Air pollution fundamentals

How air pollution forms

• Air pollutants are either emitted directly (primary) or formed in the atmosphere through chemical reactions (secondary) — and a large share of fine particulate matter is secondary, built from gases after they are released (Fuzzi et al., 2015)
• Much of that secondary fine-particle mass is secondary organic aerosol, which forms when emitted vapours are oxidised in the air and condense into particles (Hallquist et al., 2009)
• Ground-level ozone is not emitted directly: it forms when nitrogen oxides (NOx) and volatile organic compounds react together in sunlight, which is why ozone peaks on warm, sunny days (Monks et al., 2015)
• In cities, nitrogen dioxide (NO2) comes largely from traffic, and the way NOx, NO2 and ozone interconvert governs how urban concentrations behave (Erickson & Newmark, 2020)
• Many air pollutants are also short-lived climate forcers, so the same sources and chemistry connect air quality and climate change (IPCC, 2021)

How it travels, disperses and settles

• Once in the air, pollution is carried by the wind and mixed through the atmospheric boundary layer, so weather and the time of day strongly affect the concentrations people breathe (ADMLC, 2012)
• Buildings lining a street form a "street canyon" that can trap pollution close to the road, creating hyper-local hotspots that depend on wind direction, street width and building height (Lv et al., 2021)
• Because traffic emissions disperse unevenly through the built-up urban canopy, pollution levels can change sharply over just a few metres — the basis of hyper-local exposure (Liang & Chao, 2023)
• Pollutants are removed from the air by deposition onto surfaces such as soil and vegetation; dry deposition alone accounts for roughly a fifth of the ozone removed from the global lower atmosphere (Clifton et al., 2020)

Where it comes from

• Countries estimate how much of each pollutant comes from each source — traffic, industry, farming, heating and more — using standardised methods set out in the EMEP/EEA emission inventory guidebook (EEA, 2023)
• Global inventories such as the Community Emissions Data System (CEDS) reconstruct emissions of air pollutants by sector and fuel going back centuries, providing the baseline for air-quality and climate models (Hoesly et al., 2018; McDuffie et al., 2020)
• The EDGAR database maps global emissions of air pollutants and greenhouse gases onto a grid, sector by sector (Crippa et al., 2018)
• Across Europe, road transport remains a leading source of nitrogen oxides, and concentrations and exceedances are tracked and reported each year (EEA, 2024)