Lesson-specific diagram for Petrochemical smog: reaction networks and evidence. It visualizes the reasoning sequence tied to $\mathrm{NO_x+VOC+h\nu\to O_3+secondary\ products}$. Text alternative: Photochemical smog is a reaction network whose severity depends on emissions, sunlight, meteorology, and secondary chemistry. Choose a representation that matches the chemical scale and evidence; do not infer more precision or mechanism than the data support.