Which of the following pollutants causes aggravation of respiratory disease and atmospheric discolouration?

Nitrogen dioxide both worsens respiratory illness and gives the air a brown discolouration.

Carbon monoxide is a colourless gas that harms by cutting oxygen delivery, not by discolouring air.

Lead is a neurotoxin affecting the nervous system rather than tinting the atmosphere.

Ozone irritates the lungs but is itself colourless and does not brown the sky.

The reddish brown tint of polluted urban air is a signature of nitrogen dioxide.

Nitrogen dioxide also drives the formation of photochemical smog and acid rain.