Heatwave Smog: Ground-Level Ozone Formation

Heat & Sunlight

Air temperature 25°C

Sunlight / UV intensity 60%

Emissions

Traffic NOx emission 50%

Wind / ventilation 30%

Air Quality

Ozone (O₃)

0 ppb

Reaction rate

0 /s

Active NOx

PAN reservoir

How it works

Ground-level ("bad") ozone is not emitted directly — it forms in the air when nitrogen oxides (NOx, from traffic and combustion) and volatile organic compounds (VOCs) react in sunlight:

NO2 + hv (UV) → NO + O
O + O2 → O3
VOC + OH → peroxy radical (RO2)
RO2 + NO → NO2 + (VOC oxidation product)  [regenerates NO2, net catalytic O3 production]

Two heat-driven effects make this cycle a real climate–health hazard during heatwaves:

Reaction rate — the effective smog-forming rate is modelled with an Arrhenius-style temperature factor that roughly doubles every 10°C, matching the well-documented correlation between hot, sunny days and high ozone readings in cities.
Biogenic VOC emission — trees and plants emit VOCs like isoprene at a rate that also roughly doubles per 10°C of leaf temperature (the Guenther emission algorithm used in real air-quality models), so heat pumps extra fuel into the reaction even without extra traffic.
PAN as a NOx reservoir — peroxyacetyl nitrate (PAN) forms as a stable NOx "storage" molecule at cooler temperatures and decomposes back to active NOx following k = A·exp(−Ea/RT) with Ea ≈ 113 kJ/mol — so heat also rapidly releases stored NOx back into circulation.

The Ozone readout is converted to an AQI-style band using the US EPA 8-hour ozone breakpoints (ppb): Good ≤54, Moderate 55–70, Unhealthy for Sensitive Groups 71–85, Unhealthy 86–105, Very Unhealthy >105 — the same "climate change → more heatwaves → more smog days → more asthma attacks" pathway public-health agencies track.

Controls: Temperature and Sunlight drive the reaction and biogenic emission; Traffic controls how much NOx fuel is emitted; Wind/ventilation dilutes and removes all species, simulating a breeze clearing the air.

NOx (traffic) VOC (fuel / biogenic) O₃ (ozone / smog)

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