Rapid night-time nanoparticle growth in Delhi driven by biomass-burning emissions

Measurements suggest that emissions from biomass burning drive the rapid growth of particles from nanoscale into sizes relevant for haze formation during the night in Delhi. Natural and anthropogenic biomass burning are among the major sources of particulate pollution worldwide that affects air quality, climate and human health. Delhi, one of the world's most populated cities, experiences severe haze events caused by particulate pollution during winter, but the underlying pathways remain poorly understood. Here we observe intense and frequent nocturnal particle growth events during haze development in Delhi from measurements of aerosols and gases during January-February at the Indian Institute of Technology in Delhi. The particle growth events occur systematically despite the unfavourable condition for new-particle formation, including the lack of photochemical production of low-volatility vapours and considerable loss of vapours under extremely polluted conditions. We estimate that this process is responsible for 70% of the total particle-number concentration during haze. We identify that the condensation of primary organic vapours from biomass burning is the leading cause of the observed growth. The sharp decrease in night-time temperatures and rapid increase in biomass-burning emissions drive these primary organic vapours out of equilibrium, resulting in their condensation and the growth of nanoparticles into sizes relevant for haze formation. This high impact of primary biomass-burning emissions on night-time nanoparticle growth is unique compared with most urban locations globally, where low-volatility vapours formed through oxidation during the day drive particle growth and haze formation. As uncontrolled biomass burning for residential heating and cooking is rife in the Indo-Gangetic plain, we expect this growth mechanism to be a source of ultrafine particles, affecting the health of 5% of the world's population and impacting the regional climate. Our work implies that regulating uncontrolled biomass-combustion emissions may help inhibit nocturnal haze formation and improve human health in India.

Automated summary

Measurements suggest that emissions from biomass burning are responsible for the rapid growth of particles leading to haze formation during the night in Delhi. Uncontrolled biomass burning affects air quality, climate, and human health globally, with Delhi experiencing severe haze events during winter. Despite unfavorable conditions for new-particle formation, particle growth events occur systematically. The condensation of primary organic vapours from biomass burning is the leading cause of particle growth, driven by a sharp decrease in night-time temperatures and rapid increase in biomass-burning emissions. This unique mechanism may affect 5% of the world's population and regional climate. Regulating uncontrolled biomass-combustion emissions can inhibit haze formation and improve human health in India.