Study finds aerosol HONO sources drive haze chemistry more than surface levels suggest

By AI, Created 3:30 PM UTC, May 24, 2026, /AGP/ – New modeling research from the Chinese Academy of Sciences finds that aerosol-related nitrous acid sources, especially nitrate photolysis, can drive secondary organic aerosol and PAN formation far more strongly than their contribution to measured HONO levels suggests. The findings could change how China and other polluted regions assess haze chemistry and air-quality controls.

Why it matters: - HONO helps generate hydroxyl radicals that accelerate ozone, secondary organic aerosol (SOA), and peroxyacetyl nitrates (PAN) formation. - The study shows that low measured HONO at the surface can hide a much larger role in regional haze chemistry. - Aerosol-related HONO pathways may matter more for pollution control than surface emissions alone.

What happened: - Researchers led by Professor Junling An of the Institute of Atmospheric Physics, Chinese Academy of Sciences, and Professor Weigang Wang of the University of Chinese Academy of Sciences modeled HONO chemistry across China. - The team combined WRF-Chem simulations with field observations from six sites in China collected from 2015 to 2018. - The paper was made available online on February 22, 2025, and published in the Journal of Environmental Sciences on December 1, 2025. - The study is titled “Inconsistent capacity of potential HONO sources to enhance secondary pollutants: Evidence from WRF-Chem modeling.”

The details: - The model tested six HONO pathways: traffic emissions, soil emissions, indoor-outdoor exchange, nitrate photolysis, heterogeneous reactions of NO₂ on aerosol surfaces, and heterogeneous reactions of NO₂ on ground surfaces. - The researchers created an “enhancement ratio” to compare each source’s impact on secondary pollutants with its contribution to HONO itself. - Ground-surface heterogeneous reactions dominated near-surface HONO, reaching 50% to 70% in urban Beijing. - Those same ground-based sources were not the most efficient at boosting SOA and PAN. - Particulate nitrate photolysis contributed only about 1% to 12% of near-surface HONO, but its enhancement ratio for SOA and PAN reached about 7. - That makes nitrate photolysis one of the most powerful drivers of secondary pollution in the model. - Including all six HONO sources improved simulation of polluted haze events. - During winter pollution episodes, modeled PAN rose by 200% to 400%, moving predictions closer to observed values. - SOA concentrations more than doubled at several observation sites when all HONO sources were included. - Some SOA underestimation remained because the model still lacked some chemical pathways. - Traffic and ground emissions affected mainly near-surface air and weakened quickly with altitude. - Aerosol-related sources stayed active through the lower and middle troposphere, supporting OH radical production across a larger vertical range. - The study also found that traffic and indoor emissions were highly localized, limiting their regional effect. - Aerosol-related sources were more widely distributed and chemically active across multiple atmospheric layers.

Between the lines: - The results separate HONO abundance from HONO impact, which is a key shift in how haze chemistry should be evaluated. - Surface monitoring alone can miss the sources that matter most for oxidation and secondary pollutant formation. - The findings suggest that emission strategies focused only on reducing near-ground sources may leave a large part of HONO-driven pollution unchanged. - Prof. An said surface HONO measurements alone cannot represent the true atmospheric impact of different sources. - Prof. Wang said pollution control plans need to account for aerosol-driven photochemical processes, not just surface emissions.

What’s next: - The study points to a need for air-quality models that track HONO sources vertically and across particle-driven chemistry. - Future mitigation strategies may need to target aerosol-related photochemistry as well as direct emissions. - Better treatment of missing chemical pathways could further improve SOA simulation in polluted episodes.

The bottom line: - In this model, where HONO comes from mattered more than how much HONO was measured at the surface. Aerosol-linked nitrate photolysis emerged as a major hidden driver of haze-forming chemistry across China.