Photochemical smog is a harmful light blue gas formed when the primary pollutants such as hydrocarbons (HC) and nitrogen oxides (NOx), which are discharged into the atmosphere by automobile and factory, react photochemically under the action of sunlight (ultraviolet light) to generate secondary pollutants and mix with the primary pollutants.
Photochemical smog can drift hundreds of kilometers on air currents, damaging crops far from cities.
Photochemical smog mostly occurs in summer and autumn when the sunlight is strong. With the continuous progress of photochemical reactions, the reaction products accumulate continuously, and the concentration of photochemical smog increases continuously.
Photochemical smog causes many adverse effects on air, affecting plants and animals, even building materials, and greatly reducing travel visibility.
What Are Main Risks of Photochemical Smog?
- Damage to Human and Animal Health
Photochemical smog can irritate respiratory mucosa, leading to headaches, breathing disorders, worsening of chronic respiratory diseases, and abnormal lung function in children.
Photochemical smog can promote asthma attacks in asthma patients, cause chronic respiratory disease deterioration, and damage to lung function, and long-term inhalation of oxidants can reduce the metabolism of human cells and accelerate human aging.
The obvious harm of photochemical smog is its irritating effect on human eyes. In California, photochemical smog has caused pinkeye in three-quarters of the population. During the photochemical smog period in Tokyo in 1970, 8,000 people suffered from pinkeye.
Studies have shown that peroxyacetonitrate (PAN) in photochemical smoke is a strong tear inducing agent, with a tear inducing effect equivalent to 200 times that of formaldehyde.
Another strong eye irritant is peroxybenzoyl nitrate (PBN), which is about 100 times more potent than PAN.
Ozone is a strong oxidant and has a special odor at the concentration of 0.1ppm. It can reach the deep layer of the respiratory system, irritate the airway mucosa and cause chemical changes.
It acts as radiation to cause chromosomal abnormalities and age red blood cells.
- Damage to Plant Growth
When plants are damaged by ozone, the epidermis will initially fade and be waxy, and after a period of time the pigment will change and the leaves will appear reddish-brown spots.
PAN gives the underside of leaves a silver-gray or bronzer color, which affects plant growth and reduces plant resistance to pests and diseases.
- Damage to Building Material
The increase in ultraviolet radiation from sunlight due to stratospheric ozone depletion will accelerate the degradation and aging of materials, especially polymer materials, used in construction, painting, packaging and wire and cable.
The damage is especially severe in the tropics, where heat and sunshine are abundant. Losses due to this destructive effect are estimated to be in the billions of dollars each year globally.
Both artificial and natural polymers and other materials can be adversely affected. When these materials, especially plastics, are used in places that have to withstand sunlight, they can only be protected from photochemical smog by adding light stabilizers and antioxidants or by surface treatments.
- Reduction of Atmospheric Visibility
One of the important characteristics of photochemical smog is that it reduces atmospheric visibility and shortens visual range. This is mainly caused by photochemical smog aerosols formed by polluting substances in the atmosphere.
The size of such aerosol particles is generally in the range of 0.3 to 1.0μm. Because such particles are not easy to settle due to gravity, they can be suspended in the air for a long time and migrate over a long distance.
They are related to the wave length of human vision, which scatters sunlight and significantly reduces the visibility of the atmosphere, thus impairing the safe operation of vehicles such as cars and airplanes, leading to an increase in traffic accidents.
- Control of Pollution Sources
Hydrocarbons are essential components in the formation of photochemical smog. Therefore, the formation and development of photochemical smog can be effectively controlled by controlling the emission of hydrocarbons, especially those organic compounds with high reactivity (such as alkenes and aromatic hydrocarbons containing side chains).
In addition, in the photochemical reaction formed by photochemical smog, free radical reaction plays a very important role, and the free radical reaction is mainly caused by NO2 photolysis, so it is also very important to control the emission of nitrogen oxides.
Cars produced in recent years are generally equipped with exhaust gas catalytic converters, generally using high temperature resistant porous ceramics as the support, coated with a catalyst composed of fine dispersed platinum and palladium, so as to achieve efficient conversion.
- Pollution-Free Transport
For example, hydrogen is used as engine fuel, hydrogen and oxygen fuel cells are used to power transportation vehicles (electric vehicles), and electromagnetic induction is used to promote trains (superconducting suspension trains), etc.
The use of clean energy and the development of pollution-free transport has attracted the attention of scientists and governments.
- Air Purifier
A powerful air purifeir mainly includes many filter such as HEPA, activated carbon, etc. Using an air purifier at home can effectively reduce the possibility of being damdged by photochemical smog.