Photolysis is a major removal mechanism that controls the budget of organic aerosol
Fast, Jerome D — Pacific Northwest National Laboratory
Area of research
Carbon-containing vapors in the atmosphere condense into small organic particles (aerosol) that can influence climate by reflecting sunlight and initiating cloud droplet formation. Predicting the atmospheric abundances of organic particles using atmospheric models is challenging because of the complexities involved. For example, reactions that remove particles by converting them back into vapors are often not included in atmospheric models. Researchers at the Department of Energy (DOE) and the University of Washington conducted new measurements quantifying how interactions between organic aerosol and UV light remove organic particle mass from the atmosphere. They found that this chemical removal mechanism is often much more efficient than mechanical removal of particles from the atmosphere. The results are an important insight into how atmospheric models can better describe organic aerosol lifetimes.
Atmospheric models usually describe the mechanical removal of aerosol, but not the chemical loss processes. The removal of aerosol due to reactions initiated by sunlight (photolysis) has the potential to be more efficient than mechanical removal, but only limited measurements of photolysis exist. Therefore, most models do not include organic aerosol loss due to photolysis. This study quantified the aerosol loss due to photolysis for a wide range of organic aerosol. When the measurements were incorporated into a chemical transport model, organic aerosol concentrations decreased by 50% over the Amazon rain forest. These new measurements of organic aerosol photolysis will help modelers more accurately predict organic aerosol budgets and lifetimes.
DOE researchers used a large temperature-controlled environmental chamber outfitted with UV lights to simulate the life cycle of organic aerosol in the laboratory. They first prepared organic aerosol particles by reacting organic vapors common in the atmosphere, such as isoprene or α-pinene with common atmospheric oxidants. Then, they exposed the particles to chamber UV lights for at least 24 hours and measured the rate at which particles were converted back into vapors. They used data collected by several mass spectrometers to examine the chemical changes in the aerosol composition. Special care was taken during this experiment to isolate light-driven processes from concurrent chemical reactions and artifacts introduced by chamber walls. Aerosol mass loss rates quantified from the experiments were used in an atmospheric model to simulate the life cycle of organic particles in the Amazon region. The model predicted that photolysis reduced the organic aerosol concentrations by an average of 50%, indicating photolysis is a major removal mechanism that substantially shortens the lifetime of organic aerosol in the atmosphere.