PolyU Study Uncovers Climate Change and Fertilization’s Role in

Groundbreaking research emerging from The Hong Kong Polytechnic University (PolyU) has exposed a critical yet underappreciated contributor to the rise of global ozone pollution: soil nitrous acid (HONO) emissions. Traditionally, the escalation of ozone levels in the atmosphere has been primarily attributed to anthropogenic activities such as industrial emissions and vehicular exhaust. However, this new study overturns conventional wisdom by demonstrating that the soil itself, influenced by climate change and agricultural fertilisation, is a significant and escalating source of HONO, a key precursor in ozone formation. Utilizing a comprehensive global dataset spanning several decades and advanced chemistry-climate modeling, PolyU researchers reveal that rising soil HONO emissions are substantially aggravating ozone pollution worldwide, with serious repercussions for both ecosystems and human health.

Ozone, a reactive molecule located in Earth’s troposphere, has a complex role in atmospheric chemistry. While it protects life by filtering ultraviolet radiation in the stratosphere, surface-level ozone poses a toxic threat to plants, animals, and humans. Its formation is driven by photochemical reactions involving nitrogen oxides (NOₓ) and volatile organic compounds (VOCs). HONO, often overlooked in standard air quality models, plays a crucial intermediary role by releasing hydroxyl radicals (OH) upon photolysis, which accelerate these reactions, ultimately increasing ozone concentration. The PolyU research team focused their quantitative analysis on the dynamics of soil HONO emissions, seeking to clarify the mechanisms by which these emissions interact with atmospheric chemistry on a global scale.

The study’s lead, Professor Tao Wang, Chair Professor of Atmospheric Environment at PolyU’s Department of Civil and Environmental Engineering, spearheaded a multi-disciplinary team that meticulously compiled an expansive dataset of soil HONO emission measurements from global ecosystems. The data, sourced from 110 laboratory studies and field experiments, were synthesized into a novel parameterisation scheme that encapsulates the interplay of environmental and anthropogenic factors affecting HONO fluxes from soil. This integrated approach accounted for variables such as soil temperature, moisture content, fertiliser types, and application rates. For the complex and less directly measurable factors like microbial activity and soil texture, the team designed probabilistic representations based on geospatial metadata, allowing for refined simulations in diverse land use settings.

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Crucially, the model captured the temporal and spatial variability of soil HONO emissions over a 36-year period from 1980 to 2016. The findings reveal a notable increase in global soil HONO emissions, ascending from an estimated 9.4 teragrams of nitrogen (Tg N) annually to 11.5 Tg N. This escalation correlates strongly with intensified agricultural fertilisation practices and variations in climate parameters, including rising soil temperatures and altered moisture regimes due to climate change. The incorporation of these emissions into the Community Atmosphere Model with Chemistry (CAM-Chem), a state-of-the-art chemistry-climate model developed by the U.S. National Center for Atmospheric Research, allowed for an unprecedented simulation of their impacts on atmospheric composition and subsequent ozone formation.

Through these rigorous simulations, the research team discovered that soil HONO emissions contribute to an average global increase of 2.5% per year in surface ozone mixing ratios, with localized spikes reaching as much as 29%. These elevated ozone levels have the potential to inflict widespread ecological damage by impairing photosynthetic activity, reducing plant growth, and destabilizing ecosystems. Particularly, crop production is threatened by chronic ozone overexposure, which disrupts cellular functions in vegetation. The repercussions extend further to climate regulation, as diminished plant health impairs carbon dioxide absorption, thus exacerbating greenhouse gas accumulation and global warming.

The geographical distribution of emissions was found to be uneven, highlighting notable “hotspots” primarily in regions with intensive agricultural activity. Asia emerged as the largest contributor, responsible for approximately 37.2% of total soil HONO emissions, followed by significant contributions from India, eastern China, parts of North America, Europe, African savannahs, and South America. Seasonal fluctuations also emerged as a defining feature, with emissions peaking during summer months when soil temperatures elevate microbial activity and fertiliser application coincides with crop growth phases. This seasonality underscores the intrinsic linkages between human land management, microbial processes, and atmospheric chemistry.

Another revelatory aspect of the study concerns the interplay between anthropogenic emissions and soil HONO’s influence on ozone production. In regions with lower human-induced NOₓ emissions, the surface ozone chemistry is typically NOₓ-limited, meaning that increases in NOₓ concentration can disproportionately drive ozone formation. Therefore, soil HONO emissions exert a more pronounced impact on ozone levels in these cleaner air zones. As global policies aimed at reducing industrial and vehicular nitrogen emissions take effect, more regions are expected to transition into this NOₓ-sensitive regime, inadvertently heightening the relative importance of soil HONO emissions.

The PolyU researchers caution that the rise in soil HONO emissions driven by climate warming and continued fertilisation practices may counterbalance anticipated improvements achieved through reductions in conventional anthropogenic activities. This finding challenges current pollution mitigation frameworks that prioritize emission cuts from factories and traffic while neglecting biogeogenic sources. Professor Wang emphasizes the imperative to integrate soil emissions into air quality management strategies, highlighting that more comprehensive approaches are essential for effective pollution control and environmental sustainability.

To tackle this complex issue, the study deployed a synergistic methodology that fused diverse observational datasets with sophisticated climate and chemical modeling. Measurements from over a century of global soil samples were fed into MERRA2 reanalysis data—a powerful platform that reconstructs past atmospheric conditions—to anchor simulation parameters. CAM-Chem was then utilized to merge these inputs and simulate the resultant atmospheric chemical dynamics over time. This methodological rigor adds robustness and credibility to the conclusions drawn about soil HONO’s role in atmospheric processes.

Looking to the future, the research team plans to extend this pioneering work by enhancing the global observational network dedicated to soil HONO emissions. Improved field measurements, especially in under-represented regions, will refine model accuracy and predictive power. Additionally, the team aims to deepen understanding of the microbiological pathways governing HONO production in soils, illuminating the soil-atmosphere interface mechanisms that modulate emission rates. Such knowledge is critical for identifying intervention points to mitigate emissions effectively.

Furthermore, the study underscores the necessity of investigating agricultural mitigation strategies that balance fertiliser effectiveness with minimised environmental harm. Techniques such as precision deep fertiliser placement and the application of nitrification inhibitors are poised to reduce soil HONO emissions while maintaining or enhancing crop yields. These approaches could serve as practical solutions to simultaneously address food security and air quality objectives, epitomizing the integrated management required for sustainable development.

In summary, this landmark study from PolyU not only revises the understanding of ozone pollution drivers but also spotlights the complex interdependence of climate change, agricultural practices, and atmospheric chemistry. By elucidating the pathways through which soil emissions contribute to ozone formation, the research offers critical insights for policy-makers, environmental scientists, and agricultural managers alike. Addressing soil HONO emissions is now an essential frontier in the global battle against pollution and climate change, demanding coordinated efforts across scientific disciplines and societal sectors.

Subject of Research: Rising soil nitrous acid emissions driven by climate change and fertilisation and their impact on global ozone pollution.

Article Title: Increasing soil nitrous acid emissions driven by climate and fertilization change aggravate global ozone pollution

News Publication Date: 12-Mar-2025

Web References:

Nature Communications Article
DOI: 10.17632/6wmrvyp5xb.1

Image Credits:
© 2025 Research and Innovation Office, The Hong Kong Polytechnic University. All Rights Reserved.

Keywords

Climate change, Ozone, Ecosystems, Soils, Soil chemistry, Pollution

Tags: advanced chemistry-climate modelingagricultural fertilization effectsatmospheric chemistry and healthclimate change impact on soil emissionsglobal ozone levels and agricultureHONO as ozone precursorozone pollution sourcesPolyU research findingsreactive nitrogen compounds in environmentsoil health and air qualitysoil nitrous acid emissionstropospheric ozone and ecosystems

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