Long-term nitrous acid evolution modulates ozone and secondary aerosol variations in global megacities

Nitrous acid (HONO) is a crucial urban source of HOx radicals, promoting surface ozone (O3) and secondary fine particle (PM2.5) formation, yet its long-term evolution and air-quality impacts remain poorly constrained. Here, using a deep neural network established on worldwide field campaigns, we reconstruct global surface HONO distributions. Typical megacities exhibited contrasting trends in 2005–2021, marked by a sharp decline in Beijing (−47.4% in 2013–2021), an increase in Delhi (+10.0% in 2005–2018), and slight decreases in London, New York and Los Angeles. These HONO changes are identified as major driver of O3 and particulate nitrate variations, outweighing contributions of conventional NOx and volatile organic compounds abatement in Beijing and Delhi, while showing comparable impacts in the remaining cities. A fused data-driven framework further uncovers regime-dependent HONO reduction strategies, prioritizing NOx control in summer and co-reduction of NOx, PM2.5 and direct combustion emissions in winter, to accelerate O3 and PM2.5 joint mitigation. Deep learning reveals a sharp decline of HONO in China, modest drops in EU and US but an increase in India. HONO changes drive O3 and PM2.5 variations, outweighing traditional NOx and VOC control, calling for HONO-targeted air quality management.