These particles remain airborne for long periods, lowering visibility. Both can be toxic but Nitrogen dioxide is considered to be of most concern for asthmatics. The main source of the gases in urban areas are motor vehicle exhaust, gas cookers and kerosene heaters indoors. The brown haze sometimes seen over cities is mainly nitrogen oxides. These gases are also partly responsible for the generation of ozone, when acted upon by sunlight in the presence of other chemicals.
Ozone is a form of oxygen.
It is a colourless gas that has a very pungent odour. It exists naturally at low concentrations in the stratosphere, where it absorbs ultraviolet radiation. In the troposphere it exists naturally at extremely low concentrations. But these concentrations increase when sunlight acts on various gases coming mainly from vehicle exhausts, and ozone then becomes a pollutant in the troposphere. Ozone is a highly corrosive gas and is poisonous to most organisms.