Photochemical smog is that hazy, modern one from car exhausts reacting with sunlight, making ozone and PAN—it's a summer afternoon problem. Classical smog, the older 'London fog', comes from burning coal, mixing smoke with sulphur dioxide, and it's a cold, damp morning sight. The key reactions? For photochemical, it's those nitrogen oxides and VOCs getting a tune from the sun; for classical, it's sulphur turning to sulphuric acid in the fog. I picture it because my daughter’s PhD work has her tracing these very reactions in the air over our valley, while I watch the *phijigee* wheeze in the quiet.
#ipl#dc
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