I need help Explaining how nitrogen oxides are produced and their environmental impact

Nitrogen Dioxide is one of a group of highly reactive gases known as oxides of nitrogen or nitrogen oxides. Other nitrogen oxides include nitrous acid and nitric acid. Nitrogen Dioxide is used as the indicator for the larger group of nitrogen oxides. Nitrogen Dioxide primarily gets in the air from the burning of fuel.

relief sculpture - images are set against a flat background.

a coin is a good example: the inscription, the date, and the figure are slightly raised above a flat surface. the ancient egyptians sometimes carved figures into a flat surface.  

pro: they take significantly less time to make and do not require as much tools or skills as free-standing

cons: not nearly as realistic, as it is not fully 3-d, and does not have the details that a free-standing one does.

freestanding sculptures (also called  carved in the round) are not attached to any  background.   they are 3-dimensional  and fully rendered.