| Spectral aerosol light absorption is an important parameter for the
assessment of the radiation budget of the atmosphere. Although on-line
measurement techniques for aerosol light absorption, such as the
Aethalometer and the Particle Soot Absorption Photometer (PSAP), have been
available for two decades, they are limited in accuracy and spectral
resolution because of the need to deposit the aerosol on a filter substrate
before measurement. Recently, a 7-wavelength (λ) Aethalometer
became commercially available, which covers the visible (VIS) to
near-infrared (NIR) spectral range (λ=450–950 nm), and
laboratory calibration studies improved the degree of confidence in these
measurement techniques. However, the applicability of the laboratory
calibration factors to ambient conditions has not been investigated
thoroughly yet.
As part of the LBA-SMOCC (Large scale Biosphere atmosphere experiment in
Amazonia – SMOke aerosols, Clouds, rainfall and Climate) campaign from
September to November 2002 in the Amazon basin we performed an extensive
field calibration of a 1-λ PSAP and a 7-λ Aethalometer
utilizing a photoacoustic spectrometer (PAS, 532 nm) as reference device.
Especially during the dry period of the campaign, the aerosol population was
dominated by pyrogenic emissions. The most pronounced artifact of
integrating-plate type attenuation techniques (e.g. Aethalometer, PSAP) is
due to multiple scattering effects within the filter matrix. For the PSAP,
we essentially confirmed the laboratory calibration factor by Bond et al. (1999).
On the other hand, for the Aethalometer we found a multiple
scattering enhancement of 5.23 (or 4.55, if corrected for aerosol
scattering), which is significantly larger than the factors previously
reported (~2) for laboratory calibrations. While the exact reason for
this discrepancy is unknown, the available data from the present and
previous studies suggest aerosol mixing (internal versus external) as a
likely cause. For Amazonian aerosol, we found no absorption enhancement due
to hygroscopic particle growth in the relative humidity (RH) range between
40% and 80%. However, a substantial bias in PSAP sensitivity that
correlated with both RH and temperature (T) was observed for 20%<RH<30%
and 24°C<T<26°C, respectively. In addition,
both PSAP and Aethalometer demonstrated no sensitivity to gaseous
adsorption. Although very similar in measurement principle, the PSAP and
Aethalometer require markedly different correction factors, which is
probably due to the different filter media used. Although on-site
calibration of the PSAP and Aethalometer is advisable for best data
quality, we recommend a set of "best practice" correction factors for
ambient sampling based on the data from the present and previous studies.
For this study, the estimated accuracies of the absorption coefficients
determined by the PAS, PSAP and Aethalometer were 10, 15 and 20% (95%
confidence level), respectively. |