What is the main cause of brown or photochemical smog?

1. emissions from aircrafts
2. aerosol cans and coolants
3. emissions from automobiles
4. coal-burning industries
5. river pollution through discharge of industrial wastes

The main cause of brown or photochemical smog is emissions from automobiles. To determine the correct answer, we need to understand the concept of brown or photochemical smog and its causes.

Brown or photochemical smog is a type of air pollution caused by a combination of sunlight, nitrogen oxides (NOx), and volatile organic compounds (VOCs) released into the atmosphere. These pollutants are primarily generated by human activities, specifically the burning of fossil fuels. When sunlight reacts with NOx and VOCs, it produces a range of harmful pollutants, including ozone, formaldehyde, and peroxyacetyl nitrate (PAN), which contribute to the formation of smog.

Now, let's examine the options provided:

1. Emissions from aircraft: While emissions from aircraft do contribute to air pollution, they are not a significant cause of brown or photochemical smog. Aircraft emissions mainly contribute to high-altitude emissions, such as contrails and cirrus clouds.

2. Aerosol cans and coolants: Although aerosol cans and coolants can release certain chemicals that are detrimental to the environment, they are not a primary cause of brown or photochemical smog. These can contribute to other types of pollution, such as stratospheric ozone depletion.

3. Emissions from automobiles: This option is the correct answer. Automobile emissions, particularly from vehicles that burn gasoline or diesel fuel, are a major source of NOx and VOCs. When these emissions combine with sunlight, they contribute significantly to the formation of brown or photochemical smog.

4. Coal-burning industries: Coal-burning industries are major contributors to air pollution, primarily through the release of sulfur dioxide (SO2) and particulate matter. However, they are not the main cause of brown or photochemical smog, as this type of smog is primarily formed due to vehicle emissions.

5. River pollution through discharge of industrial wastes: While river pollution can have detrimental effects on water quality and aquatic life, it is not directly linked to the formation of brown or photochemical smog.

Therefore, the correct answer to the question is option 3: emissions from automobiles.