Q.Which is always present in photochemical smog?

Sulphur dioxide chiefly damages the chloroplast/chlorophyll of plant cells, leading to chlorosis and necrosis.

Sulphur dioxide (SO2) is a major air pollutant released by burning fossil fuels. In plants it enters through the stomata and injures the photosynthetic machinery: it breaks down chlorophyll, damages the chloroplast, and produces chlorosis (yellowing) and necrotic (dead) patches on leaves. Lichens, which are extremely sensitive to SO2 …

Photochemical smog always contains ozone (O3).

Photochemical smog forms in warm, sunny cities when nitrogen oxides and hydrocarbons from vehicle exhaust react in sunlight. The reactions produce ozone (O3) along with PAN (peroxyacetyl nitrate) and other oxidants. Ozone at ground level is a harmful, always-present component of phot …

Acid rain has a pH below ~5.6, so the answer is (A).

Even unpolluted rainwater is slightly acidic (pH about 5.6) because atmospheric CO2 dissolves to form weak carbonic acid. Rain is termed acid rain when oxides of sulphur and nitrogen (SO2, NO2) from burning fossil fuels dissolve to form sulphuric and nitric acids, lowering the pH …

Photochemical smog is defined by PAN and oxides of nitrogen (plus ozone), so the intended answer is (B).

Photochemical smog forms in warm, sunny, urban areas from the action of sunlight on primary pollutants — mainly nitrogen oxides (NOx) and unburnt hydrocarbons from vehicles. Sunlight drives reactions that produce secondary pollutants, chiefly ozone (O3) and peroxyacetyl nitrate (PAN), together with the oxides of nitrogen themselves. This makes it an oxidising, irritating smog.

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Carbon monoxide is chiefly an air pollutant, so the answer is (A).

Carbon monoxide (CO) is a colourless, odourless toxic gas produced by the incomplete combustion of carbon-containing fuels, mainly in automobile exhaust. It is a major air pollutant. In the body it binds strongly to haemoglobin (forming carboxyhaemoglobin), reducing …

Lichens are good indicators of SO2 pollution — they disappear from areas with high sulphur dioxide levels.

Lichens (a symbiotic association of a fungus and an alga) are extremely sensitive to atmospheric sulphur dioxide. In heavily polluted industrial areas with high SO2, lichens fail to grow, so their presence or absence i …

Photochemical (oxidising) smog = Ozone + PAN + NO2.

Photochemical smog forms in warm, sunny cities with heavy automobile traffic. Sunlight acts on the primary pollutants (nitrogen oxides and unburnt hydrocarbons from vehicles) to generate secondary pollutants: ozone (O3), peroxyacetyl nitrate (PAN) and nitrogen dioxide (NO2). These are strong oxidising agents that irritate th …