A silent force is altering our atmosphere—rising soil emissions of nitrous acid, driven by climate change and fertiliser use, are fueling dangerous levels of ozone pollution that could derail global climate and crop stability.
Key Points at a Glance
- Soil nitrous acid (HONO) emissions have risen sharply since 1980
- These emissions significantly contribute to the formation of harmful ground-level ozone
- Agricultural fertilisation and rising soil temperatures are major drivers
- Regions with low industrial pollution are paradoxically more vulnerable
- PolyU’s modelling can reshape global air pollution mitigation strategies
For decades, ozone pollution has been blamed primarily on anthropogenic sources—tailpipes, smokestacks, and chemical industries. But a new groundbreaking study from The Hong Kong Polytechnic University (PolyU) adds a hidden and potent culprit to the list: the soil beneath our feet.
Led by Professor Wang Tao and published in Nature Communications, the research reveals a substantial rise in emissions of nitrous acid (HONO) from soils worldwide, driven largely by increasing fertiliser use and rising temperatures due to climate change. The consequence? A consistent climb in ozone levels that threatens crops, ecosystems, and the delicate climate balance.
Soil microbes and agricultural practices have long been known to release gases, but this is the first time that their global influence on atmospheric chemistry has been quantified with such precision. Using a sophisticated chemistry-climate model and data spanning from 1980 to 2016, researchers found that HONO emissions from soil have increased from 9.4 to 11.5 teragrams of nitrogen, contributing to a global average rise in surface ozone mixing ratio of 2.5% annually—with some hotspots experiencing up to 29% increases.
This ozone isn’t the protective kind that shelters life from UV rays—it’s the ground-level pollutant linked to respiratory diseases, lower crop yields, and ecological damage. It also reduces vegetation’s ability to absorb carbon dioxide, setting up a destructive feedback loop that could undermine climate mitigation efforts.
The team’s advanced modelling incorporated a wide range of variables, including fertiliser application rates, soil temperature, water content, and land use data. Crucially, their findings show that HONO emissions are not evenly distributed. Summer months see a peak due to higher soil temperatures, with Asia—especially agricultural regions in India and China—accounting for 37.2% of emissions. Other hotspots include the African savannahs, central North America, and parts of South America and Europe.
Ironically, areas with low human-generated pollution are the most affected. These are typically NOₓ-limited regions, where even modest increases in nitrogen oxide from soil HONO can dramatically amplify ozone formation due to abundant volatile organic compounds (VOCs). As industrial NOₓ emissions fall globally, more regions could fall into this vulnerable regime.
“Climate change and fertiliser use are on the rise. Without intervention, soil HONO emissions may offset gains from reducing industrial pollution,” warns Prof. Wang. He urges that these emissions be included in global air pollution strategies and that agricultural practices be adapted—such as employing deep fertiliser placement or nitrification inhibitors—to balance productivity and sustainability.
To support their conclusions, the PolyU team tapped into a dataset built from 110 prior laboratory experiments and atmospheric reanalysis data (MERRA2). They also used the CAM-Chem climate-chemistry model from the U.S. National Center for Atmospheric Research to simulate how HONO influences atmospheric ozone and vegetation exposure globally.
Looking ahead, the team plans to expand observational networks and delve deeper into microbial mechanisms behind HONO emissions. These steps will be critical for more accurate predictions and for crafting practical strategies to prevent further atmospheric damage from the ground up.
