8.3.3—Fossil fuel combustion
- Syllabus
- First assessment 2026
- Objective
- 8.3.3
- Level
- HL
Direct Emission Is Not the Same as Formation
Combustion can emit PM2.5, PM10, CO and SO₂ directly, while tropospheric ozone is secondary and forms in the atmosphere from precursors.
Label a pollutant by where it appears in the pathway. NOx from a tailpipe can react in sunlight later; ozone is therefore not simply an exhaust gas even when traffic supplies its precursor.
A car emits NOx directly at 8 a.m.; sunlight can help form ozone downwind later in the morning.
No. It may show secondary ozone formed from traffic precursors and sunlight.
‘From fossil fuels’ describes the source pathway; it does not make every pollutant primary.