Nitrogen Dioxide (NO₂) is a reddish-brown gas with a sharp, acrid smell. It is part of a group of gases known as nitrogen oxides, which are produced mainly from burning fossil fuels, such as in cars and power plants. NO₂ can contribute to air pollution and is harmful to human health, causing respiratory problems and other issues. When nitrogen dioxide reacts with moisture in the atmosphere, it can form acid rain, which can damage buildings, forests, and aquatic ecosystems. Additionally, NO₂ plays a role in the formation of ground-level ozone, a key component of smog that can affect air quality and public health.