PNNL team details role of NOx in formation of urban brown haze
29 September 2015
Researchers at Pacific Northwest National Laboratory (PNNL) have detailed the role of NOx in the formation of the brown haze that hovers over cities on sunny days. A paper on their work was published recently in the RSC journal Physical Chemistry Chemical Physics.
The PNNL team examined the chemistry of atmospheric brown carbon (BrC)—a significant contributor to light absorption and climate forcing. However, little has been known about the fundamental relationship between the chemical composition of BrC and its optical properties.
They focused on light-absorbing secondary organic aerosols (SOAs) generated from the photo-oxidation of toluene—a common pollutant—in the presence of NOx (Tol-SOA). They found that the chemical composition of BrC chromophores and the light absorption properties of toluene SOA (Tol-SOA) depend strongly on the initial NOx concentration.
Specifically, the found that Tol-SOA generated under high-NOx conditions (initial NOx/toluene of 5/1) appeared yellow with a mass absorption coefficient of the bulk sample nearly 80 fold higher than that measured for Tol-SOA generated under low-NOx conditions.
They identified 15 compounds, most of which are nitrophenols, as major BrC chromophores responsible for the enhanced light absorption of Tol-SOA material produced in the presence of NOx. The integrated absorbance of these fifteen chromophores accounted for 40–60% of the total light absorbance by Tol-SOA at wavelengths between 300 nm and 500 nm.
Resources
Peng Lin, Jiumeng Liu, John E. Shilling, Shawn M. Kathmann, Julia Laskin and Alexander Laskin (2015) “Molecular characterization of brown carbon (BrC) chromophores in secondary organic aerosol generated from photo-oxidation of toluene” Phys. Chem. Chem. Phys., 17, 23312-23325 doi: 10.1039/C5CP02563J
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