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Spectral aerosol light absorption is an important parameter for theassessment of the radiation budget of the atmosphere. Although on-linemeasurement techniques for aerosol light absorption, such as theAethalometer and the Particle Soot Absorption Photometer (PSAP), have beenavailable for two decades, they are limited in accuracy and spectralresolution because of the need to deposit the aerosol on a filter substratebefore measurement. Recently, a 7-wavelength (λ) Aethalometerbecame commercially available, which covers the visible (VIS) tonear-infrared (NIR) spectral range (λ=450–950 nm), andlaboratory calibration studies improved the degree of confidence in thesemeasurement techniques. However, the applicability of the laboratorycalibration factors to ambient conditions has not been investigatedthoroughly yet.As part of the LBA-SMOCC (Large scale Biosphere atmosphere experiment inAmazonia – SMOke aerosols, Clouds, rainfall and Climate) campaign fromSeptember to November 2002 in the Amazon basin we performed an extensivefield calibration of a 1-λ PSAP and a 7-λ Aethalometerutilizing a photoacoustic spectrometer (PAS, 532 nm) as reference device.Especially during the dry period of the campaign, the aerosol population wasdominated by pyrogenic emissions. The most pronounced artifact ofintegrating-plate type attenuation techniques (e.g. Aethalometer, PSAP) isdue 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 multiplescattering enhancement of 5.23 (or 4.55, if corrected for aerosolscattering), which is significantly larger than the factors previouslyreported (~2) for laboratory calibrations. While the exact reason forthis discrepancy is unknown, the available data from the present andprevious studies suggest aerosol mixing (internal versus external) as alikely cause. For Amazonian aerosol, we found no absorption enhancement dueto hygroscopic particle growth in the relative humidity (RH) range between40% and 80%. However, a substantial bias in PSAP sensitivity thatcorrelated 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 gaseousadsorption. Although very similar in measurement principle, the PSAP andAethalometer require markedly different correction factors, which isprobably due to the different filter media used. Although on-sitecalibration of the PSAP and Aethalometer is advisable for best dataquality, we recommend a set of "best practice" correction factors forambient sampling based on the data from the present and previous studies.For this study, the estimated accuracies of the absorption coefficientsdetermined by the PAS, PSAP and Aethalometer were 10, 15 and 20% (95%confidence level), respectively

atmospheric chemistry and physics

Spectral light absorption by ambient aerosols influenced by biomass burning in the Amazon Basin. I: Comparison and field calibration of absorption measurement techniques