A dual-wavelength photothermal aerosol absorption monitor: design, calibration and performance
There exists a lack of aerosol absorption measurement techniques with low uncertainties and without artefacts. We have developed the two-wavelength Photothermal Aerosol Absorption Monitor (PTAAM-2λ), which measures the aerosol absorption coefficient at 532 and 1064 nm. Here we describe its design, c...
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Published in | Atmospheric measurement techniques Vol. 15; no. 12; pp. 3805 - 3825 |
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Main Authors | , , , , , , , , , , , , , , , , |
Format | Journal Article |
Language | English |
Published |
Katlenburg-Lindau
Copernicus GmbH
27.06.2022
Copernicus Publications |
Subjects | |
Online Access | Get full text |
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Summary: | There exists a lack of aerosol absorption measurement techniques with low
uncertainties and without artefacts. We have developed the two-wavelength
Photothermal Aerosol Absorption Monitor (PTAAM-2λ), which measures
the aerosol absorption coefficient at 532 and 1064 nm. Here we describe its design, calibration and mode of operation and evaluate its applicability, limits and uncertainties. The 532 nm channel was calibrated with ∼ 1 µmol mol−1 NO2, whereas the 1064 nm channel was
calibrated using measured size distribution spectra of nigrosin particles
and a Mie calculation. Since the aerosolized nigrosin used for calibration
was dry, we determined the imaginary part of the refractive index of
nigrosin from the absorbance measurements on solid thin film samples. The
obtained refractive index differed considerably from the one determined
using aqueous nigrosin solution. PTAAM-2λ has no scattering
artefact and features very low uncertainties: 4 % and 6 % for the
absorption coefficient at 532 and 1064 nm, respectively, and 9 % for the
absorption Ångström exponent. The artefact-free nature of the
measurement method allowed us to investigate the artefacts of filter
photometers. Both the Aethalometer AE33 and CLAP suffer from
cross-sensitivity to scattering – this scattering artefact is most
pronounced for particles smaller than 70 nm. We observed a strong dependence
of the filter multiple scattering parameter on the particle size in the
100–500 nm range. The results from the winter ambient campaign in Ljubljana
showed similar multiple scattering parameter values for ambient aerosols and
laboratory experiments. The spectral dependence of this parameter resulted
in AE33 reporting the absorption Ångström exponent for different
soot samples with values biased 0.23–0.35 higher than the PTAAM-2λ measurement. Photothermal interferometry is a promising method for reference
aerosol absorption measurements. |
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ISSN: | 1867-8548 1867-1381 1867-8548 |
DOI: | 10.5194/amt-15-3805-2022 |