Environ. Sci. Technol. 2008, 42, 859–863
Measurements of Nanoparticles of Organic Carbon and Soot in Flames and Vehicle Exhausts L E E A . S G R O , * ,† A N T O N I O B O R G H E S E , ‡ LUCIANO SPERANZA,† ALBERTO C. BARONE,† PATRIZIA MINUTOLO,§ ANNALISA BRUNO,| ANDREA D’ANNA,† AND ANTONIO D’ALESSIO† Dipartimento di Ingegneria Chimica, Università “Federico II” di Napoli, P. Tecchio 80, 80125 Naples, Italy, Istituto Motori, CNR, Naples, Italy, Istituto di Ricerche sulla Combustione, CNR, Naples, Italy, and Dipartimento di Fisica, Università “Federico II” di Napoli, P. Tecchio 80, 80125 Naples, Italy
Received February 26, 2007. Revised manuscript received September 14, 2007. Accepted November 15, 2007.
We measured the size distribution and UV extinction spectra of carbonaceous nanoparticles present in the size range of 1–100 nm in the exhausts of 2004 model gasoline and diesel powered vehicles and compared the results with those obtained in premixed flames. In addition to soot particles, nanoparticles of organic carbon (NOC) were measured in the emissions of these test vehicles in significant number and mass concentrations. The number and mass concentration of NOC was higher than soot in gasoline vehicle emissions. In diesel emissions, NOC had a higher number concentration than soot in terms of number concentration, but in terms of mass concentration, soot was higher than NOC. The size (1–3 nm) and extinction spectra in the UV–visible (strong in the UV and transparent in the visible) of macromolecules/nanoparticles collected in water samples from the vehicles are similar to those measured in laboratory hydrocarbon-air flames, suggesting that these nanoparticles are formed in hydrocarbon combustion reactions. We advance the hypothesis that NOC in vehicle emissions are produced by high-temperature combustion processes and not by low-temperature condensation processes.
Introduction Atmospheric particulate matter (PM) is decisively influenced by the concentration, size, and chemical nature of PM emissions from combustion sources. This is particularly true for the ultrafine fraction of PM smaller than 100 nm, which is mostly constituted by carbonaceous matter (1, 2) and is thought to play a role in observed health effects associated with pollution (3, 4). Atmospheric carbonaceous PM is typically divided into two classes, elemental and organic carbon (EC and OC) by thermogravimetric measurements, but the relative amounts of OC/EC depend operationally on the sampling/measurement methods (5). * Corresponding author phone: +39-768-2221; fax: +39-081-5963963; e-mail:
[email protected]. † Dipartimento di Ingegneria Chimica, Università “Federico II” di Napoli. ‡ Istituto Motori, CNR. § Istituto di Ricerche sulla Combustione, CNR. | Dipartimento di Fisica, Università “Federico II” di Napoli. 10.1021/es070485s CCC: $40.75
Published on Web 01/01/2008
2008 American Chemical Society
Previous work examining laboratory flames with in situ UV–visible optical diagnostics showed that two classes of carbonaceous particles are produced in fuel-rich flames: graphitic-like soot particles, with sizes that range from 20 nm to some microns that have continuous absorption spectra throughout the entire UV–visible wavelength range, and smaller particles approaching molecular dimensions with typical sizes between 1 and 10 nm which absorb prevalently in the far UV and are almost transparent to near UV and visible wavelengths (6). We define this class of macromolecules/nanoparticles as nanosized organic carbon (NOC) particles. Further studies showed that the size distribution of combustion-generated NOC, determined by combined light scattering and extinction in situ measurements (7), atomic force microscopy of thermophoretically sampled particles (8), and online differential mobility analysis (DMA) have modal diameters smaller than 3 nm (9). In addition, it was shown that NOC particles can be captured in water without showing appreciable agglomeration (6, 9) and have a very low coagulation rate at high flame temperature (1700 K), as long as their size is smaller than 5 nm (7, 10). Measurements of the UV extinction spectra of atmospheric aerosols in air (11) and rain (12) showed similarities to those of flame-generated NOC, indicating the presence of NOC in urban atmospheres. More recent measurements found that both the size and extinction spectra of NOC in atmospheric fog samples were similar to that of NOC in flames (13). In situ UV–visible extinction measurements in vehicle and stationary source emissions also indicate the presence of NOC in addition to soot (12, 14, 15), but these earlier studies did not measure the size distribution of emitted particles. It has been hypothesized that combustion-generated NOC may be emitted from vehicles and contribute significantly to atmospheric OC, and that the small size of NOC in emissions may prevent its detection (16). In this study, we measured the UV extinction and size distribution of particles directly in the exhausts of 2004 model vehicles and in water samples collected from those vehicles in a first attempt to quantify the size and relative amounts of NOC and soot in modern vehicle emissions and to evaluate the use of water sampling for NOC collection. We applied measurement strategies used extensively to study NOC and soot in flames to the exhausts of engines as a first step toward quantifying the magnitude of macromolecules/nanoparticles in vehicle emissions with sizes slightly smaller than the detection limit of commercial instruments (as small as to 1–2 nm).
Materials and Methods We measured the size, concentration, and UV extinction spectra of nanoparticles emitted from two gasoline- and two diesel-powered 2004 model vehicles and compared them with those in laminar atmospheric pressure premixed laboratory flat flames. Laboratory flames, supported on a McKenna burner, serve as a highly simplified, well controlled and characterized experimental model of possible combustion conditions (defined by their fuel/air ratio, residence time, and temperature) that occur in applied combustion systems, and are used to study reaction kinetics or investigate new diagnostics. A detailed description/discussion of the diagnostics and methods briefly discussed here used can be found in cited works and in the online Supporting Information for this article. The four cars were run with commercial low-sulfur (
