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| Last Updated:: 12/07/2024

Suspended Particulate Matter

Suspended Particulate Matter
Atmospheric particulate matter – also known as particulate matter (PM) or particulates – are microscopic solid or liquid matter suspended in the Earth's atmosphere. Particulate matter is the sum of all solid and liquid particles suspended in air many of which are hazardous. This complex mixture includes both organic and inorganic particles, such as dust, pollen, soot, smoke, and liquid droplets. These particles vary greatly in size, composition, and origin. It consists of microscopically small solid particles or liquid droplets suspended in the air. The smaller the particles, the deeper they can penetrate into the respiratory system and the more hazardous they are to breathe. Particulate matter is primarily a problem in the wintertime in the Bay Area, when seasonal wood-burning makes a substantial contribution.
Particulate matter is often divided into two main groups, based on their size
  • Inhalable coarse particles: These particles range from 2.5 micrometers to 10 micrometers in diameter (PM10 – PM2.5).
  • Fine particles: These particles are found in smoke and haze with a size up to 2.5 µm (PM2.5).
While Inhalable coarse particle are found near roadways and dusty industries, fine particles can be directly emitted from sources such as forest fires, or they can form when gases emitted from power plants, industries and automobiles react in the air.
This diagram shows types and size distribution in micrometres of atmospheric particulate matter.
Sources of particulate matter
Particles originate from a variety of stationary and mobile sources and may be directly emitted (primary emissions) or formed in the atmosphere (secondary emissions) by transformation of gaseous emissions.
Primary PM sources are derived from both human and natural activities. A significant portion of PM sources is generated from a variety of human (anthropogenic) activity. These types of activities include agricultural operations, industrial processes, combustion of wood and fossil fuels, construction and demolition activities, and entrainment of road dust into the air. Natural (non anthropogenic or biogenic) sources also contribute to the overall PM problem. These include windblown dust and wildfires.
Secondary PM sources directly emit air contaminants into the atmosphere. Hence, these pollutants are considered precursors to PM formation. These secondary pollutants include SOx, NOx, VOCs, and ammonia. Control measures that reduce PM precursor emissions tends to have a beneficial impact on ambient PM levels.
How are particles formed?
The size of suspended particles in the atmosphere varies over four orders of magnitude, from a few nanometres to tens of micrometres. The largest particles, called the coarse fraction (or mode), are mechanically produced by the break-up of larger solid particles. These particles can include wind-blown dust from agricultural processes, uncovered soil, unpaved roads or mining operations. Traffic produces road dust and air turbulence that can stir up road dust. Near coasts, evaporation of sea spray can produce large particles. Pollen grains, mould spores, and plant and insect parts are all in this larger size range. The amount of energy required to break these particles into smaller sizes increases as the size decreases, which effectively establishes a lower limit for the production of these coarse particles of approximately 1 µm. Smaller particles, called the fine fraction or mode, are largely formed from gases. The smallest particles, less than 0.1 µm, are formed by nucleation, that is, condensation of low-vapour-pressure substances formed by high-temperature vaporization or by chemical reactions in the atmosphere to form new particles (nuclei). Four major classes of sources with equilibrium pressures low enough to form nuclei mode particles can yield particulate matter: heavy metals (vaporized during combustion), elemental carbon (from short C molecules generated by combustion), organic carbon and sulfates and nitrates. Particles in this nucleation range or mode grow by coagulation, that is, the combination of two or more particles to form a larger particle, or by condensation, that is, condensation of gas or vapour molecules on the surface of existing particles. Coagulation is most efficient for large numbers of particles, and condensation is most efficient for large surface areas. Therefore the efficiency of both coagulation and condensation decreases as the particle size increases, which effectively produces an upper limit such that particles do not grow by these processes beyond approximately 1 µm. Thus particles tend to “accumulate” between 0.1 and 1 µm, the so-called accumulation range.
Sub micrometre-sized particles can be produced by the condensation of metals or organic compounds that are vaporized in high-temperature combustion processes. They can also be produced by condensation of gases that have been converted in atmospheric reactions to low- vapour-pressure substances. For example, sulphur dioxide is oxidized in the atmosphere to form sulphuric acid (H2SO4), which can be neutralized by NH3 to form ammonium sulfate. Nitrogen dioxide (NO2) is oxidized to nitric acid (HNO3), which in turn can react with ammonia (NH3) to form ammonium nitrate (NH4NO3). The particles produced by the intermediate reactions of gases in the atmosphere are called secondary particles. Secondary sulphate and nitrate particles are usually the dominant component of fine particles. Combustion of fossil fuels such as coal, oil and petrol can produce coarse particles from the release of non-combustible materials, i.e. fly ash, fine particles from the condensation of materials vaporized during combustion, and secondary particles through the atmospheric reactions of sulphur oxides and nitrogen oxides initially released as gases.
Causes of Particulate Matter
Natural Causes
  • Volcanoes – Erupting volcanoes eject large quantities of particulates including volcanic ash and gases into the atmosphere, volcanic eruptions have been directly associated with climate change since studies began.
  • Dust storms– Strong winds can pick up vast clouds of dust which in turn are dispersed into the atmosphere and can take years to return to the surface.
  • Forest and grassland fires – Wood and grass smoke contain a complex mixture of particulates such as carbon monoxide and hydrogen cyanide, which are lifted into the air and rest in the atmosphere.
  • Living vegetation– Vegetation that emits particles to the air, such as isoprene, methanol and spores. These particles can be carried upwards by the wind and add to the level of particulates in the atmosphere.
  • Sea spray – Due to the large amounts of plastics that have broken down to Nano scale, particles and can be found in ocean water all over the world. These hazardous particles can be thrown into the air by strong sea spray.
  • Tornado’s and hurricanes – These powerful weather systems can pick up large quantities of resting dust and pollutants just from the countryside, let alone when they pass through cities and encounter cement dust and higher levels of overall pollutants.
Man Made Causes
  • Coal Combustion – Coal burning is still used in the majority of countries to generate heat and supplying energy, the burning of coal directly increases the amount of carbon monoxide and other hazardous particles into the atmosphere.
  • Oil Combustion– Used for fueling vehicles, which in turn emit a large number of exhaust fumes containing hazardous particulates all over the globe, in huge quantities. Due to the large amounts if these pollutants in cities, countless deaths are caused by particulates.
  • Wood combustion – The burning of wood is a wide scale cause of particulates, used for many purposes such as heating and generating power, the combustion process sends many toxic cocktails of pollutants into the atmosphere, such as soot.
  • Construction– Cement dust is a large portion of overall global pollutants, because of the dust’s small particle size, it can hang around in the air for quite some time. The use of vehicles in the construction industry and other known pollutants makes the construction industries in dire need of reforming.
  • Demolition – Huge amounts of dust are thrown into the air during even the smallest demolition project, these particles are picked up into the wind, and again due to the small size of said particles, they can stay airborne for a very long time.
  • Road dust – Roads are covered in microscopic dust and pollutants which are sent airborne by the air pressure changes and wind caused when a car uses a road, this happens all over the planet.
  • Power plants – Plants that burn fossil fuels for energy and even nuclear plants disperse particulates on a huge scale, vast plumes of smoke will be found at most power plants, dispersing hundreds of cubic feet of pollutants every hour.
  • Industrial – Manufacturing plastics and other materials which create toxic fumes are dispersed into the oceans, air and land. Which in turn adds to the huge amount of man-made pollutants.
  • Agricultural – Pesticides and other volatile chemicals are sent into the air via sprayers and liquid jets. Again the agricultural industries use a large number of vehicles running on fuels such as petrol and diesel, which all attribute to the level of air pollution.
  • Livestock– The livestock industry creates a huge amount of particulates which are dispersed into the air, ground and oceans. Even the animals themselves have been found to disperse great quantities of methane into the atmosphere.
  • Deforestation – Felling trees for various industries impacts the rate at which trees would naturally produce carbon dioxide worldwide, however recent trends of replanting and harvesting cycles have reduced this.
  • Poor condition of anti-pollution technology – In recent years the world has made a strong effort to combat the high levels of lethal pollutants, but unfortunately, some countries have produced very minimal reductions in annual pollution output.
  • Tobacco smoke – Hundreds of toxic chemicals are present in tobacco smoke, and due to the millions of smokers worldwide, this leads to further pollution.
Effects of Particulate Matter
Climate effects
  • Volcanic eruptions – These have been linked with changes in the earth’s climate. For example, in the 1600s a volcanic eruption in Peru (Huaynaputina) is believed to have caused a devastating famine in Russia which resulted in nearly 2 million deaths.
  • Graph of Solar radiation reduction due to volcanic eruption.
  • Eruption of Mount Pinatubo– The eruption of 1991, the second largest terrestrial eruption of the 20th century, led to a world-wide temperature reduction of 0.5 degrees Celsius which lasted several years.
  • Weather – Particulates are thought to affect weather on a regional scale and have been linked to the failure of the Indian Monsoon. Due to suppression of levels of evaporation of water from the Indian Ocean.
  • Drought– Aerosol haze and particulates are believed to be pushing tropical rainfall southward, leading to a number of droughts across the world. Droughts worldwide have been occurring much more often since recording began.
  • Rainfall declines– A decline in Australian rainfall have led researchers to believe the increase of pollutants from Asia have shifted multi-latitude systems southward.
  • Greenhouse Gasses – Our atmosphere’s molecular make up has changed dramatically since the industrial revolution. The increase in global industries has led to a build-up of so called greenhouse gasses in the atmosphere, which prevent heat from escaping the planet leading to global warming.
  • Global dimming – Reductions in the earths direct irradiance have led researchers to believe that the increase in particulates in the atmosphere has impacted this, global dimming also creates a cooling effect, counteracting the heating of the greenhouse gases.
  • Ocean acidification – Due to the higher levels of carbon dioxide released by human activity. An estimated 30-40% of carbon dioxide dissolves into the oceans, causing harmful effects to ocean life such as coral bleaching.
Health Effects
  • Asthma – A rising rate of diagnoses have been linked to the increased levels of fine pollutants in countries worldwide, particularly in areas with higher pollution.
  • Lung cancer– Fine particles that penetrate deep into the human respiratory system and attack the bronchi, affecting the health of the lungs and leading to cancerous growths.
  • Cardiovascular disease – Numerous different particulates have drastic effects on the heart and its functions, again caused by the fine particulates that easily pass into the human system unfiltered.
  • Premature delivery – Exposure to high levels of air pollutants has led to an increase in the amount of failed pregnancy’s, especially in towns and cities with higher levels of pollution.
  • Birth defects – Particulates pass through the mother and into the child at any point of the pregnancy and can lead to a wide range of birth defects.
  • Premature death – Typically higher in regions with high levels of air pollutants and aerosols.
  • Vascular inflammation – Caused by a plaque build-up in the arteries, directly caused by particulates inhaled.
  • Atherosclerosis – Hardening of the arteries that reduces elasticity, leading to heart problems, also caused by plaque build-up.
  • Radiation exposure– A large number of particulates are formed up of radioactive material such as uranium and thorium, which is then inhaled or finds its way into crops which in turn are consumed.
  • One Million Deaths– Every year are associated directly to the air pollution caused by the coal industry alone.
  • 5 Million Deaths – Every year are believed to be caused by particulates worldwide.
Vegetation Effects
  • Mortality – Stomatal openings are clogged, leading to failures during the photosynthesis process.