Morphology, composition, and mixing state of primary particles from combustion sources - crop residue, wood, and solid waste

Abstract

Morphology, composition, and mixing state of individual particles emitted from crop residue, wood, and solid waste combustion in a residential stove were analyzed using transmission electron microscopy (TEM). Our study showed that particles from crop residue and apple wood combustion were mainly organic matter (OM) in smoldering phase, whereas soot-OM internally mixed with K in flaming phase. Wild grass combustion in flaming phase released some Cl-rich-OM/soot particles and cardboard combustion released OM and S-rich particles. Interestingly, particles from hardwood (pear wood and bamboo) and softwood (cypress and pine wood) combustion were mainly soot and OM in the flaming phase, respectively. The combustion of foam boxes, rubber tires, and plastic bottles/bags in the flaming phase released large amounts of soot internally mixed with a small amount of OM, whereas the combustion of printed circuit boards and copper-core cables emitted large amounts of OM with Br-rich inclusions. In addition, the printed circuit board combustion released toxic metals containing Pb, Zn, Sn, and Sb. The results are important to document properties of primary particles from combustion sources, which can be used to trace the sources of ambient particles and to know their potential impacts in human health and radiative forcing in the air.

Figure 1

TEM images and EDS spectra of individual primary particles from the combustion of maize straw. (a) Low magnification TEM image of gel-like OM particles containing K-rich cores in the smoldering phase; (b) Low magnification TEM image of a mixture of OM-K and soot-OM-K particles in the flaming phase; (c and d) Soot particles internally mixed with OM and KCl in the flaming phase; (e) An OM-K particle including KCl and K₂SO₄ cores in the flaming phase. The corresponding EDS spectra are indicated by the number in square brackets.

Figure 2

TEM images and EDS spectra of individual primary particles from the combustion of wheat straw. (a) Gel-like OM internally mixed with K₂SO₄/soot; (b) Gel-like OM without any inclusion; (c) One irregular particle comprised of soot, K₂SO₄, and OM coating. The combustion phase was a mixture of smoldering and flaming phases. The insets show the electron diffraction patterns of K₂SO₄ with different K/S ratio. The corresponding EDS spectra are indicated by the number in square brackets.

Figure 3

TEM images and EDS spectra of individual primary particles from the combustion of cotton straw. (a) Low magnification TEM image of gel-like OM in the smoldering phase; (b) A gel-like and transparent OM particle with dark and spherical OM inclusions in the smoldering phase; (c) Low magnification TEM image of a mixture of OM-K and soot-OM-K in the flaming phase; (d) An OM particle internally mixed with KCl and externally mixed with satellite ring of tiny KCl particles in the flaming phase; (e) An OM particle internally mixed with soot in the flaming phase. The corresponding EDS spectra are indicated by the number in square brackets.

Figure 4

TEM images and EDS spectra of individual primary particles from the combustion of wild grass. (a) Low magnification TEM image of the gel-like OM in the smoldering phase; (b) Low magnification TEM image of a mixture of OM and soot-OM in the flaming phase; (c) Two irrugualr OM and one internally mixed with soot in the flaming phase; (d) Two Cl-rich particles with thin OM coating and one internally mixed with soot in the flaming phase; (e) An irregular Cl-rich particle internally mixed with OM and soot in the flaming phase. The corresponding EDS spectra are indicated by the number in square brackets.

Figure 5

TEM images and EDS spectra of individual primary particles from the combustion of common reeds. (a) Low magnification TEM image of gel-like OM particles internally mixed with KCl in the flaming phase; (b and c) Two rectangle and one square KCl particles corresponding to red square 1 and 2 in (a), respectively. The inset shows the electron diffraction patterns of KCl. The corresponding EDS spectra are indicated by the number in square brackets.

Figure 6

TEM images and EDS spectra of individual primary particles from the combustion of apple wood. (a) Two spherical gel-like OM in the smoldering phase; (b) An irregular OM particle internally mixed with KCl and soot in the flaming phase; (c) A soot particle coated by gel-like and transparent OM in the end of flaming (burnout) phase. The corresponding EDS spectra are indicated by the number in square brackets.

Figure 7

TEM images and EDS spectra of individual primary particles from the combustion of cypress, pine, and pear wood, and bamboo. Low magnification and high resolution TEM images of (a and b) irregular OM-soot particles from cypress wood, (c and d) OM-K particles from pine wood, (e and f) bare cluster-like soot particles from pear wood, and (g and h) cluster-like soot particles internally mixed with KCl and spherical OM from bamboo, in the flaming phase, respectively. The corresponding EDS spectra are indicated by the number in square brackets.

Figure 8

TEM images and EDS spectra of individual primary particles from the combustion of cardboard, foam boxes, waste rubber tires, and plastic bottles/bags. (a) Low magnification TEM image of OM and S-rich-OM from cardboard in the flaming phase; (b) An irregular OM particle from cardboard in the flaming phase; (c) An S-rich particle internally mixed with OM from cardboard in the flaming phase; (d) A bare chain-like soot particle from foam boxes in the flaming phase; (e) Soot particles coated by thin OM from waste rubber tires in the flaming phase; (f) A soot particle coated by thick OM from plastic bottles/bags in the flaming phase. The corresponding EDS spectra are indicated by the number in square brackets.

Figure 9

TEM images and EDS spectra of individual primary particles from the combustion of printed circuit boards and copper-core cables. (a) Br-rich particles internally mixed within bubble-like OM (Sn, Sb) from printed circuit boards in the flaming phase; (b and c) Bubble-like OM (Sn, Sb) internally mixed with Br-rich, Pb-rich, P-rich and Zn-rich from printed circuit boards in the flaming phase; (d and e) Low magnification and high resoluton TEM images of OM-Br-soot particles from copper-core cables in the flaming phase. The corresponding EDS spectra are indicated by the number in square brackets. The Cu* peaks in the EDS spectra are not analyzed because of the interference from the copper TEM grids.

Figure 10

Percentages of typical primary particles emitted from different materials’ combustion with the smoldering and flaming phases. The number of the analyzed particles is shown above each column. The particle number of common reeds cannot be counted because the gel-like OM particles were in contact with each other.