Photochemical Smog

Photochemical smog is the haze of ozone, oxidants, and secondary particles produced when sunlight drives reactions among nitrogen oxides and volatile organic compounds in polluted air.

Definition

A secondary air-pollution condition in which photochemical reactions among NOx and volatile organic compounds generate ozone, oxidants, and fine particles.

Scope

This topic covers the precursor emissions, the sunlight-driven radical chemistry that converts them to ozone, peroxyacetyl nitrate, and aerosol, the diurnal cycle of oxidant buildup, and the meteorological conditions that trap smog over urban areas.

Core questions

• Which precursor emissions create photochemical smog?
• How does sunlight initiate the radical chemistry that builds ozone?
• Why do oxidant levels peak in the afternoon?
• What is peroxyacetyl nitrate and why does it matter?

Key theories

NOx-VOC photochemical ozone production

Sunlight photolyzes NO2 and drives OH-initiated oxidation of volatile organics, cycling NO to NO2 and accumulating ozone and oxidized products characteristic of smog.

Mechanisms

After sunrise, photolysis of NO2 and the OH oxidation of hydrocarbons produce peroxy radicals that convert NO to NO2 without consuming ozone, allowing ozone to build through the day. Acetyl peroxy radicals combine with NO2 to form peroxyacetyl nitrate, an eye irritant and NOx reservoir, while partially oxidized organics nucleate secondary aerosol.

Clinical relevance

Photochemical smog is the dominant warm-season urban air-quality problem, with ozone and fine particles affecting respiratory health and vegetation; its chemistry informs whether controlling NOx or VOC emissions is more effective in a given airshed.

History

Haagen-Smit identified the photochemical origin of Los Angeles smog in the early 1950s, distinguishing it from the sulfurous London-type smog and launching modern urban air-quality chemistry.

Key figures

• Arie Jan Haagen-Smit

Seminal works

• finlaysonPitts2000
• vanLoon2017

Frequently asked questions

How does photochemical smog differ from classic London smog?

London-type smog is a sulfur-and-soot pollution from coal burning, whereas photochemical smog is an oxidant haze produced by sunlight acting on NOx and hydrocarbon emissions, largely from traffic.