Effects of different correction algorithms on absorption coefficient – a comparison of three optical absorption photometers at a boreal forest site
2021
Abstract. We present a comparison between three absorption photometers that measured
the absorption coefficient ( σabs ) of ambient aerosol particles in
2012–2017 at SMEAR II (Station for Measuring Ecosystem–Atmosphere Relations II), a measurement station located in a boreal forest
in southern Finland. The comparison included an Aethalometer (AE31), a multi-angle absorption photometer (MAAP), and a particle soot absorption
photometer (PSAP). These optical instruments measured particles collected on
a filter, which is a source of systematic errors, since in addition to the
particles, the filter fibers also interact with light. To overcome this
problem, several algorithms have been suggested to correct the AE31 and PSAP
measurements. The aim of this study was to research how the different
correction algorithms affected the derived optical properties. We applied
the different correction algorithms to the AE31 and PSAP data and compared
the results against the reference measurements conducted by the MAAP. The
comparison between the MAAP and AE31 resulted in a multiple-scattering correction factor ( Cref ) that is used in AE31 correction algorithms to
compensate for the light scattering by filter fibers. Cref varies
between different environments, and our results are applicable to a boreal
environment. We observed a clear seasonal cycle in Cref , which was
probably due to variations in aerosol optical properties, such as the
backscatter fraction and single-scattering albedo, and also due to
variations in the relative humidity (RH). The results showed that the
filter-based absorption photometers seemed to be rather sensitive to the
RH even if the RH was kept below the recommended value of 40 %. The
instruments correlated well ( R≈0.98 ), but the slopes of the
regression lines varied between the instruments and correction algorithms:
compared to the MAAP, the AE31 underestimated σabs only
slightly (the slopes varied between 0.96–1.00) and the PSAP overestimated
σabs only a little (the slopes varied between 1.01–1.04 for a
recommended filter transmittance >0.7 ). The instruments and
correction algorithms had a notable influence on the absorption
Angstrom exponent: the median absorption Angstrom exponent
varied between 0.93–1.54 for the different algorithms and instruments.
Keywords:
- Correction
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