The Sources and Impacts of Reduced Nitrogen in the Atmosphere

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Human changes to Earth's nitrogen cycle are known to be one of the most dramatic disruptions to the planetary boundaries that represent our safe operating space. In order to feed all the humans on the planet, we rely on fertilizer nitrogen for our crops, and produce even more by raising animals to eat. These perturbations to the nitrogen cycle have consequences for air quality, climate forcing, ozone depletion, acid deposition, ecosystem productivity, nutrient leaching, and eutrophication. In 2021, the World Health Organization tightened its guidelines on the exposure to ambient particulate matter, in an attempt to better protect public health. There is an increasing recognition that agricultural emissions, driven largely by the molecule ammonia, are responsible for a significant amount of fine particulate matter in the air around the world and that reductions of 50% or more could save hundreds of thousands of lives, with associated economic benefits in the billions of dollars. However, estimates like these rely on highly uncertain emissions inventories, which are often found to be biased, compromising our ability to accurately quantify the problem and develop solutions that protect public health without threatening food security. A better understanding of the sources of ammonia may also be important for managing stratospheric ozone depletion and climate forcing caused by nitrous oxide. Ammonia does not just contribute to the total mass of particles in the atmosphere, it, along with a similar family of molecules called amines, also changes the rate at which new particles are formed. These new particles can grow large enough that they eventually act as seeds for cloud droplets to form on. In very clean parts of the atmosphere, for example in the Arctic, over the ocean, or at the tops of mountains, small changes in the rate of new particle formation can have big impacts on cloud cover. Because clouds are an important part of the climate system, understanding the ingredients that change their abundance is important for predicting future changes. And since clean parts of the atmosphere might also be good analogues for the pre-industrial atmosphere, they might also help us understand how particles and clouds impacted climate in the past, which is a major uncertainty in our understanding of Earth's climate sensitivity. To better quantify reduced nitrogen compounds (ammonia and amines) in the atmosphere and to evaluate their roles in processes that impact climate forcing and air quality, we will 1)Develop techniques to quantify trace levels of organic acids and amines in air and water 2)Determine the role of organic acids and amines and ammonia in new particle formation and growth in remote environments 3)Constrain regional emissions of ammonia and their dependence on climate and management drivers 4)Quantify the role of ammonia and organic acids in particle pollution in cold urban and rural environments