Tropospheric pollution arises due to the presence of undesirable substances in the lowest layer of the atmosphere. Oxides of sulphur, nitrogen, carbon, and hydrocarbons are the major gaseous pollutants. Oxides of sulphur (${\mathrm{SO}}_2$ and ${\mathrm{SO}}_3$) and nitrogen (${\mathrm{NO}}_2$, NO) are produced as a result of burning of fossil fuels (coal, automobile fuel). These oxides react with water in the presence of atmospheric oxygen to form nitric acid (HNO3) and sulphuric acid

(${\mathrm{H}}_2{\mathrm{SO}}_4$), which leads to the formation of ‘Acid rain’.

                     $2S{O}_{2\left(s\right)}+O_{2\left(s\right) }+ 2H_2{O}_{\left(i\right) }\to 2H_{2}S{O}_{4\left(aq\right)}$
$4N{o}_{2\left(g\right)}+O_{2\left(g\right) }+ 2H_2{O}_{\left(i\right) \to 4 NH{O}_{3\left(aq\right)}}$

Acid rain causes harm to agriculture, plants, and trees. It also leads to various respiratory ailments. Hydrocarbons are carbon and hydrogen containing compounds that burn to produce oxides of carbon. Hydrocarbons are carcinogenic and their products are also major pollutants. Carbon monoxide (CO) is poisonous in nature as it reacts with the haemoglobin of blood, which can even result in death. Though carbon dioxide (${\mathrm{CO}}_2$) is not toxic in nature, yet it contributes towards global warming by trapping the reflected IR rays. This results in the heating up of the Earth’s atmosphere, thereby leading to the melting of icebergs and glaciers. Particulates of smoke, dust, mist, and fume are harmful for human health as they are likely to block the nasal passage of a person, causing respiratory ailments. Smoke and fog combine to produce smog during a cool, humid day, thereby reducing visibility to a large extent. Photochemical smog is formed due to the presence of PAN, ozone, formaldehyde, and acrolein. It causes eye irritation, headaches, and chest pain. It also leads to the cracking of rubber and does damage to plants.