Scientists studying chemical makeup of brown smog


In an ongoing effort to better understand the serious health consequences associated with brown haze, ranging from mild eye and respiratory irritation to asthma and deadly pulmonary diseases such as lung cancer, scientists at Pacific Northwest National Laboratory (PNNL) are studying the chemical makeup of the smog that hovers over cities on sunny days. This haze, known as atmospheric brown carbon (BrC), has been linked to climate change because it acts as a warming blanket, absorbing sunlight and trapping surface heat. The scientists determined that high concentrations of nitrogen oxides influence the creation of the brown haze, providing new insights that can create higher accuracy climate and atmospheric models. A paper describing their findings was the cover story in the journal “Physical Chemistry Chemical Physics.”


The PNNL team examined the chemistry of atmospheric brown carbon. Although the BrC is a known contributor climate change, little has been known about the fundamental relationship between the chemical composition of BrC and its optical properties.


The team 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 adding a bit of nitrogen oxide, released in combustion engine car exhaust, resulted in particles that trapped some heat. Increasing the nitrogen oxides level, however, resulted in particles that held significantly more heat and caused the mix to turn yellowish brown.


They identified fifteen compounds, most of which are nitrophenols. Nitrophenols are a chemical known to cause nose and throat irritation if breathed in large quantities. It is the combination of these nitrophenols that combine to absorb specific wavelengths of light resulting in the brownish color we see.


The team is doing further research on the properties of brown haze to assess its impact on the climate.

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