Soil Emissions Drive Surge in Global Ozone Pollution, PolyU Study Reveals.

A groundbreaking study conducted by researchers from the Hong Kong Polytechnic University (PolyU) has identified rising soil emissions of nitrous acid (HONO) as a critical factor accelerating global ozone pollution. The findings, recently published in Nature Communications, underscore the environmental consequences of climate change and agricultural fertilization on air quality and ecosystem health.

Ozone pollution, long recognized as a major threat to human health, crop yields, and global warming, has traditionally been linked to anthropogenic pollutants such as vehicle emissions and industrial activity. However, this new research sheds light on a less acknowledged yet potent contributor—soil emissions of HONO.

The international research team, led by Prof. Wang Tao, Chair Professor of Atmospheric Environment in the Department of Civil and Environmental Engineering at PolyU, compiled and analyzed global soil HONO emission data from 1980 to 2016. The study incorporated this dataset into a chemistry-climate model to quantify the influence of soil emissions on atmospheric ozone levels and their cascading impact on vegetation.

Dr. Yanan Wang, a PolyU Postdoctoral Fellow, and Dr. Qinyi Li from Shandong University served as co-first authors of the study. Their research found that microbial soil processes and fertilizer use play major roles in emitting gases, including HONO, into the atmosphere. Previous investigations have shown that soil sources can contribute up to 80% of the atmospheric HONO concentration.

HONO plays a vital role in atmospheric chemistry, especially in enhancing the production of ozone through its interaction with other pollutants and by increasing levels of nitrogen oxides (NOx), a key ozone precursor.

Using advanced parameterization techniques, the research team developed a comprehensive model that integrates variables such as soil temperature, water content, fertilizer type, and application rates. For factors difficult to quantify—like microbial activity, land use, and soil texture—the team applied data-driven estimates based on geographical and land-use information.

Their analysis revealed that global soil HONO emissions increased from 9.4 teragrams of nitrogen (Tg N) in 1980 to 11.5 Tg N in 2016. This growth contributed to an average 2.5% annual increase in global surface ozone levels, with localized surges reaching up to 29%.

These elevated ozone levels pose a serious threat to vegetation, potentially disrupting ecosystems and lowering agricultural productivity. Moreover, the ozone-induced stress reduces the ability of plants to absorb carbon dioxide, amplifying the greenhouse effect.

The research also highlighted the regional disparities in emission patterns. Emissions peak during the summer due to higher soil temperatures and active crop growth. Two-thirds of global emissions occur in the Northern Hemisphere, with Asia accounting for 37.2% of the total. Key hotspots include agricultural zones in India, eastern China, central North America, Europe, South America, and parts of Africa.

Importantly, the study found that regions with lower industrial emissions are more vulnerable to ozone increases caused by soil HONO. These areas tend to have low NOx but high volatile organic compound (VOC) concentrations, placing them in a NOx-limited regime. Here, even a modest increase in NOx from soil emissions can lead to a disproportionate rise in ozone levels.

With a global trend of declining anthropogenic emissions, more regions may shift into NOx-limited regimes, magnifying the relative impact of soil-sourced emissions.

Prof. Wang emphasized the importance of addressing this overlooked source: “Rising soil HONO emissions due to climate change and increased fertilizer usage could counteract the benefits of reducing industrial pollution. Effective air quality management must take these emissions into account.”

The study utilized data from over 110 laboratory experiments and the MERRA2 climate reanalysis dataset. Simulations were carried out using the CAM-Chem model developed by the National Center for Atmospheric Research (NCAR).

Looking forward, the research team plans to expand the global observation network for soil HONO and delve deeper into the microbial mechanisms behind these emissions. They also advocate for strategies such as improved fertilizer practices and the use of nitrification inhibitors to mitigate emissions without compromising crop yields.