Particulate matter and coal dust

Airborne Particles

Airborne particles––also known as particulate matter (PM), particulates, or aerosols––refer to any mixture of solids and liquid droplets in the atmosphere. Particles can be directly emitted into the air or formed through chemical reactions of gaseous pollutants.

Particles are often distinguished by their size. The terms PM₁, PM_2.5, and PM₁₀ are common ways particles are described, measured, and regulated, with the number indicating the diameter of the particles in micrometers. Particles can also be discussed in the terms of very-fine (PM₁), fine (PM_2.5), and coarse (PM₁₀) particles, with PM₁₀ generally referring to as dust. Particle size, composition, and shape affect how particles are regulated under the U.S. Clean Air Act and their health impacts.

Coal Dust

Coal dust is a type of particulate matter that contains coal. Coal dust particles tend to be larger in size, including PM_2.5 and PM₁₀, as well as even bigger PM that is visible with the unaided eye. Coal dust is lost from the atmosphere through settling and deposition, leaving black dust films on outdoor and indoor surfaces. Coal dust contains toxic heavy metals that have serious health impacts at ambient concentrations and exposures below the U.S. Environmental Protection Agency National Ambient Air Quality Standards (NAAQS).

Coal Dust Emissions

Coal dust is produced through physical processes that wear on the brittle coal rocks during mining, transport, and storage to generate coal particles that are then uplifted to the atmosphere and carried by winds. There are various physical disturbances that can create small coal particles and uplift them into the air. The distance traveled by coal dust in the atmosphere depends on the size of particles, with smaller particles generally reaching farther distances as they settle more slowly because they less affected by gravity, and the conditions of the atmosphere. Physical disturbances influencing coal dust emissions and downwind impacts include weather and wind patterns and the transporting and handling of coal.

Wind and weather disturbances

Wind: In Southeast Newport News, coal is stored in large open piles, and these stockpiles are susceptible to wind erosion with coal particles easily becoming airborne. The wind speeds over the coal piles influence the coal dust emissions and transport downwind, with higher wind speeds associated with higher airborne particle concentrations. As wind speeds increase, atmospheric particulate matter in the air increases, and there is an overall increase in dust fluxes to the atmosphere of particulates such as coal. The direction of airborne dust travel is determined by the wind direction. Higher particulate matter and black carbon concentrations have been observed in the neighboring community when winds blow from of the Curtis Bay coal export terminal in Baltimore, Maryland.

In January 1987, the Virginia Air Pollution Control Board (APCB) published a study on coal dust emissions from the terminals in Southeast Newport News: Control of Fugitive Emissions from Open Coal Storage in Newport News, Virginia. As part of this study, researchers developed a mathematical formula for the effective wind forces (K_t) on the terminals' coal piles using readily available meteorological measurements.

$K_t=SP\left(\frac{T}{RH}\right)\left(\frac{P}{\mu \times 1.68}\right)$

The relationship between the measured total airborne particle concentration and the daily summed Kt for those sample days. This line is labeled TSPunc/t. The second line, labeled CEunc, shows only the un-combusted coal portion of those measured particles. The title reflects that the total particles were measured using a hi-volume sampler and "EK_t" should read "ΣK_t". This is figure 3 in the 1987 study Control of Fugitive Emissions from Open Coal Storage in Newport News, Virginia.

SP is the wind speed in miles per hour, T is air temperature in °F, RH is relative humidity (%), P is the air density in pounds (lbs) per cubic feet, and μ is the air viscosity in lbs per cubic feet hours. The constant of 1.68 is the value of P divided by μ at standard conditions of 70°F, RH of 60%, and P of 29.92 inches Hg. The term T/RH reflects the tendency of the coal piles to emit particles when temperatures are high and/or RH is low. Hot, dry, and windy atmospheric conditions present a worst case scenario for coal dust emissions.

The APCD developed the equation for K_t to guide and optimize wet dust suppression with sprayers in Southeast Newport News. Their 1987 APCB study found a strong relationship between the daily summed K_t (ΣK_t) and the amount of particles measured using a high-volume air sampler. The 1987 APCB study cited a 1984 study from the U.S. Environmental Protection Agency (EPA) Office of Air, Noise and Radiation, and the Office of Air Quality Planning and Standards as having reported that dust emissions can become significant at winds of 12 miles per hour. We note that we only found a related report from 1985.

Clouds, rain, and fog: Coal dust emissions are reduced on cloudy, rainy, foggy, and humid days. This is similar to the use of water sprayers for wet dust suppression.

• The presence of clouds reduces the radiant energy received by the coal piles, meaning there is less heating of the piles by the sun.
• Rain cools the piles, forms a water tension seal for fine particulate, and adds weight to larger particles meaning stronger winds are required to uplift them into the atmosphere. Rain events are associated with lower airborne coal dust concentrations, as the water droplets reduce both dust emissions and force airborne particles out of the air and back to the surface through a process called wet deposition. If significant rainfall soaks the interior of the pile, the drying process can be slow, reducing dust emissions even after the rains have stopped.
• Like rain, fog cools the piles, and the very small fog droplets contribute to particle encapsulation that limit emissions.
• Relative humidity influences coal dust particle size, with higher humidity associated with larger particles as the dust particles clump together. Lower humidity increases the overall concentration and the amount of fine dust particles specifically.

In Control of Fugitive Emissions from Open Coal Storage in Newport News, Virginia, the APCD developed a weight (F_c) that is multiplied by K_t to include the effects of rain and fog on coal dust emissions. F_c is a value between 0 and 1, with heavy fog (visibility less than 4 miles) or rain (more than 0.3 inches) giving a value of F_c equal to zero, meaning K_t x F_c = 0.

Coal transporting and handling disturbances

Transportation of coal: The transport of coal in uncovered, open-top railcars results in significant amounts of coal dust being emitted into the air by the jostling of the railcars as they travel and/or because of wind and other weather. The railroad company BNSF has conducted studies that indicate 500 lbs to 1 ton of coal can escape from a single coal car during travel along a railway.

Coal handling: Loading and unloading coal from railcars into storage piles can unsettle dust particles that are released into the air. Equipment at the terminal that manages and moves coal piles, such as bulldozers and conveyor belts, can release coal dust into the air. The transfer of coal onto ships at the port can similarly unsettle dust particles. In 1989, the APCD set limits on the emissions from each ship unloading operation, conveyor belts, bucket elevators, silo airslides, and the truck loadout system to 0.3 pounds per hour for PM_2.5 and PM₁₀ for each respective coal handling operation. In July 1994, this emission limit was increased to 1.6 pounds per hour.

Handling size and frequency: The amount of dust varies with the amount material passing through the facilities. More coal, more dust. The age of the coal in the pile is also important. When new material is loaded onto a storage pile, its potential for dust emissions is at a maximum, and particles are easily created and released. As the coal weathers, the potential for dust is reduced. Some particles are already lost by winds and exposure to moisture causes the aggregation and cementation of particles to bulk coal surfaces.

Evidence of coal dust deposition in Hampton Roads, Virginia

Soil samples soils collected in Lambert’s Point, Virginia measured elevated arsenic in local soils. This research reported that soils at a depth of 4 centimeters were 7.1 percent by weight un-combusted coal with a mean arsenic concentrations of 13.3 milligrams per kilogram soil (mg/kg). The percent by weight of un-combusted coal decreased with increasing distance from the Norfolk Southern Terminal.

Documents

• EPA Office of Air, Noise and Radiation, and the Office of Air Quality Planning and Standards, Compilation of Air Pollution Emission Factors, January 1985
• Virginia Air Pollution Control Board, Control of Fugitive Emissions from Open Coal Storage in Newport News, Virginia, 1987
• Bounds, The Distribution of Airborne Coal Dust in Soil Profiles of Norfolk, VA and Its Implication for Arsenic Loading to These Soils, Thesis, Old Dominion University, ODU Digital Commons, 2001
• Aneja et al., Particulate matter pollution in the coal-producing regions of the Appalachian Mountains: Integrated ground-based measurements and satellite analysis, Journal of the Air & Waste Management Association, 67, 4, 421–430, doi:10.1080/10962247.2016.1245686, 2019
• Coal Dust Residues on Homes in Southeast Newport News, 2024–2025 Photographs

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  Coal Dust Residues
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  Coal Dust Residues
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  Coal Dust Residues