What is Particulate Matter and How does it Vary? What is Particulate Matter? How Does PM Vary? The Influence of Emissions, Dilution and Transformations Resource Links Contact: Rudolf Husar,

Figure 1. Aerosol Size Distribution and Morphology Purposes of the illustration: 1.Size spectra over 4 decades, modes 2.Particle shapes, electron micrographs 3.Chemical composition by size Click figure to enlarge

What is Particulate Matter? The term Particulate Matter or aerosol, refers to liquid or solid particles suspended in the air. Depending on their origin and visual appearance, aerosols have acquired different names in the everyday language. Dust refers to solid airborne material, dispersed into aerosol from grainy powders such as soil. Combustion processes produce smoke particles, but the incombustible residues of coal are called flyash. In the early days, air pollution had the appearance of both smoke and fog, so the term smog was created. In the open atmosphere, the visibility may often be reduced by regional haze, originating from various natural or anthropogenic sources. Neither water droplets of fog and clouds, snow, rain, sleet (hydrometors) nor dust particles larger than 100 um (blowing sand) are considered to be particulate matter

On the Origin PM2.5 in the Atmosphere - [Fragment] At this time, the most reliable means of identifying the origin of PM2.5 is the chemical analysis of the the PM samples which reveals that most of PM2.5 is composed of secondary sulfates, organics and nitrates. Identifying the PM2.5 precursor sources is elusive since they occur 100 or 1000 miles from the receptor. Also, the chemical transformations involve many factors including photochemical oxidants and cloud interactions. Primary PM2.5 such as soot and fine dust can be traced based on chemical signatures since each PM source type produces particles with specific physical, chemical and optical signature. Contact: Rudolf Husar,

Properties of Particulate Matter Physical, Chemical and Optical Properties Size Range of Particulate Matter Mass Distribution of PM vs. Size: PM10, PM2.5 Fine and Coarse Particles Fine Particles - PM2.5 Coarse Particle Fraction - PM10-PM2.5 Chemical Composition of PM vs. Size Optical Properties of PM Resource Links Contact: Rudolf Husar,

Physical, Chemical and Optical Properties PM is characterized by its physical, chemical and optical properties The physical properties include particle size and particle shape. The particle size refers to particle diameter or ‘equivalent’ diameter for odd shaped particles. The particle shape of may be liquid droplets, regular or irregular shaped crystals or aggregates of odd shape. Their chemical composition may also vary from dilute water solution of acids or salts, organic liquids, to earth's crust materials (dust), soot (unburned carbon) and toxic metals. The optical properties determine the visual appearance of dust, smoke and haze and include light extinction, scattering and absorption. The optical properties are determined by the physical and chemical properties of the ambient PM. Each PM source type produces particles with specific physical, chemical and optical signature. Hence, PM may be viewed as several pollutants since each aerosol type has its own properties, sources and requires different control control

Size Range of Particulate Matter The size of aerosol particles ranges from about tens of nanometers (nm) which corresponds to molecular aggregates to tens of microns of the size of human hair. The smallest particles are generally more numerous and the number distribution of particles generally peaks below 0.1 um. The size range below 0.1 um is also referred to as ultrafine range. The largest particles ( um) are small in number but contain most of the aerosol volume (mass). The volume (mass) distribution can have two or tree peaks (modes). The bi-modal mass distribution has two peaks. The peak of the aerosol surface area distribution is always between the number and the volume peaks

Mass Distribution of PM vs. Size: PM10, PM2.5 Usually, the PM mass is plotted vs. the log of particle diameter The mass distribution tends to be bi-modal with the saddle in the 1-3 um size rage PM10 refers to the fraction of the PM mass less than 10 um in diameter PM2.5 or fine mass are less than 2.5 um in size. The difference between PM10 and PM2.5 constitutes the coarse fraction The fine and coarse particles have different sources, properties and effects. Many of the known environmental impacts (health, visibility, acid deposition) are attributed to PM2.5.

Fine and Coarse Particles There is a natural division of atmospheric particulates into Fine and Coarse fraction based on particle size. The fine and coarse particles originate from different sources, their formation mechanisms, transport distance, their properties and effects are also different. Many of the known environmental impacts on health, acid deposition, visibility, and corrosion are associated with the fine particles

Fine Particles - PM2.5 The majority (over 90%) of the PM2.5 mass over the US is of secondary origin, formed within the atmosphere through gas-particle conversion of precursor gases such as sulfur oxides, nitrogen oxides and organics. The resulting secondary aerosol products are sulfates, organics and nitrates. Some PM2.5 is emitted as primary emissions from industrial activities and motor vehicles including soot (unburned carbon), trace metals and oily residues. Fine particles are mostly droplets except for soot which is in the form of chain aggregates. Over the industrialized regions of the US anthropogenic emissions from fossil fuel combustion contribute most of the PM2.5. In remote areas biomass burning, windblown dust, and sea salt also contribute.

Coarse Particles - PM10-PM2.5 Coarse particles are primary in that they are emitted as windblown dust and sea spray in coastal areas. Anthropogenic coarse particle sources include flyash from coal combustion and road dust from automobiles The chemical composition of the coarse particle fraction is similar to that of the earth's crust or the sea but sometimes coarse particles also carry trace metals and nitrates. Coarse particles are removed from the atmosphere by settling, impaction to surfaces and by precipitation. Their atmospheric residence time is generally less than a day, and their typical transport distance is below a few hundred km. Some dust storms tend to lift the dust to several km altitude, which increase the transport distance to many thousand km.

Chemical Composition of PM vs. Size The chemical species that make up the PM occur at different sizes. For example in Los Angeles, ammonium and sulfate occur in the fine mode, <2.5 um in diameter. Carbonaceous soot, organic compounds and trace metals tend to be in the fine particle mode The sea salt components, sodium and chloride occur in the coarse fraction, > 2.5 um. Wind-blown and fugitive dust also mainly in the coarse mode. Nitrates may occur in fine and coarse modes.

Optical Properties of PM Particles effectively scatter and absorb solar radiation. The scattering efficiency per aerosol mass is highest at about 0.5 um. This is why, say, 10 ug of fine particles ( um)

How Does PM Vary? Spatially, temporally, with particle size and by chemical composition As all pollutants, the ambient aerosol concentration patterns contain endless variability in space and time. However, unlike gaseous pollutants, particulate matter also depends on particle size, shape and chemical composition. The chemically rich aerosol mix arises from the multiplicity of PM sources, each having a unique chemical signature at the source. The primary aerosol chemical composition is further enriched by the addition of secondary species during atmospheric transport. The effective mixing in the lower atmosphere stirs these primary and secondary particles into an externally mixed batch with various degrees of homogeneity, depending on location and time. Lastly, repeated cloud scavenging and evaporation tends to mix the particles from different sources internally into particles with mixed composition. The result is a heterogeneous PM pattern that is probably unparalleled in the domain of atmospheric sciences. For instance it is common to find soot particles within sulfate droplets, or nitrate deposited on sea salt particles.

The Influence of Emissions, Dilution and Transformations The PM concentration, C, at any given location and time is determined by the combined interaction of emissions, E, atmospheric dilution, D, and chemical transformation and removal, T, processes: C = f (E, D, T) Each of the three processes has its own pattern at secular, yearly, weekly, synoptic, diurnal and micro time scales. The yearly, weekly and the diurnal scales are periodic

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