Assessing the aging of aerosols
Our planet’s lower atmosphere is one very busy place, chemically speaking, as climate researchers have discovered. They are developing a new appreciation of complex processes such as cloud formation and radiative forcing, which determines how much light from the sun can reach the earth’s surface and how much heat can escape. More specifically, the latest research is beginning to take stock of the many constituents that contribute to these processes.
Wilfrid Laurier University chemistry Professor Hind Al-Abadleh has focused her attention on the intricate surface interactions that are taking place amongst aerosols in the sky. Given the obvious difficulty of studying these reactions in situ, she has created laboratory conditions to model them in an accurate way.
“We know from single-particle analyses that aerosol particles undergo phase transitions at different temperatures and relative humidity,” she says.
What has remained unknown, she adds, is the role of metals in the chemical aging of these aerosols. Sea spray, desert winds, and air from roads and industry regularly hurl particles of iron, manganese, and copper compounds into the atmosphere. These redox active and photoactive agents break down to liberate metal atoms, which are then free to react directly with secondary oxidations products of volatile organic compounds (VOCs) around them.
In a recent paper for Environmental Science and Technology Al-Abadleh and her colleagues at the University of California Irvine outlined their efforts to model the behavior of iron with two common secondary organics that yielded products on the same order of molecular magnitude as black carbon and aerosols generated by burning biomass. That makes them large enough to strongly absorb visible radiation and qualify as participants in some of the most elusive features of climate dynamics.
She was then invited to give a talk on the paper at the Geochemical Society’s prestigious Goldschmidt Conference, where she found herself describing efforts to weigh the practical limitations of bulk chemistry against the much wider array of reactions that undoubtedly occur in the atmosphere.
“We want to increase the complexity of our model system to mimic real environments,” she says, noting that the impact on climate could vary with the presence of commonly found pollutants like sulphates or the relative organic or inorganic fractions of aerosols, as well as their pH or water content. Her lab has also recently acquired a quartz crystal microbalance that will enable investigators to consider mass deposition on the order of nanograms, just as it would be taking place among aerosols in a real-world setting.
“It’s hard to run surface chemistry on systems like these,” she concludes, pointing to a challenge she shares with members of the climate modelling community. “The biggest uncertainly in climate models is secondary particle formation and how that affects cloud formation.”