Ammonia (NH3) in the marine atmosphere and impact on aerosols

Atmospheric particles (aerosols) are key to the Earth's climate and affect air quality. Ammonia (NH3) is one of the principal precursor gases to aerosols (forming particulate ammonium, NH4+) and the primary base in the atmosphere, neutralising acids and facilitating new particle formation. Over the last few decades, pollution regulations (e.g. Clean Air Acts) have significantly reduced the emissions of acidic aerosol precursors in Europe and North America, including sulphur and nitrogen oxides emitted from fossil fuel combustion. However, NH3 emissions, driven largely by agriculture, have hardly decreased. This has resulted in long-term changes in aerosol pH and chemistry that are not well understood. Additionally, we lack understanding of natural NH3 sources. Oceanic NH4+ may be a key source of NH3 to the marine atmosphere, depending on biogeochemistry, water temperature and water pH. Estimates of air-sea NH3 flux on a regional/global scale are severely hampered by a) a paucity of marine atmospheric NH3 observations, and b) a lack of mechanistic studies of air-water NH3 exchange processes. As a result, NH3 is not well represented in global climate models and its impact is poorly constrained or neglected. It is envisaged that this PhD project will combine long-term ambient observations, laboratory experiments,
and an Earth System Model to significantly improve our understandings in atmospheric NH3 cycling, oceanic NH3 emissions, and the impact of NH3 on aerosols. 1. What are the concentrations of atmospheric NH3 and aerosol NH4+in different airmasses and how do they vary seasonally? 2. Does air-sea NH3 exchange behave as predicted according to existing theory and is the ocean a large source of NH3 to the marine atmosphere? 3. What are the impacts of (ocean-derived) NH3 on marine aerosols and clouds? 4. How will oceanic NH3 emission change in the future and what feedback may that have on marine aerosols, clouds and climate?