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.

自然和人为生物质燃烧是全球颗粒物污染的主要来源之一，会影响空气质量、气候和人类健康。德里是世界上人口最多的城市之一，在冬季经历了由颗粒物污染引起的严重雾霾事件，但人们对其中的潜在途径仍知之甚少。在这里，我们通过德里印度理工学院 1 月至 2 月期间对气溶胶和气体的测量，观察到德里雾霾发展期间强烈而频繁的夜间粒子生长事件。尽管新粒子形成的条件不利，包括缺乏低挥发性蒸气的光化学产生以及在极度污染的条件下蒸气大量损失，但粒子生长事件系统地发生。我们估计这个过程占雾霾期间总粒子数浓度的 70%。我们确定生物质燃烧产生的主要有机蒸气的冷凝是观察到的生长的主要原因。夜间温度的急剧下降和生物质燃烧排放的快速增加使这些主要有机蒸气失去平衡，导致它们凝结并使纳米粒子生长到与雾霾形成相关的尺寸。与全球大多数城市地区相比，初级生物质燃烧排放物对夜间纳米粒子生长的这种高影响是独一无二的，在这些地区，白天通过氧化形成的低挥发性蒸汽驱动粒子生长和雾霾形成。由于住宅供暖和烹饪不受控制的生物质燃烧在印度-恒河平原很普遍，我们预计这种生长机制将成为超细颗粒物的来源，影响全球 5% 人口的健康并影响区域气候。我们的工作表明，调节不受控制的生物质燃烧排放可能有助于抑制夜间雾霾的形成并改善印度的人类健康。