Air Pollution's Secondary Impact: Understanding The Haze

A secondary air pollutant is formed when a primary pollutant comes into contact with other primary pollutants or with naturally occurring substances. Ground-level ozone, for example, is a major secondary air pollutant that forms when volatile organic compounds (VOCs) and nitrous oxides (NOx) react with sunlight in the presence of heat. Smog is another example of a secondary pollutant, formed by chemical reactions involving sunlight, air, automobile exhaust, and ozone.

Ground-level ozone

Ozone is a colourless gas that occurs naturally in the upper atmosphere, where it is beneficial as it absorbs ultraviolet radiation from the sun. However, at ground level, it becomes a harmful pollutant. Ground-level ozone is not emitted directly into the atmosphere but is formed through complex chemical reactions involving primary pollutants. These reactions can take time, and ground-level ozone is typically found downstream of primary emission sources.

The formation of ground-level ozone is influenced by various factors, including temperature, sunlight intensity, and the presence of specific chemicals in the atmosphere. On hot, sunny days, nitrogen oxide gases (NOx) and hydrocarbons emitted from sources such as vehicle engines and industrial processes, react with sunlight and water vapour to form ground-level ozone. This process is known as photochemical smog formation, where sunlight reacts with NO2, leading to the creation of smog.

The control and mitigation of ground-level ozone are challenging due to the complex and varied nature of its formation. Unlike primary pollutants, which are emitted directly from specific sources, secondary pollutants like ground-level ozone have multiple pathways of synthesis that are not yet fully understood. This lack of understanding makes it difficult to implement effective control measures, highlighting the importance of ongoing research and the development of smart technologies to address this pressing environmental issue.

Acid rain

Secondary air pollutants are formed in the lower atmosphere by chemical reactions. They are harder to control than primary pollutants because they have different ways of forming and are not yet fully understood. They form naturally in the environment and contribute to problems like photochemical smog, which is a type of secondary pollutant that includes ground-level ozone and nutrient enrichment compounds.

Photochemical smog is formed when sunlight reacts with nitrogen dioxide (NO2), which then interacts with other molecules in the air to create smog. This process is influenced by hot temperatures and ultraviolet light, which facilitate photochemical reactions. These reactions produce a yellow cloud cover that is hazardous to human health.

Nitrogen oxide gases and hydrocarbons contribute to the formation of secondary gaseous pollutants. When primary pollutants react with oxygen in the environment, they form secondary pollutants. Examples of primary pollutants include ammonia, sulphur dioxide, nitrogen dioxide, and carbon monoxide.

While primary pollutants, such as carbon monoxide, are emitted directly from sources like combustion activities and industrial processes, secondary pollutants are formed indirectly. This occurs when gases from burning fuels react with sunlight and water vapour.

Nitric acid

In addition to its environmental impacts, nitric acid can have harmful effects on human health. It can irritate the lungs and aggravate respiratory conditions such as asthma and bronchitis, increasing susceptibility to respiratory infections. When inhaled, nitric acid can damage lung tissue and contribute to the development of chronic respiratory diseases.

Overall, nitric acid is a concerning secondary air pollutant due to its adverse effects on the environment and human health, as well as its contribution to the formation of photochemical smog.

Peroxyacyl nitrates (PANs)

The most common peroxyacyl radical is peroxyacetyl, which can be formed from the free radical oxidation of acetaldehyde, various ketones, or the photolysis of dicarbonyl compounds such as methylglyoxal or diacetyl. Peroxyacetyl nitrate is the most prevalent peroxyacyl nitrate, comprising 75-90% of total atmospheric emissions. Other peroxyacyl nitrates include peroxypropionyl nitrate (PPN), peroxybenzoyl nitrate (PBzN), and methacryloyl peroxynitrate (MPAN). PANs are important in tropospheric ozone production as they transport NOx to regions where it can more efficiently produce ozone.

PANs are formed from other pollutants by chemical reactions in the atmosphere. Free radical reactions catalysed by ultraviolet light from the sun oxidize unburned non-methane hydrocarbons to aldehydes, ketones, and dicarbonyls, whose secondary reactions create peroxyacyl radicals. PANs are more toxic than ozone and are a major cause of eye irritation from photochemical smog. They can be synthesized in the gas phase via photolysis of acetone and NO2 with a mercury lamp, with methyl nitrate (CH3ONO2) as a byproduct.

At lower temperatures, PANs are stable and can be transported over long distances, providing nitrogen oxides to otherwise unpolluted areas. At higher temperatures, they decompose into NO2 and peroxyacetyl radicals. The decay of PANs in the atmosphere is primarily thermal, with long-range transport occurring through cold regions and decomposition at warmer levels. PANs can also be broken down by UV radiation and serve as a source and sink of ROx- and NOx radicals.

Smog

The formation of smog begins when ultraviolet rays from the sun interact with nitrogen oxides present in the atmosphere. Nitrogen oxide gases and hydrocarbons contribute to the formation of these gaseous pollutants. In the presence of ultraviolet light on hot days, these chemical compounds undergo photochemical reactions, resulting in the formation of smog.

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