Invisible, overlooked, climate relevant? Unprecedented levels of ultrafine particles and the hydrological cycle
Authors: Junkermann Wolfgang , Hacker Jorg 
Published: March 2024

Current extremes within regional water cycles, extensive drought periods and torrential flooding, are&#160;&#160; associated in literature and media to first indicators of greenhouse gas driven climate change. Indeed, they are among major threats to be expected from climate change model results. The main obvious physical process behind such global warming water cycle extremes, is the temperature dependent water vapor content of air (Clausius Clapeyron, 1834, CC) and it&#8217;s increase by ~ 7% per degree C. Naturally the water vapor input into the atmosphere via evapotranspiration is dependent on shortwave radiation reaching the surface, a process controlled partially by fine particles, partially by clouds. Here the ultrafine, invisible, fraction of the aerosols is becoming important.Ultrafine particles (UFP) acting as cloud condensation nuclei (CCN) are the driving force behind cloud modification and changing rainfall patterns. However, the sources and budgets of anthropogenic primary and secondary particles were not well known. Based on airborne measurements we identified as a major contribution modern fossil fuel flue gas cleaning techniques to cause a doubling of global primary UFP number emissions. The subsequent enhancement of CCN numbers has several side effects. It&#8217;s changing the size of the cloud droplets and delays raindrop formation, suppressing certain types of rainfall and increasing the residence time of water vapor in the atmosphere. This additional latent energy reservoir is directly available for invigoration of rainfall extremes. Additionally it&#8217;s a further contribution to the column density of water vapor as a greenhouse gas and important for the infrared radiation budget. The localized but ubiquitous fossil fuel related UFP emissions and their role in the hydrological cycle, may thus contribute to regional or continental climate trends, such as increasing drought and flooding, observed within recent decades.We discuss the impact of the ultrafine fraction on the hydrological cycle and its historical timeline. Ultrafine particles (UFP) initially don&#8217;t interact with radiation like fine ones. However, a significant increase of the ultrafine particle burden may serve similar to CC to more water vapor molecules, respectively more latent energy in the troposphere, especially in the altitude range of convective clouds. We also discuss the origin of the majority of UFP, whether a simple dependence of ultrafine particles on the atmospheric sulphur load is a reasonable and valid assumption and what should be taken additionally into account for future UFP szenarios.Junkermann, W. &amp; Hacker, J., 2022, Unprecedented levels of ultrafine particles, major sources, and the hydrological cycle, Nature Scientific Reports, 12:7410 https://doi.org/10.1038/s41598-022-11500-5Junkermann, W. (2022). Ultrafine particle emissions in the Mediterranean region. In F. Dulac, S. Sauvage, &amp; E. Hamonou (Eds.), Atmospheric chemistry in the Mediterranean region (Vol. 2, From air pollutant sources to impacts). Springer, 21 pp. https://doi.org/10.5445/IR/1000154173