AE31 7-wavelength "Spectrum" Aethalometer Data Example: Ambient Air in Berkeley, California

The Model AE31 "Spectrum" Aethalometer acquires data in seven channels at wavelengths ranging from the ultraviolet to near-infrared, namely 370, 450, 571, 615, 660, 880 and 950 nm. The algorithm converts the incremental optical signal in each channel to a mass of material using a cross-section assuming spectrally-uniform absorbance, i.e. absorption proportional to photon energy, SIGMA = k / LAMBDA. If the aerosol consisted only of extremely small pure black spheres, the signals in all channels would be interpreted identically as the same mass of material, and the seven data chart traces would be perfectly superimposed. Deviations from this spectral uniformity could be caused by several factors. Examples include: large particle size (more absorption per unit mass in red, less in blue); presence of aromatic organic compounds (onset of enhanced absorbance in yellow and blue channels); colored mineral dust (extra absorption in particular channel); and other effects due to specific details of the aerosol composition.

This page presents two charts showing measurements taken of ambient air in Berkeley, California. On-shore winds from the Pacific Ocean result in concentrations lower than 100 ng/m3 at times, even in an urban area of several millions' population. Most of the time, the seven channel traces are superimposed, indicating that the aerosol absorbance is spectrally "pure", i.e. most likely due to micrographitic elemental carbon. Deviations from this can indicate the presence of other aerosol constituents.

The first chart shows 'clean' air with events of neighborhood automobile traffic. This was taken with an older AE30 model, i.e. not extended to 370 nm in the ultraviolet.



The second chart shows the spectral response to local smoke from wood-burning fireplaces. This data was taken with a 'newer' AE31 model that includes the 370 nm ultraviolet channel. During the daytime, all seven traces are superimposed. In the evening, the blue and especially the UV traces show enhancement due to the presence of short-wavelength-absorbing organic compounds associated with wood smoke.



 


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