Potential Climatic Impact of Organic Haze on Early Earth
We have explored the direct and indirect radiative effects on climate of organic particles likely to have been present on early Earth by measuring their hygroscopicity and cloud nucleating ability. The early Earth analog aerosol particles were generated via ultraviolet photolysis of an early Earth analog gas mixture, which was designed to mimic possible atmospheric conditions before the rise of oxygen. An analog aerosol for the present-day atmosphere of Saturn's moon Titan was tested for comparison. We exposed the early Earth aerosol to a range of relative humidities (RHs). Water uptake onto the aerosol was observed to occur over the entire RH range tested (RH = 80–87%). To translate our measurements of hygroscopicity over a specific range of RHs into their water uptake ability at any RH < 100% and into their ability to act as cloud condensation nuclei (CCN) at RH > 100%, we relied on the hygroscopicity parameter κ, developed by Petters and Kreidenweis. We retrieved κ = 0.22 ± 0.12 for the early Earth aerosol, which indicates that the humidified aerosol (RH < 100 %) could have contributed to a larger antigreenhouse effect on the early Earth atmosphere than previously modeled with dry aerosol. Such effects would have been of significance in regions where the humidity was larger than 50%, because such high humidities are needed for significant amounts of water to be on the aerosol. Additionally, Earth organic aerosol particles could have activated into CCN at reasonable—and even low—water-vapor supersaturations (RH > 100%). In regions where the haze was dominant, it is expected that low particle concentrations, once activated into cloud droplets, would have created short-lived, optically thin clouds. Such clouds, if predominant on early Earth, would have had a lower albedo than clouds today, thereby warming the planet relative to current-day clouds.
© 2011 Mary Ann Liebert, Inc. Submitted 24 August 2010; Accepted 8 December 2010. Published Online Ahead of Print: March 9, 2011. This material is based on work supported by NASA Exobiology Grant NNXOAAV55G issued through the Office of Space Science. C.A.H. was supported with a National Science Foundation Graduate Research Fellowship. M.A.F. acknowledges support from the NOAA Climate and Global Change Postdoctoral Fellowship Program administered by the University Corporation for Atmospheric Research. M.R.B. was supported by an EPA-STAR fellowship.
Published - Hasenkopf2011p13412Astrobiology.pdf