainly four types of PM_2.5 particles in air, named: fluffy soot aggregate, elongated minerals, spherical fly ash, and others. The data indicate that fluffy soot aggregate,  show unusually large adhesiveness, and they show the ability of aggregating all other types of PM_2.5, leading to a mixed chemical composition with increased toxicity. The larger stickiness of these particles may lead to longer exposure times when interacting with the body, and they may become an obstruction for the airways. For these reasons these particles, which are rich in carbon and are originated from incomplete combustion of hydrocarbons, may be the most noxious for human health.

This work is supported by Major State Basic Research Development Program of China, and National Natural Science Foundation of China (NSFC).

Figure: (a) and (b), SEM and topographic AFM images of PM_2.5 soot aggregate with high surface roughness (respectively). (c), Example of force-distance curves measured on particles with different surface roughness (SR). (d), Weibull probability plot of the adhesion force collected on two groups of PM_2.5 particles (with low and high surface roughness). (e), EDAX Chemical composition histogram of the particles collected with SEM/EDAX classified by surface roughness. (f) Example of an aggregate, in which the soot particle trap the fly ash and elongated mineral.

Source: College of Engineering
Edited by: Zhang Jiang