(248aq) Carbonaceous Aerosol: Physical Nanostructure and Chemical Analysis

Electron microscopic and spectroscopic methods have great potential for carbonaceous aerosol characterization. High-resolution transmission electron microscopy (HRTEM) provides information of the carbon nanostructure (or relative lack thereof).  Image quantification allows extraction of various statistical parameters including lamella length, separation distance and tortuosity.  These may be summarized in the form of histograms.  X-ray photoelectron spectroscopy (XPS) provides a means for probing the surface chemistry of materials.  It provides not only the elemental composition but also the bonding state of the constituent elements.

Combustion produced soot is a highly variable material.  Physically the nanostructure can range from amorphous to graphitic to fullerenic.  Chemically nearly any element could be included, though the surface functional groups are predominantly oxygen-based.  Results will be presented for analysis of HRTEM images of the physical nanostructure and XPS analysis of the chemical composition of soots collected from oil-fired boilers of different scale, diesel engines, gas-turbine combustor (jet engine produced soots) and biofuel combustion.  Soots from these emission source classes may be differentiated physically based on carbon nanostructure: lamella length, mean separation, tortuosity and overall ordering.  These soots may also be distinguished chemically by elemental composition, types of surface (oxygen) functional groups and carbon bonding.

Detailed characterization of source (particulate) emissions is integral to refining emission inventories and improving source profile databases. Moreover the physical and chemical measures provide a fingerprint of the soot by which sources can be distinguished.  Such detailed source characterization would also improve testing receptor models for source contributions to monitored sites and areas and permit development of more detailed models. Atmospheric and local environmental impacts can then be more accurately assessed, regulatory statutes more specifically targeted and human exposure and related health risks better estimated. 

To resolve detailed morphological and nanostructural changes induced in the soot by pulsed high intensity laser light, high resolution transmission electron microscopy (HRTEM) was used to examine the laser-heated soot. The annealing process accentuated the recognizable structural differences and led to graphitization of lamellae, but the spatial organization of lamellae was quite different across these soots. Initial nanostructure in conjunction with the chemistry of construction governed the material transformation under pulsed laser annealing.

Epidemiological studies have shown that inhalation of soot particles can cause pulmonary disease, cardiovascular damage and mortality.  New findings show soot may be contributing to changes happening near the North Pole, such as accelerating melting of sea ice and snow and changing atmospheric temperatures.   Soot surface chemistry may determine whether it can act as cloud nuclei, its participation in atmospheric heterogeneous reactions, thereby significantly affecting the atmospheric radiative balance. These properties are further modified by soot’s nanostructure, along with its oxidation characteristics.  With increasing knowledge of the detrimental health effects of soot and environmental impacts, identification of emission sources is of increasing importance.