4. Airborne acid may help soot turn into cloud seeds
Carbon soot aerosols from combustion of fossil fuels and forest fires directly influence the Earth-atmosphere heat balance by absorbing sunlight. Fresh soot particles repel water and hence have little effect on properties and lifetimes of clouds. As soot particles age, they are thought to undergo a weathering process that allows them to absorb water, potentially transforming particles into cloud condensation nuclei. To learn more about how soot develops an affinity for water, Khalizov et al. examine the properties of soot aerosols exposed to gaseous sulfuric acid. They find that although fresh soot does not change below water saturation, soot particles exposed to sulfuric acid increase in mass when relative humidity rises because the acid-coated soot absorbs water. An increase in particle mass is often accompanied by a decrease in size, suggesting that conventional measurement methods based on particle size may underestimate the impact of soot aging on clouds. Because sulfuric acid, a pollutant and the driving agent in acid rain, is increasing in the atmosphere due to industrial activities, the authors expect that this mechanism of water absorption by acid-coated soot significantly influences cloud formation.
Title: Formation of highly hygroscopic soot aerosols upon internal mixing with sulfuric acid vapor
Authors: Alexei F. Khalizov, Renyi Zhang, Dan Zhang, and Huaxin Xue: Department of Atmospheric sciences, Texas A&M University, College Station, Texas, U.S.A.;
Joakim Pagels: Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, U.S.A.; also at Division of Ergonomics and Aerosol Technology, Faculty of Engineering, Lund University, Lund, Sweden;
Peter H. McMurry: Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, U.S.A.