Low temperature and humidity slow down photochemical reactions.
Sunlight plays a major role in degrading toxic pollutants in atmospheric particles, but how environmental conditions affect photodegradation rates is unclear. A recent study demonstrated for the first time that low temperatures and low humidity increase the viscosity of atmospheric particles and slow down photodegradation of toxic pollutants trapped in particles.
The findings suggest that air pollutants trapped within particles in polar regions or at high altitudes will have longer lifetimes than those in warm, humid regions. By shedding new light on reactions happening inside and on surfaces of atmospheric particles, such as those produced in large cities on smoggy days, the study could lead to the development of more accurate air pollution models.
Atmospheric chemistry is almost entirely driven by sunlight. Scientists understand photochemical reactions that happen with gaseous molecules in air, but reactions happening inside and on surfaces of atmospheric particles, such as those produced in large cities on smoggy days, remain unexplored. To address this unknown, a team of researchers from the University of California at Irvine, University of British Columbia, Environmental Molecular Sciences Laboratory [EMSL; a Department of Energy (DOE) national scientific user facility], and Pacific Northwest National Laboratory explored the effect of environmental conditions on photodegradation rates of atmospherically relevant pollutants embedded in a film of secondary organic material (SOM). The researchers used liquid chromatography (LC) coupled to a photodiode array detector and electrospray ionization high-resolution mass spectrometer (LC-PDA-MS) measurements at EMSL to study three types of SOM. Photodegradation rates of the pollutant 2,4 dinitrophenol were slower at lower temperatures and lower relative humidity—conditions that make SOM more viscous. Additional analyses suggested increased viscosity hinders motion of the molecules in SOM, thereby slowing down the rate of their photodegradation. The findings show that pollutants trapped inside viscous particles, which are more abundant in cold, dry parts of the atmosphere, may take longer to decompose than expected. Future efforts to expand the scope of the study could reveal how environmental conditions influence the photodegradation of compounds known to affect human health.
BER PM Contact
Paul Bayer, SC-23.1, 301-903-5324
Environmental Molecular Sciences Laboratory/Pacific Northwest National Laboratory
This work was supported by DOE’s Office of Science, Office of Biological and Environmental Research, including support of EMSL; Department of Commerce; National Science Foundation; and Natural Sciences and Engineering Research Council of Canada.
Hinks, M. L., M. V. Brady, H. Lignell, M. Song, J. W. Grayson, A. K. Bertram, P. Lin, A. Laskin, J. Laskin, and S. A. Nizkorodov. 2016. “Effect of Viscosity on Photodegradation Rates in Complex Secondary Organic Aerosol Materials,” Physical Chemistry Chemical Physics 18, 8785–93. DOI: 10.1039/C5CP05226B. (Reference link)
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