An air pollutant is a substance in the air that can have adverse effects on humans and the ecosystem. The substance can be solid particles, liquid droplets, or gases. A pollutant can be of natural origin or man-made. Secondary pollutants are not emitted directly.

Secondary pollutants
Secondary pollutants are formed in the atmosphere through the reactions of primary pollutants and other compounds in the presence of sunlight. They are not emitted directly into the atmosphere but are formed due to chemical reactions involving primary pollutants and atmospheric constituents. The products resulting from atmospheric reactions of hydrocarbons and nitrogen oxides in the presence of sunlight are classified as secondary pollutants.

Atmospheric reactions
When hydrocarbons and nitrogen oxides are released into the atmosphere through various anthropogenic activities such as industrial processes, vehicle emissions, and fossil fuel combustion, they undergo a series of chemical reactions in the presence of sunlight. These reactions occur in the atmosphere and involve various atmospheric constituents such as oxygen, ozone, and other reactive species. The key reactions involved in the formation of secondary pollutants include photochemical reactions and oxidation reactions.

Role of sunlight
Sunlight plays a crucial role in the formation of secondary pollutants. It provides the energy required for the initiation of chemical reactions between hydrocarbons and nitrogen oxides. Sunlight contains ultraviolet (UV) radiation, which has sufficient energy to break the chemical bonds in these compounds, leading to the formation of highly reactive free radicals. These free radicals then react with other atmospheric constituents to form secondary pollutants.

Examples of secondary pollutants
The atmospheric reactions of hydrocarbons and nitrogen oxides can lead to the formation of various secondary pollutants. Some examples include:
- Ozone (O3): It is formed through the reaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Ozone is a harmful pollutant at ground level and contributes to the formation of smog.
- Nitrogen dioxide (NO2): It is formed through the oxidation of nitrogen monoxide (NO) in the atmosphere. Nitrogen dioxide is a brownish gas and a major component of photochemical smog.
- Peroxyacetyl nitrate (PAN): It is formed through the reaction of nitrogen oxides and volatile organic compounds. PAN is a highly irritating and reactive compound that contributes to the formation of smog.

Conclusion
The products resulting from atmospheric reactions of hydrocarbons and nitrogen oxides in the presence of sunlight are classified as secondary pollutants. These pollutants are formed through various chemical reactions involving primary pollutants and atmospheric constituents. They have harmful effects on human health, ecosystems, and the environment.