Understanding Smog by Measuring Light Absorption

Haze lingering over cities may look fairly similar from place to place, but its composition can vary greatly. In some regions, industrial coal smoke predominates, whereas in others, automotive exhaust makes significant contributions. Researchers from Paul Scherrer Institut in Villigen and from the University of Bern, both in Switzerland, used aerosol light absorption to elucidate regional variations in sources of smog.

A photograph of the Alpine city of Roveredo, Switzerland, shows low-lying smog cover. Courtesy of André S.H. Prévôt, Paul Scherrer Institut.

The scientists conducted their investigation in Roveredo, a mountain-locked village that relies on wood as a source of heat in winter. A two-lane highway passes through the town.

During field studies in January, March and November - December, the researchers estimated airborne black carbon mass concentration and aerosol light absorption coefficients with a Magee Scientific aethalometer. For measuring the fraction of modern carbon, they employed a Digitel Elektronik high-volume sampler with quartz fibre filters. An Aerodyne Research quadrupole aerosol mass spectrometer permitted chemical composition and mass size distribution measurements, and a Grimm Aerosol Technik optical particle counter yielded particle size distribution. Organic carbon and elemental carbon concentrations were assessed with a Sunset Laboratory analyser.

For determining the relative contributions of wood burning and vehicle emissions to the aerosol light absorption, the researchers used known absorption exponents for pure wood smoke and traffic fumes computed between 370 and 950 nm. Fossil and nonfossil carbon concentrations were quantified by examining carbon isotope ratios. Because of a lack of industry in the area, the scientists assumed negligible particulate mass contributions from other sources.

Selecting absorption exponent values of 1.1 and 1.8 to 1.9 for wood and traffic, respectively, the researchers observed good agreement between aerosol absorption and fossil/nonfossil calculations. On average, wood burning comprised 88 per cent of carbonaceous particulate mass and traffic, 12 per cent. Wood combustion products, the scientists found, made up 51 per cent of total black carbon, whereas traffic aerosols comprised 49 per cent — a high ratio compared with that in most regions. The results of the study thus indicate not only the accuracy of the aethalometer models but also the need for methods to reduce emissions from wood burning.

(Environmental Science & Technology, 1 May 2008, pp. 3316-3323)