Abstract
Abstract. Deriving absorption coefficients from Aethalometer attenuation data requires different corrections to compensate for artifacts related to filter-loading effects, scattering by filter fibers, and scattering by aerosol particles. In this study, two different correction schemes were applied to seven-wavelength Aethalometer data, using multi-angle absorption photometer (MAAP) data as a reference absorption measurement at 637 nm. The compensation algorithms were compared to five-wavelength offline absorption measurements obtained with a multi-wavelength absorbance analyzer (MWAA), which serves as a multiple-wavelength reference measurement. The online measurements took place in the Amazon rainforest, from the wet-to-dry transition season to the dry season (June–September 2014). The mean absorption coefficient (at 637 nm) during this period was 1.8 ± 2.1 Mm−1, with a maximum of 15.9 Mm−1. Under these conditions, the filter-loading compensation was negligible. One of the correction schemes was found to artificially increase the short-wavelength absorption coefficients. It was found that accounting for the aerosol optical properties in the scattering compensation significantly affects the absorption Ångström exponent (åABS) retrievals. Proper Aethalometer data compensation schemes are crucial to retrieve the correct åABS, which is commonly implemented in brown carbon contribution calculations. Additionally, we found that the wavelength dependence of uncompensated Aethalometer attenuation data significantly correlates with the åABS retrieved from offline MWAA measurements.
Highlights
Aerosol particles scatter and absorb solar radiation in the atmosphere and have an important impact on the Earth’s radiative budget and climate (Andreae and Ramanathan, 2013; IPCC, 2013; Penner et al, 1992; Yu et al, 2006).Published by Copernicus Publications on behalf of the European Geosciences Union.J
The artifacts that affect the åABS retrieval from filter-based multi-wavelength absorption measurements could be avoided by using photoacoustic spectroscopy (PAS) methods that have been successfully implemented to measure light absorption by suspended aerosol particles (e.g., Ajtai et al, 2010)
We applied two different correction algorithms to compensate for the various Aethalometer absorption measurement artifacts
Summary
Aerosol particles scatter and absorb solar radiation in the atmosphere and have an important impact on the Earth’s radiative budget and climate (Andreae and Ramanathan, 2013; IPCC, 2013; Penner et al, 1992; Yu et al, 2006).J. Aerosol particles scatter and absorb solar radiation in the atmosphere and have an important impact on the Earth’s radiative budget and climate (Andreae and Ramanathan, 2013; IPCC, 2013; Penner et al, 1992; Yu et al, 2006). Light absorption by atmospheric aerosols is dominated by black carbon (BC), an aerosol species that is emitted by incomplete combustion of biomass or fossil fuels (Bond and Bergstrom, 2006). High uncertainties still remain regarding the aerosol interactions with solar radiation (Andreae and Ramanathan, 2013; Bond et al, 2013), especially because ambient aerosol absorption is often measured over a limited wavelength range or at only one wavelength
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