Ammonia: A Cost-Effective Priority for Cleaner Air
Baojing Gu
Zhejiang University, China
“Mitigating nitrogen emissions into the atmosphere is vital for keeping nitrogen levels within the safe limits of planetary boundaries, which not only aids in decreasing aerosol loading but also plays a significant role in conserving biodiversity and decelerating the pace of global warming”
Nitrogen, comprising 78% of Earth's atmosphere, is essential for DNA and protein synthesis in living organisms. However, atmospheric nitrogen gases cannot be directly utilized by most life forms. To meet human food production needs, synthetic nitrogen fertilizers like urea are extensively employed, significantly contributing to over half of the global food and feed production. However, more than half of these fertilizers do not end up in food or feed but are instead lost to the environment as ammonia (NH3), nitrate (NO3-), nitrogen oxides (NOx), and nitric oxide (N2O). Additionally, the combustion of fossil fuels releases substantial amounts of reactive nitrogen, predominantly NOx. These emissions are major contributors to air and water pollution, biodiversity loss, soil acidification, and global warming. Managing nitrogen effectively is a pressing global challenge, essential for maintaining safe planetary boundaries on nitrogen and ensuring sufficient food and energy production with minimal environmental impact.
NH3 and NOx are key precursors to fine particulate matter (PM2.5), particles with a diameter less than 2.5 micrometers. Worldwide, PM2.5 pollution is responsible for over 6 million premature deaths annually, particularly in urban areas. Historically, urban-sourced NOx and other pollutants like sulfur dioxides (SO2) were considered primary contributors to PM2.5 pollution. Consequently, global efforts to mitigate SO2 and NOx emissions have led to reductions in many regions, enhancing air quality. However, as NOx emissions from industries and transport continue to decrease, the marginal cost of further reductions has risen sharply, imposing a significant economic burden on sustainable development. Moreover, in some regions, the continuous decrease in NOx emissions no longer translates into substantial air quality improvements. In this context, NH3, mainly from agricultural sources, emerges as a significant PM2.5 precursor.
To thoroughly assess the contributions of NOx and NH3 to PM2.5 formation globally, we introduce and quantify an index called N-share. This index measures PM2.5 concentration changes when emissions of NH3, NOx, or both are halted. The findings are striking: NH3 contributes more to PM2.5 formation than NOx, especially in developed regions like the European Union and the United States, where NOx levels have been significantly reduced. Crucially, the cost of reducing a unit of NH3-N (1.5 USD per kg N) is less than a tenth of that for NOx-N (16 USD per kg N). This disparity is due to the historical lack of control over NH3 emissions, previously thought to be insignificant in urban PM2.5 pollution due to their predominantly rural sources. These results suggest a paradigm shift: controlling NH3 emissions can be much more cost-effective, offering greater benefits for air quality improvement, particularly in countries where NOx is already tightly regulated, but NH3 is not.
Given the relatively low cost of reducing NH3 emissions, implementing practical measures is quite feasible. These measures include using more efficient fertilizers, improving fertilization methods, employing low-protein feed, enhancing manure management, reducing food waste, and decreasing the share of animal-based foods. For example, urease inhibitors can slow down the conversion of urea to NH3, thereby reducing NH3 emissions when spreading urea on croplands. Alternatively, switching from surface broadcasting to deep placement of fertilizers can cut NH3 emissions by more than half. Another effective method is fertigation, which combines fertilization with irrigation, applying nutrients directly to the subsurface and minimizing emissions. Lowering the protein content in animal feed and adopting better manure management practices can reduce the overall nitrogen content in manure, thereby decreasing NH3 emissions from livestock operations.
The consumer aspect of agricultural production also plays a pivotal role in NH3 emissions, especially with regards to food waste, overconsumption and meat consumption. Discarding a single piece of meat, for example, represents a loss through the entire production chain, encompassing nitrogen fertilizer production, fertilizer application, grain harvesting, feed production and transport, manure management, meat processing, and finally, the meat reaching the consumer. Thus, reducing food waste not only decreases NH3 emissions but also conserves resources and promotes human health indirectly. Moreover, overconsumption contributes similarly to NH3 emissions through the overproduction of food, while also directly causing health issues such as obesity and hyperglycemia. In many developed nations, over half of the grain produced is used for animal feed rather than human consumption, resulting in a lengthier food production chain and increased NH3 emissions from both crop and animal production. High consumption of animal-based foods is associated with similar health concerns as overconsumption. Therefore, minimizing food waste and reducing the reliance on animal-based foods are effective strategies for lowering NH3 emissions, leading to cleaner air and healthier diets.
Our research outlines a clear, cost-effective strategy for reducing NH3 emissions and contributing to cleaner air. This process involves a range of stakeholders, from industry and government to the general public. We propose a Nitrogen Credit System (NCS) to incentivize farming practices that lower NH3 emissions, benefiting farmers through increased income and society at large through a more sustainable food supply with reduced environmental impact. The development of new fertilizers with reduced NH3 emission potential, coupled with advanced application techniques, is vital not only for air quality but also for ensuring food security. Similar innovations are needed in livestock production, including new feed formulas and better manure management facilities. Governments play a pivotal role in this endeavor, as reducing NH3 emissions serves the public interest. Without robust policy measures, farmers and agricultural industries have little motivation to independently reduce NH3 emissions. Additionally, social constraints like small farm sizes and an aging farming population can hinder the implementation of these advanced methods. Therefore, optimizing farm sizes and promoting new farming models, particularly those involving younger farmers, are essential for reducing NH3 emissions at the grassroots level. For the general public, behavioral changes such as reducing food waste and adopting healthier diets can drive the agricultural system towards cleaner air and enhanced food security.
Addressing NH3 emissions is crucial for maintaining nitrogen within safe planetary boundaries. It also aids in reducing aerosol loading and nitrogen deposition in ecosystems, thereby supporting biodiversity conservation. Effectively managing nitrogen can also curtail emissions of N2O, a potent greenhouse gas, thereby slowing global warming. Beyond maintaining planetary health, nitrogen management significantly contributes to achieving over half of the global sustainable development goals (SDGs). Hence, ensuring cleaner air is just the beginning of nitrogen management; comprehensive efforts are required to align nitrogen usage with the SDGs. The United Nations has advocated for halving nitrogen waste as part of a broader strategy for global sustainable development. Achieving this goal necessitates collaboration among various stakeholders across all global regions. A habitable Earth is within reach if we continuously manage our environment within safe planetary boundaries and collectively advance global sustainable development with equity and justice.
Further readings
Gu, B., Zhang, L., Van Dingenen, R., Vieno, M., Van Grinsven, H. J., Zhang, X., ... & Sutton, M. A. (2021). Abating ammonia is more cost-effective than nitrogen oxides for mitigating PM2. 5 air pollution. Science, 374(6568), 758-762.
Gu, B., Zhang, X., Lam, S. K., Yu, Y., Van Grinsven, H. J., Zhang, S., ... & Chen, D. (2023). Cost-effective mitigation of nitrogen pollution from global croplands. Nature, 613(7942), 77-84.
Ren, C., Zhou, X., Wang, C., Guo, Y., Diao, Y., Shen, S., ... & Gu, B. (2023). Ageing threatens sustainability of smallholder farming in China. Nature, 616(7955), 96-103.
Cheng, L., Zhang, X., Reis, S., Ren, C., Xu, J., & Gu, B. (2022). A 12% switch from monogastric to ruminant livestock production can reduce emissions and boost crop production for 525 million people. Nature Food, 3(12), 1040-1051.
Gu, B., van Grinsven, H. J., Lam, S. K., Oenema, O., Sutton, M. A., Mosier, A., & Chen, D. (2021). A credit system to solve agricultural nitrogen pollution. The Innovation, 2(1).
Gu, B., Zhang, X., Bai, X., Fu, B., & Chen, D. (2019). Four steps to food security for swelling cities. Nature, 566(7742), 31-33.
Wu, Y., Xi, X., Tang, X., Luo, D., Gu, B., Lam, S. K., ... & Chen, D. (2018). Policy distortions, farm size, and the overuse of agricultural chemicals in China. Proceedings of the National Academy of Sciences, 115(27), 7010-7015.
