Photochemical smog is primarily formed when sunlight catalyzes the reaction between nitrogen dioxide (NO2) and volatile organic compounds (VOCs), leading to the production of ozone (O3) and secondary pollutants like peroxyacetyl nitrate. The presence of sunlight is crucial as it triggers these photochemical reactions, contributing to a complex mixture of harmful pollutants that can degrade air quality and visibility, particularly in urban areas. This understanding is essential for tackling air pollution issues, aligning with UPSC's focus on environmental science and its implications for public health and policy.