Causes of Haze

Formation of Haze

Haze is an accumulation of microscopic particles suspended in the Earth’s atmosphere, scientifically known as atmospheric aerosols. It reduces the visibility of air, as the tiny pollution particles make it difficult for light to reach the observer. Some of the light is absorbed by particles, and others are being scattered away. The more massive the amount of pollution in the atmosphere, the more the visibility is worsened. Furthermore, haze introduces some types of particles such as sulfates, that scatter more light, particularly during humid conditions, resulting in poor vision.

Haze consists of sufficient smoke, photochemical smog, dust, moisture, and vapour suspended in air. The major components of this pollution includes a mixture of carbon, sulfates and nitrates compounds. Some of these haze-causing particles are directly emitted to the air, while others are transported from various emission sources. The sources can be natural or man-made. Natural sources include windblown dust, and soot from wildfires. Man-made sources include motor vehicles, electric utility and industrial fuel burning, and manufacturing operations.

Components of Haze

Smoke

Formation of smoke is basically a process of conversion of molecules of hydrocarbon fuels into particles of soot. If soot is unable to find air (Oxygen) in the combustion cycle, it will pass as exhaust. In addition, if the quantity is sufficient, it will be visible and that is called smoke. The colour of the smoke depends on the size of soot particles.

Photochemical smog

Photochemical smog is a mixture of pollutants that are formed through solar radiation reaction with airborne pollution, such as nitrogen oxides and volatile organic compounds (VOCs) react with sunlight, creating a brown haze above cities.

Photochemical smog formation proceeds through a sequence of reactions, all involving a free radical mechanism.

(1) Nitrogen dioxide is photochemically decomposed by sunlight to form nitric oxide (NO) and oxygen atom (O).

NO2 + sunlight → NO + O

(2) Oxygen atoms then react with atmospheric oxygen (O2) to form ozone (O3)

O + O2 → O3  

(3) This is known as scavenging reaction. Ozone is consumed by nitric oxide to produce nitrogen dioxide and oxygen.

O3 + NO→ NO2 + O

(4) Oxygen atoms react with water to form hydroxyl radicals

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(4) The hydroxyl radicals in turn react with hydrocarbons to form hydrocarbon radicals.

Untitled                                                                                     RH = Any hydrocarbon (i.e. CH3CH2CH3 or CH3CH3),

(5) Oxidation of hydrocarbons by the hydroxyl radical leads to the formation of aldehydes. In the process, more nitrogen dioxide is being produced.

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(6) The aldehydes are oxidized further to form aldehyde peroxides and aldehyde peroxyacids. 

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whereby

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Soot

Soot is the common name by which we refer to as “particulate matter” or particle pollution (PM 2.5)—a complex mixture of tiny particles and liquid droplets formed in our air from metals, acids, and chemicals like sulfur dioxides and nitrogen oxides. The most significant sources of soot are diesel engines, vehicle tailpipes, coal plant smokestacks, oil refineries, and fires. The burning of large forests for agricultural purposes produces soot, and so does the use of grills, fireplaces and cook stoves for cooking and heating.

Soot formation

During the combustion of hydrocarbons, for example (C12H23), the majority of the fuel undergoes oxidation to form carbon monoxide, carbon dioxide and water vapour. However, there is another reaction path whereby soot is being produced via the pyrolysis reactions. These reactions break down the fuel molecules into unsaturated radicals and intermediates like acetylene (C2H2), known as soot precursors. The soot precursors then react to form small polycyclic aromatic hydrocarbons also known as PAH. The growth of the PAH species continues through reactions in which acetylene is added to aromatic ring structures. These PAH collide with each other and form larger PAH, until they become liquid particles known as incipient soot particles. These incipient soot particles continue to grow by reactions with acetylene and smaller PAH, a process known as surface growth. They also collide and coalesce, resulting in formation of soot particles.

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Definition of the terms used

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. Pyrolysis is also known as thermal cracking, cracking, thermolysis deploymerization, etc.

Polycyclic aromatic hydrocarbons, abbreviated as PAH, is a common term for higher molecular, aromatic hydrocarbons, which means they contain more than two unsubstituted benzene rings. Some examples of PAH include Phenanthrene (C14H10), Pyrene ( C16H10), Naphthalene (C10H8), etc

images       Untitledsdfdsf        Naphthalene Structure

The formation of soot depends strongly on the fuel composition. The rank ordering of sooting tendency of fuel components is: naphthalenes > benzenes > aliphatics. For instance, aromatic species (like benzene and toluene) produce more soot than aliphatic species (like n-heptane and iso-octane).However, the order of sooting tendencies of the aliphatics (alkanes, alkenes, alkynes) varies dramatically depending on the flame type. The difference between the sooting tendencies of aliphatics and aromatics is thought to result mainly from the different routes of formation. Aliphatics appear to first form acetylene and polyacetylenes; aromatics can form soot both by this route and also by a more direct pathway involving ring condensation or polymerization reactions building on the existing aromatic structure.

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Particulate Matter

Particulate matter are being classified into two sizes, PM10, whereby particle diameters are less than or equal to 10 microns in size, and PM2.5, also called “fine particulates,” whereby particle diameters that are less than or equal to 2.5 microns in size.

pm2_5_graphic_lg

Particulate matter (PM) in smog has direct emission sources from cars and trucks, but these are overwhelmed by the PM that is chemically formed in the atmosphere. PM contains various chemical compounds: organics, sulfate, nitrate, ammonium and elemental carbon make up most of the mass. Organics have direct sources from cars and trucks, but are mostly formed in the atmosphere from hydrocarbons emitted from man-made and natural sources. Sulfate is formed from sulfur dioxide that has large sources from coal-fired power plants. Nitrate is formed from nitrogen oxides from vehicles and power plants. Ammonium is formed from ammonia from agriculture and vehicles. Nitrate is more abundant at cooler temperatures,  as it forms a volatile salt with ammonium,  ammonium nitrate, that is more stable at lower temperatures. Elemental carbon, also called black carbon, is directly emitted from predominantly diesel vehicles.

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References:

http://en.wikipedia.org/wiki/Haze

http://www.epa.gov/airquality/pdfs/haze.pdf

Soot Pollution Facts

http://www.greenfacts.org/en/particulate-matter-pm/level-3/01-presentation.htm#0p0

http://patents.justia.com/patent/20130137618

https://books.google.com.sg/books?id=pOMwznXaGNUC&pg=PA176&lpg=PA176&dq=PAH+reactions+to+form+soot&source=bl&ots=hjSjy9ssu7&sig=crQJfn543JlKpUPMSsDFs0mThlI&hl=en&sa=X&ei=75wOVf23CtONuATyhYHAAQ&ved=0CDMQ6AEwBA#v=onepage&q=PAH%20reactions%20to%20form%20soot&f=false