Sunlight plays key role in airborne aerosol gases

September/October 2015
ATMOSPHERIC CHEMISTRY

The behaviour of aerosols in the atmosphere remains largely unaccounted for in climate modelling — with good reason. This complex atmospheric component is exceedingly difficult to study in the field and almost as problematic to replicate under laboratory conditions.

Such challenges have not daunted Wilfrid Laurier University chemistry Professor Hind Al-Abadleh, who has outfitted her laboratory with an infrared spectrometer and various accessories dedicated to analyzing gas-solid and liquid-solid reactions. Al-Abadleh also uses a suite of bulk techniques to analyze samples that have high concentrations of chemicals to mimic reactions in surface water. Her goal has been to tease apart what happens when aerosols with variable water content meet the other constituents of the atmosphere and what the result might mean for the absorption of solar energy. “We’re talking about aerosols that are emitted from either natural sources like dust storms or industrial activities,” she says, noting that these small particles offer a massive surface area for interfacial reactions. “Surface chemistry, particularly that which takes place at the gas-solid interface with islands of water, proceeds by different mechanisms and different pathways than bulk phase beaker chemistry.”

Al-Abadleh selected two organics: the aromatic oil derivative guaiacol and the natural anti-oxidant catechol, as model compounds in atmospheric aerosols. In the absence of light, such chemicals interact with iron, another component of aerosols. Their interaction yields complex sunlight-absorbing polymers that are also reactive towards atmospheric gases like ozone or nitrogen oxides (NOx).

These “dark reactions” are crucial, Al-Abadleh says, since they will determine the components that will be present once the sun comes back up. “These ‘control experiments’ help us better understand the role of sunlight in driving chemistry in aerosols,” she notes.  
The findings of Al-Abadleh’s collaboration with colleagues at the University of Waterloo and the University of California, Irvine, appeared this year in Environmental Science & Technology. She emphasizes that the emergence of these intricate polymers is not only unanticipated by most versions of atmospheric chemistry but the production of these “secondary” aerosols by transition metals appears to be much more efficient than expected. “This potentially important pathway is currently unaccounted for in climate and atmospheric chemistry models,” she says. “These results will help fill the gap in our understanding of the surface chemistry driven by iron in atmospherically relevant surfaces.”