Abstract
Abstract. A new photometer is described for continuous determination of the aerosol light absorption coefficient, optimized for long-term studies of the climate-forcing properties of aerosols. Measurements of the light attenuation coefficient are made at blue, green, and red wavelengths, with a detection limit of 0.02 Mm−1 and a precision of 4 % for hourly averages. The uncertainty of the light absorption coefficient is primarily determined by the uncertainty of the correction scheme commonly used to convert the measured light attenuation to light absorption coefficient and ranges from about 20 % at sites with high loadings of strongly absorbing aerosols up to 100 % or more at sites with low loadings of weakly absorbing aerosols. Much lower uncertainties (ca. 40 %) for the latter case can be achieved with an advanced correction scheme.
Highlights
Reliable observations of aerosol light absorption are crucial for quantifying the radiative forcing of climate
Light absorption measurements are recommended for all stations in the Global Atmosphere Watch network, which is coordinated by the World Meteorological Organization (World Meteorological Organization, 2016)
The constrained two-stream (CTS) method reported by Müller et al (2014), yields much lower uncertainties for weakly absorbing aerosols, with uncertainties reduced to about 30 % for a single-scattering albedo of 0.98
Summary
Reliable observations of aerosol light absorption are crucial for quantifying the radiative forcing of climate. NOAA has used filter-based instruments for measuring the aerosol light absorption coefficient at baseline observatories for over two decades Both Aethalometers (Hansen et al, 1984) and Particle Soot Absorption Photometers (PSAP; Bond et al, 1999) have been used. The MAAP only measures at a single wavelength, the PSAP can require frequent (hourly to daily) filter changes, and the Aethalometer does not yet have a widely accepted correction scheme (see, e.g., Collaud Coen et al, 2010) To address these issues, NOAA developed and built a filter-based instrument, the Continuous Light Absorption Photometer (CLAP), with the following design features:. A simple user interface and menu system are provided for manual or computerized control of all CLAP functions
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