Microphysical explanation of the RH‐dependent water affinity of biogenic organic aerosol and its importance for climate

• Published in: Geophysical research letters 2017-05, Vol.44 (10), p.5167-5177
• Main Authors: Rastak, N., Pajunoja, A., Acosta Navarro, J. C., Ma, J., Song, M., Partridge, D. G., Kirkevåg, A., Leong, Y., Hu, W. W., Taylor, N. F., Lambe, A., Cerully, K., Bougiatioti, A., Liu, P., Krejci, R., Petäjä, T., Percival, C., Davidovits, P., Worsnop, D. R., Ekman, A. M. L., Nenes, A., Martin, S., Jimenez, J. L., Collins, D. R., Topping, D.O., Bertram, A. K., Zuend, A., Virtanen, A., Riipinen, I.
• Format: Article
• Language: English
• Subjects: aerosol-climate interactions; aerosol-water interactions; Aerosols; Aerosols and Particles; Affinity; Airborne particulates; Anthropogenic factors; Atmosphere; atmospheric aerosol; Atmospheric Composition and Structure; Atmospheric models; Atmospheric precipitations; Atmospheric Processes; Atmospheric Science; Bioavailability; Climate; Climate effects; Climate models; Clouds and Aerosols; Complexity; Composition; Composition of Aerosols and Dust Particles; Data processing; Ecosystems; ENVIRONMENTAL SCIENCES; Forests; Frameworks; Geochemistry; Global Change; Global Climate Models; Human influences; Humidity; Hygroscopicity; Isoprene; Laboratories; Laboratory experiments; Marine ecosystems; Mass; miljövetenskap; Mineral nutrients; Modelling; Molecular modelling; Monoterpenes; Nutrients; Oceanography: Biological and Chemical; Paleoceanography; Particulate emissions; Particulate matter; Phase separation; Pollution: Urban and Regional; Precipitation; Profiles; Public health; Research Letter; Research Letters; Secondary aerosols; secondary organic aerosol; Separation; Solubility; State of the art; Suspended particulate matter; Terpenes; Uptake; Water; Water uptake
• ISSN: 0094-8276; 1944-8007; 1944-8007
• DOI: 10.1002/2017GL073056

A large fraction of atmospheric organic aerosol (OA) originates from natural emissions that are oxidized in the atmosphere to form secondary organic aerosol (SOA). Isoprene (IP) and monoterpenes (MT) are the most important precursors of SOA originating from forests. The climate impacts from OA are currently estimated through parameterizations of water uptake that drastically simplify the complexity of OA. We combine laboratory experiments, thermodynamic modeling, field observations, and climate modeling to (1) explain the molecular mechanisms behind RH‐dependent SOA water‐uptake with solubility and phase separation; (2) show that laboratory data on IP‐ and MT‐SOA hygroscopicity are representative of ambient data with corresponding OA source profiles; and (3) demonstrate the sensitivity of the modeled aerosol climate effect to assumed OA water affinity. We conclude that the commonly used single‐parameter hygroscopicity framework can introduce significant error when quantifying the climate effects of organic aerosol. The results highlight the need for better constraints on the overall global OA mass loadings and its molecular composition, including currently underexplored anthropogenic and marine OA sources. Plain Language Summary The interaction of airborne particulate matter (“aerosols”) with water is of critical importance for processes governing climate, precipitation, and public health. It also modulates the delivery and bioavailability of nutrients to terrestrial and oceanic ecosystems. We present a microphysical explanation to the humidity‐dependent water uptake behavior of organic aerosol, which challenges the highly simplified theoretical descriptions used in, e.g., present climate models. With the comprehensive analysis of laboratory data using molecular models, we explain the microphysical behavior of the aerosol over the range of humidity observed in the atmosphere, in a way that has never been done before. We also demonstrate the presence of these phenomena in the ambient atmosphere from data collected in the field. We further show, using two state‐of‐the‐art climate models, that misrepresenting the water affinity of atmospheric organic aerosol can lead to significant biases in the estimates of the anthropogenic influence on climate. Key Points Phase separation effects explain differences in water affinity of biogenic secondary organic aerosol (SOA) at subsaturation and supersaturation Laboratory data for monoterpene and isoprene SOA are represe