What Are Particulate Emissions?

Particulate matter in the air is a complex mixture of organic and inorganic matter, including natural sea salt, soil particles, soot generated by combustion, and sulfate and nitrate produced by secondary conversion in the air. ^{[1] In} theory, particulate matter in atmospheric pollutants generally refers to particulate matter larger than molecules, but the minimum limit is actually 0.01um. In engineering technology, particulate matter is generally referred to simply as dust. ^[2]

Due to natural processes and human activities, particles continue to enter the air. Physical and chemical processes in the air also produce particles. Air particles generally refer to material particles (dispersed phase) in air (continuous phase), while aerosol refers to the entire material system composed of material particles (dispersed phase) and air (continuous phase). Therefore, air particulate matter is not the same as aerosol. ^[3]

The main components of air particulate matter include sulfate, nitrate, ammonia, hydrogen ions, moisture around the particles, elemental carbon, a variety of organic compounds, crustal matter, and sea salt. It is easy to observe sulfur, ammonia and hydrogen ions in fine particles, elemental carbon, secondary organic compounds and primary organic substances and certain transition metal elements produced by cooking or combustion. Components such as potassium and nitrate are easily observed in both coarse and fine particles, but the potassium in the coarse particles mainly comes from the soil, and the potassium in the fine particles mainly comes from emissions from burning wood or cooking meat. Nitrate in fine particles mainly comes from granular ammonium nitrate formed by the reaction of gaseous nitric acid and gaseous ammonia. Nitrate in coarse particles mainly comes from the reaction between gaseous nitric acid and previously existing coarse particles.

(1) Classification according to the source of particulate matter

Single dust source category: Particulate matter discharged or produced by similar sources, such as coal smoke dust, soil dust, wind dust and so on.

Mixed Dust Sources: Mixed particulates emitted or generated by multiple sources, usually referring to various types of dust.

(2) Classification according to the positional relationship between the source of pollution and the study area

External urban dust: refers to the dust that is transported from outside the city to the urban area.

Dust produced in the city: refers to the dust generated by the emission sources in the area within the city.

(3) Classification according to whether the particulate matter discharged from the pollution source passes through a fixed exhaust pipe or building structure

According to whether the particulate matter discharged from the pollution source passes through the fixed exhaust pipe or the building structure, it is divided into organized emission source and unorganized emission source.

(4) Classification according to whether the pollution source of the discharged particulate matter is fixed

The stationary source category refers to an emission source that is fixed at a certain location and cannot be moved. Fixed sources are divided into two types: organized sources and unorganized sources. The fixed sources of organized emissions can also be divided into overhead sources, intermediate sources (overhead sources and intermediate sources are also called point sources), and low-level sources according to the height of the exhaust cylinder.

The mobile source category refers to the emission source that moves along a certain route, and is also called the line source category. It can be divided into the following types.

Ground mobile sources include motor vehicle emission sources and train emission sources. Motor vehicle emission sources include large, medium and small passenger cars, large, medium and small trucks, special vehicles and motorcycles. Train emission sources include steam locomotives, diesel locomotives, and electric trains.

Air mobile sources include large, medium and small civil and military aircraft.

Water mobile sources include large, medium and small civil and military vessels.

The open source category refers to sources with unorganized emissions in the open air environment. The open source category can be divided into the following types.

Bare soil wind erosion type includes bare wasteland, farmland, mountains, roads, etc.

Yard type includes industrial stockpile, coal pile, ash yard, building material yard, garbage dump, etc.

Road paving type includes various paved roads and buildings.

Building construction types include roads and bridges under construction and various industrial and civil buildings.

The above is a method roughly classified according to the many emission pathways of particulate matter. The classification has a certain overlap and should be flexibly mastered according to the actual needs of local environmental management. ^[4]

The particle size and distribution of particulate matter are the main parameters of pollutant control. They have a great impact on the mechanism of the dust removal process, the design of the dust collector and its operating effect, so it is of great significance to study them.

(1) Particle size

Particle size is one of the basic parameters of particulate matter. Different particle sizes have different physical and chemical properties, and affect the dust removal mechanism and performance.

The particle size of the particulate matter refers to a representative size representing the size of the particulate matter. For spherical particles of uniform size, the diameter can be taken as the representative size of the particles, that is, the particle size of the particles. However, for practical particles, not only their sizes are different, but also their shapes are different, so their diameter cannot be taken as their particle size. A certain method is required to determine a representative size representing the size of the particulate matter as the particle size of the particulate matter.

The particle size is generally divided into a single particle size that represents a single particle that is too small and an average particle size that represents a particle group consisting of particles of different sizes. ^[5]

Respirable particulate matter comes from human direct emissions, mainly from combustion processes, such as the combustion of fossil fuels (coal, gasoline), biomass (straw, firewood), and secondly from gases that are converted into respirable particulates in the air The pollutants are mainly SO2, NO2, NH3 and TVOC. Other sources such as road dust, kitchen smoke, forest fires, pollen, fungal spores, etc. can also cause increased levels of inhalable particulate matter in the air.

Inhalable particulate matter can accumulate in the respiratory system and cause many diseases. The smaller the diameter of the particles, the deeper the area into the respiratory tract. Particles with a diameter of 10um are usually deposited in the upper respiratory tract. Particles with a particle diameter of more than 10um will be blocked outside the human nose. Particles with a particle diameter of 2.5-10um can enter the upper respiratory tract, but some can be discharged through sputum In vitro, the harm to human health is relatively small.

The main source of fine particulate matter PM _2.5 is man-made. Residues emitted by combustion during daily power generation, industrial production, automobile exhaust emissions, etc., mostly contain toxic substances such as heavy metals, including dust, sulfuric acid, nitric acid, organic hydrocarbons and other particles dispersed into the air. PM _2.5 can penetrate the human respiratory tract and enter the body, causing physical diseases; it has a toxic effect on the cardiovascular system. PM _2.5 has a strong extinction ability, which greatly shortens the field of view and significantly reduces sunlight. ^[6]