When volatile organic compounds (VOCs) and nitrogen dioxide (NO2) are acted upon by sunlight in the atmosphere, they form ground-level ozone. This process is known as photochemical smog.

Here's a breakdown:

* VOCs: These are organic compounds that easily evaporate at room temperature, like gasoline, paint thinner, and cleaning supplies.

* Nitrogen dioxide (NO2): A reddish-brown gas produced by burning fossil fuels.

* Sunlight: Provides the energy needed for the chemical reactions to occur.

Sunlight breaks down NO2 into nitric oxide (NO) and a highly reactive oxygen atom (O). This oxygen atom then reacts with oxygen molecules (O2) in the air to form ozone (O3). The ozone then reacts with VOCs, leading to a complex chain reaction that creates other harmful pollutants, including aldehydes and ketones.

Ground-level ozone is a major component of smog and is harmful to human health, causing respiratory problems, irritation, and damage to lungs. It also damages plants and ecosystems.