Optical Methods for Air Quality Measurements
Soot, or carbon, in the air is both an environmental and a health problem. Not only do carbon-based aerosols contribute to the greenhouse effect, but also breathing in the byproducts of combustion processes can have serious physical effects. Despite several decades of research in the field of soot measurement, there is still no standard method for measuring the light-absorbing (black carbon) or thermally refractive (elemental carbon) fraction of atmospheric carbonaceous aerosols.
The average mass concentrations of elemental and black carbon obtained with various methods shows that none of the methods is completely unacceptable for monitoring air quality [VDI = a method used mainly in Germany that involves solvent extraction of filters with subsequent thermal analysis; EC-IC = Cachier (corrected for carbonates); LTM = light transmission method; TO = thermo-optical method; IS = integrating sphere; MAAP = multiangle absorption photometer; AET = aethalometer]. Reprinted with permission of Environmental Science & Technology.
Because current methods of measuring these aerosols — optical and thermal — produce method-specific results, one can never be sure whether concentrations of black carbon or elemental carbon measured with different techniques really are comparable. In extreme cases, method-specific differences can be a factor of two. Researchers from the University of Vienna in Austria, the Institut für Physik der Atmosphäre in Oberpfaffenhofen, Germany, and Umweltbundesamt in Berlin therefore chose a pragmatic approach and evaluated the major measurement techniques used in Europe all at one site — the roof of their laboratory at the university.
“Our aim was to set up conversion tables so that concentrations measured by different optical and thermal methods under urban summer conditions can be more easily compared to each other,” said Regina Hitzenberger, a professor at the institute. Four of the methods studied were optical — the aethalometer, light transmission, multiangle absorption photometer (MAAP) and an integrating sphere. The team found that they were easy to operate compared with the thermal methods.
An aethalometer, in this case from Magee Scientific in Berkeley, Calif., measures light transmission through a particle-loaded filter. Any decrease in light transmission is attributed to absorption. It cannot measure black carbon concentrations directly, but uses a conversion factor to infer them from the absorption signal. It is an online instrument and can provide concentration values quasi-continuously.
The light transmission method works in a similar way — it is simply a lighttight instrument with a white light source on one side of a filter and a photodetector on the other. The main difference between this and the aethalometer is that it is an off-line method, where air is drawn through a filter for 24 hours and the deposited material is analyzed later in a laboratory.
The multiangle absorption photometer is an online instrument that also measures light transmittance through particle-loaded filters, but it has several detectors (including ones in the backscattering region) and can therefore measure the radiation field in both forward and back hemispheres. Quasi-continuous black carbon concentrations are obtained from radiative transfer modeling of this radiation field.
The researchers detailed their work in the Oct. 15 issue of Environmental Science and Technology. Because no reference method exists for the determination of elemental or black carbon, they calculated the average concentration over the time of the study and its standard deviation for each method separately. As can be seen in the figure, the average values of black and elemental carbon agree within their standard deviations. “This shows that none of the methods is completely unacceptable for air quality monitoring purposes,” Hitzenberger said. “Among the optical methods, the MAAP performed very well. It does not suffer from the scattering losses usually encountered in optical methods.”
However, because the optical methods measure black carbon via light absorption, a procedure is always needed to convert optical absorption to black carbon concentration. “This conversion is definitely not straightforward,” Hitzenberger said. “It depends on the aerosol type, size distribution, chemical composition and mixing state.”
She added that the group could not conclude which method was the best overall for measuring black and elemental carbon. “The only thing we can say is that every method has its advantages and its drawbacks. It is really up to the user to decide on a method, but the user should be aware that there is no ‘true’ method for black carbon or elemental carbon measurement.”
The conditions under which the measurements are taken are also very important, Hitzenberger emphasized. The study was performed under summer conditions in an urban background aerosol without direct impact from traffic and other combustion sources but in a country with a high percentage of diesel automobiles.
According to Hitzenberger, the conversion factors would be quite different at street level. An earlier comparison between an aethalometer and the integrating sphere method on aerosol sampled at sidewalk level in June 1998 in Vienna showed that the aethalometer gave only about 50 percent of the black carbon measured with the integrating sphere method, even though the same integrating sphere and calibration curve had been used. In the data set obtained at roof level, the aethalometer data was comparable to that from the integrating sphere.
The researchers will soon publish results from tests under winter conditions. Those tests showed large differences among the methods depending on the amount of wood combustion aerosol in the air.
Setting environmental standards for elemental and black carbon has long been under discussion but has not been implemented because of the difficulty of selecting a universally accepted standard technique, Hitzenberger said.
“I don’t see that this can be resolved soon. Maybe one simply has to be pragmatic at some point and define one of the methods as standard, but we have not yet progressed this far.”
Contact: Regina Hitzenberger, University of Vienna, Austria; e-mail: firstname.lastname@example.org.
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