Traffic and combustion emission controls leave cooking as a dominant local source of primary PM₁ in megacities

Megacities frequently suffer from poor air quality, with PM_2.5 concentrations routinely exceeding WHO guidelines and elevating the risk of cardiovascular and respiratory disease. Air quality legislation typically targets primary emissions from traffic, a dominant urban source of PM₂.₅. Here, we present direct, chemically resolved quantification of PM₁ emissions from three contrasting megacity neighbourhoods-London, Beijing and Delhi-revealing the magnitude and diversity of urban sources by applying a novel source attribution approach to eddy-covariance flux measurements using aerosol mass spectrometry. In Delhi, traffic and combustion dominate primary PM₁ emissions. In contrast, cooking is the largest source during mealtimes in London and Beijing, contributing up to 83% of midday emissions and 87% in the early evening in London, and up to 61% and 76%, respectively, in Beijing. These findings suggest that air quality legislation in London and Beijing has effectively reduced non-refractory transport and combustion emissions, allowing cooking to emerge as the largest local source of primary non-refractory PM₁. Although cooking is not included in national, European or global emission inventories, recent estimates from the London Atmospheric Emissions Inventory align well with our observations. Together, these results show that the sources dominating primary non-refractory PM₁ emissions differ from those controlling ambient PM₁ mass. Although cooking is now the leading primary source in London and Beijing, further reductions in urban PM₁ will depend on mitigating secondary formation processes.