Which one of the following is associated with the formation of brown air in traffic congested cities ?

The formation of 'brown air' in traffic-congested cities is primarily associated with nitrogen oxides (NOx). In urban environments, vehicle exhaust is the largest source of nitrogen dioxide (NO2) [t3]. While nitric oxide (NO) is the predominant species initially emitted, it rapidly oxidizes in the atmosphere to form nitrogen dioxide [t1]. Nitrogen dioxide is a characteristic reddish-brown gas with a pungent odor that gives photochemical smog its distinct brownish haze [t1][t3][t5]. This 'brown air' or photochemical smog forms when these nitrogen oxides and volatile organic compounds react in the presence of sunlight to produce ground-level ozone [c1][t3]. In contrast, other pollutants like sulphur dioxide, carbon monoxide, and carbon dioxide are typically colorless gases [t1][t3]. Therefore, the visible brown coloration of the atmosphere in heavy traffic areas is specifically due to the presence of nitrogen dioxide.

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > Y{r7 $ EilVIAONMEHT > p. 65