Introduction:

Looking up at the sky, we would never guess that our atmosphere contains between one and three billion tons of dust and other particles at any given time. Wind assists in keeping this dust airborne, but gravity wins most of the time, forcing the dust particles earthward, proving the old adage: “what goes up, must come down.”

Dust comes from many different sources. Some, like the byproducts of the combustion of fossil fuels, are man-made. Others
come from natural sources – like sea-spray blowing off the ocean, or dust blowing in from the desert.  Dust comprises inorganic matter, such as sand particles, as well as a large amount of organic matter, including pollen, spores, moulds and viruses. These minute particles, ranging in size from around 100 micro metres (µm) to a few nano metres (nm)3, invade our airspace every day, a part of life that we aren’t even aware of, except when we dust the furniture!

Definitions

Aerodynamic diameter - the diameter of a spherical particle that has a density of 1g/cm3 and which has the same terminal settling velocity as the particle of interest.

Atmospheric dust – Dust that is in the atmosphere.

Brownian Motion – The continual random movement, due to molecular agitation, of fine particles suspended in a gas or a liquid.

d50 – In a sample of dust the d50 diameter is the diameter above which fifty percent of the particles are larger and below which fifty percent of the particles are smaller.

Dry deposition – The collection of precipitant dust during periods with no rainfall.

Export bucket – The export bucket can be the North, South, East or West bucket that is closest to the dust sources. When the wind blows over the dust source towards the sampling location then the export bucket is open and dust from the dust source is collected in the bucket.

Fugitive dust – Dust that is not emitted from a point source that can be easily defined such as stacks. Sources are open fields, travel-ways, stockpiles and process-buildings.

Meteorology – the earth science dealing with phenomena of the atmosphere (especially weather).

Occult deposition – Increasing particle size due to moisture that results in deposition due to increased mass of particle.

PM2 .5 – Sampling of atmospheric dust where the aerodynamic d50 diameter is 2.5 µm.

PM10 – Sampling of atmospheric dust where the aerodynamic d50 diameter is 10 µm.

Precipitant dust – Any particulate matter that has an aerodynamic diameter below 100 µm.

Total deposition – The sum of wet and dry deposition. Occult deposition is also included.

Wet deposition – The collection of precipitant dust and any soluble substances in the rainwater during periods of rainfall.

Definition of monitoring:

Keep the focus on using the tool to create awareness and achieve the objective of monitoring which is to create awareness of environmental pollution.

The design of a monitoring program must therefore have regard for the final use of the data before monitoring starts.

Environmental monitoring - The process of checking, observing, or keeping track of something for a specified period of time or at specified intervals.

Environmental monitoring must have a goal in mind and be linked to a problem or potential problem.

Keep it simple initially and then as the monitoring identifies problems, the results can be used to include more complicated results.  Reports should be easy to read and understand.

It is important to keep the big picture in focus when running a monitoring programme, while still maintaining high levels of quality control.
 

What is Precipitant Dust

Precipitant dust, as the word implies, is dust that precipitates or falls down. It refers to any particle with an aerodynamic diameter less than 100 µm and precipitant dust is broadly defined as particulate that ranges in size up to 100 µm in diameter. 

1. Inhalable particulate mass is for those materials that are hazardous when deposited anywhere in the respiratory tract.  This is the particulate that will pass from the air into the nose or mouth and will travel up to the beginning of the throat.
2. Thoracic Particulate mass is for those materials that are hazardous when deposited anywhere within the lung airways and the gas-exchange region.  This is the particulate that will pass through the throat and up to the small bronchiole.
3. Respirable particulate mass is for those materials that are hazardous when deposited in the gas-exchange region.

 

 

  The Mass % of Inhalable Dust that can be deposited.
The bold numbers in this table indicate how many of the particles with an aerodynamic diameter above 10 micron are inhaled into the body.  This provides information regarding the health implications of fallout dust, which is often regarded as benign.  If particles are breathed into the body then the contents of the dust can be absorbed into the body.

Mass % that can be Deposited in the Lung Airways and the Gas-exchange Region
The bold numbers in this table indicate how many of the particles with an aerodynamic diameter above 10 micron are inhaled into the body.  This provides information regarding the health implications of fallout dust, which is often regarded as benign.  If particles are breathed into the body then the contents of the dust can be absorbed into the body.

The fall-out dust standards from STANDARDS SOUTH AFRICA are shown in the table below.  (SANS 1929:2005)

How to collect fallout dust.

The present method used to establish precipitant dust levels is the ASTM (American Standard Test Method) D-1739 of 1998 “Standard Method for Collection and Analysis for Dust Fall (Settleable particulates)” 
While single open buckets partly-filled with a capture medium will accumulate all precipitating dust, this does not establish precipitant dust emanating from a given direction unless the bucket is closed to any dust from other directions. Such open buckets are also subject to inaccuracies due to wind turbulence within the buckets, lower air densities over the bucket and other factors.

The single-bucket precipitant dust collection method “is a crude and nonspecific test method, but it is useful in the study of long term trends.”

Image Courtesy of http://www.meted.ucar.edu/mesoprim/dust/print.htm

http://www.meted.ucar.edu/mesoprim/dust/print.htm

  Dust Storm Approaching Melbourne, Australia. - For more information see http://www.meted.ucar.edu/mesoprim/dust/print.htm

http://www.meted.ucar.edu/mesoprim/dust/aeolian.htm - Wind-Driven Movement of Sand and Dust particles by Creep, Saltation, and Suspension.  Some of these are collected in dust monitoring equipment

http://www.meted.ucar.edu/mesoprim/dust/shear_plume.htm

http://www.meted.ucar.edu - Register and get access to their material.  An excellent resource.

Initially keep it simple and then if required go to more complex scenarios. 

 

How to Calculate fallout dust Results and Interpretation.

1.0               CALCULATIONS

·        The cross-sectional area of the buckets is a standard constant in all of the calculations representing the area over which fall-out collection has been made:  = 0.02545m²

·         The actual mass collected is derived by subtraction of the mass of the filter (mass₁) from the combined mass of the filter and filtrate (mass₂):
 Mass₂ – mass₁ = collected mass of dust sample 

·         All units should be expressed in milligrams and the value of milligram/square metre/day derived from the formula:
 Fall-out rate (mg/m²/day) = (collected mass x 1)/(0.02545 X days)

2.0               LIMITATIONS OF SAMPLING AND FILTER MATERIALS 

·         Generally finer suspended dust (2.5µm > 5µm) will remain airborne almost indefinitely due to the dynamic nature of the air currents and thermal activities on any given day, even if there is no wind at all.  A rapid increase in humidity together with an absence of wind will result in precipitation of less than 5 micron particulate. 

·         Particulate larger than about 5µm will settle on a very still day and this material is collected within the DustWatch buckets in varying amounts depending on the wind velocities. 

·         Particulate of a size about 0.5mm carried by high wind velocities will not be collected within the buckets due to the aerodynamic shape and wind screen effect of the selector disk.  This acts much like the wing of an aircraft, which produces a high-pressure area below the disk, keeping the gritty particulate from falling into the buckets.  The disk optimum efficiency occurs at wind velocities exceeding 2.0m/s.   At velocities below 3.0 m/s no particulate of this size is lifted higher than a maximum of about 2.0m. 

·        Once the wind changes direction the particulate starts precipitating and this gets captured in the buckets.  We thus note that no dust gets captured during very windy conditions but actually when the wind drops.   Once the wind changes, the maximum precipitation rate is reached when the air mass movement is totally arrested and then starts to move in the opposite direction.