Mark A. Garro

Emissions from the laboratory combustion of wildland fuels : Particle morphology and size

[1] The morphology of particles emitted by wildland fires contributes to their physical and chemical properties but is rarely determined. As part of a study at the USFS Fire Sciences Laboratory (FSL) investigating properties of particulate matter emitted by fires, we studied the size, morphology, and microstructure of particles emitted from the combustion of eight different wildland fuels (i.e., sagebrush, poplar wood, ponderosa pine wood, ponderosa pine needles, white pine needles, tundra cores, and two grasses) by scanning electron microscopy. Six of these fuels were dry, while two fuels, namely the tundra cores and one of the grasses, had high fuel moisture content. The particle images were analyzed for their density and textural fractal dimensions, their monomer and agglomerate number size distributions, and three different shape descriptors, namely aspect ratio, root form factor, and roundness. The particles were also probed with energy dispersive X-ray spectroscopy confirming their carbonaceous nature. The density fractal dimension of the agglomerates was determined using two different techniques, one taking into account the three-dimensional nature of the particles, yielding values between 1.67 and 1.83, the other taking into account only the two-dimensional orientation, yielding values between 1.68 and 1.74. The textural fractal dimension that describes the roughness of the boundary of the two-dimensional projection of the particle was between 1.10 and 1.19. The maximum length of agglomerates was proportional to a power a of their diameter and the proportionality constant and the three shape descriptors were parameterized as function of the exponent a.

Light scattering and absorption by fractal-like carbonaceous chain aggregates: comparison of theories and experiment.

This study compares the optical coefficients of size-selected soot particles measured at a wavelength of 870 nm with those predicted by three theories, namely, Rayleigh-Debye-Gans (RDG) approximation, volume-equivalent Mie theory, and integral equation formulation for scattering (IEFS). Soot particles, produced by a premixed ethene flame, were size-selected using two differential mobility analyzers in series, and their scattering and absorption coefficients were measured with nephelometry and photoacoustic spectroscopy. Scanning electron microscopy and image processing techniques were used for the parameterization of the structural properties of the fractal-like soot aggregates. The aggregate structural parameters were used to evaluate the predictions of the optical coefficients based on the three light-scattering and absorption theories. Our results show that the RDG approximation agrees within 10% with the experimental results and the exact electromagnetic calculations of the IEFS theory. Volume-equivalent Mie theory overpredicts the experimental scattering coefficient by a factor of approximately 3.2. The optical coefficients predicted by the RDG approximation showed pronounced sensitivity to changes in monomer mean diameter, the count median diameter of the aggregates, and the geometric standard deviation of the aggregate number size distribution.

Strong radiative heating due to wintertime black carbon aerosols in the Brahmaputra River Valley

values of BC mass concentration were 9–41 mgm � 3 , with maxima over 50 mgm � 3 during evenings and early mornings. Median BC concentrations were higher than in mega cities of India and China, and significantly higher than in urban locations of Europe and USA. The corresponding mean cloud-free aerosol radiative forcing is � 63.4 Wm � 2 at the surface and +11.1 Wm � 2 at the top of the atmosphere with the difference giving the net atmospheric BC solar absorption, which translates to a lower atmospheric heating rate of � 2 K/d. Potential regional climatic impacts associated with large surface cooling and high lower-atmospheric heating are discussed. Citation: Chakrabarty, R. K., M. A. Garro, E. M. Wilcox, and H. Moosmuller (2012), Strong radiative heating due to wintertime black carbon aerosols in the Brahmaputra River Valley, Geophys. Res. Lett., 39, L09804, doi:10.1029/2012GL051148.

Observation of Superaggregates from a Reversed Gravity Low-Sooting Flame

Copyright 2012 American Association for Aerosol Research

Simulation of Aggregates with Point-Contacting Monomers in the Cluster–Dilute Regime. Part 1: Determining the Most Reliable Technique for Obtaining Three-Dimensional Fractal Dimension from Two-Dimensional Images

Analysis of electron microscopy images of fractal-like aggregates involves extraction of three-dimensional (3-d) structural and geometrical properties of aggregates, which are commonly unknown, from their two-dimensional (2-d) projected images. The fractal dimension Df of an aggregate is considered to be the key property for characterizing fractal-like aggregates. The nested squares method (NSM) (also known as the cumulative-intersection method and concentric circles method), the perimeter grid method (PGM), and the ensemble method (EM) have found wide use as techniques for determination of Df of both individual and ensemble aggregates in the cluster-dilute regime. However, no study has so far compared the validity and accuracy of these three most commonly used analysis methods. In this article, using the fractal simulation package FracMAP, these methods were individually tested by applying them to a statistically significant (∼2500 per fractal dimension) number of projected images of all stable orientations of computer-generated 3-d fractal aggregates with Df ranging between 1.0 and 3.0 in increments of 0.1. Results show that of the three methods, the only method that can be used to reliably determine Df from 2-d images is the EM. Both the NSM and the PGM yield many overlapping values of 2-d Df for differing values of 3-d Df resulting in a non-one-to-one relationship and large margins of error. A correction factor has been formulated as a piece-wise function of linear functions for calibrating EM measured values of 2-d Df to actual 3-d Df values.

Simulation of Aggregates with Point-Contacting Monomers in the Cluster-Dilute Regime. Part 2: Comparison of Two- and Three-Dimensional Structural Properties as a Function of Fractal Dimension

In the past two decades, several experimental and simulation studies have proposed simple empirical relations between projected two-dimensional (2-D) and three-dimensional (3-D) structural properties of fractal-like aggregates in the cluster–dilute regime. These empirical relations have found extensive use in inferring the 3-D structural properties of aggregates from their projected (i.e., 2-D) properties—measurable from aggregate electron microscopy images. This study probes the limitations and nongeneralizability of these simple and straightforward empirical relationships and proposes replacing them with new empirical formulas. A straightforward empirical relationship for directly determining the 3-D fractal dimension (Df ) of an aggregate from the knowledge of its 2-D aspect ratio is also identified. These new relationships were derived by comparing the ratios of several 2-D and 3-D structural properties of a statistically significant number of simulated aggregates with point-contacting monomers as a function of their 3-D Df ranging from 1.0 to 3.0 in increments of 0.1.

FracMAP: A user-interactive package for performing simulation and orientation-specific morphology analysis of fractal-like solid nano-agglomerates !

Abstract Computer simulation techniques have found extensive use in establishing empirical relationships between three-dimensional (3d) and two-dimensional (2d) projected properties of particles produced by the process of growth through the agglomeration of smaller particles (monomers). In this paper, we describe a package, FracMAP, that has been written to simulate 3d quasi-fractal agglomerates and create their 2d pixelated projection images by restricting them to stable orientations as commonly encountered for quasi-fractal agglomerates collected on filter media for electron microscopy. Resulting 2d images are analyzed for their projected morphological properties. Program summary Program title: FracMAP Catalogue identifier: AEDD_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEDD_v1_0.html Program obtainable from: CPC Program Library, Queens University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 4722 No. of bytes in distributed program, including test data, etc.: 27 229 Distribution format: tar.gz Programming language: C++ Computer: PC Operating system: Windows, Linux RAM: 2.0 Megabytes Classification: 7.7 Nature of problem: Solving for a suitable fractal agglomerate construction under constraints of typical morphological parameters. Solution method: Monte Carlo approximation. Restrictions: Problem complexity is not representative of run-time, since Monte Carlo iterations are of a constant complexity. Additional comments: The distribution file contains two versions of the FracMAP code, one for Windows and one for Linux. Running time: 1 hour for a fractal agglomerate of size 25 on a single processor.