Abstract
Abstract. We present a new correction scheme for filter-based absorption photometers based on a constrained two-stream (CTS) radiative transfer model and experimental calibrations. The two-stream model was initialized using experimentally accessible optical parameters of the filter. Experimental calibrations were taken from the literature and from dedicated experiments for the present manuscript. Uncertainties in the model and calibration experiments are discussed and uncertainties for retrieval of absorption coefficients are derived. For single-scattering albedos lower than 0.8, the new CTS method and also other correction schemes suffer from the uncertainty in calibration experiments, with an uncertainty of about 20% in the absorption coefficient. For high single-scattering albedos, the CTS correction significantly reduces errors. At a single-scattering albedo of about 0.98 the error can be reduced to 30%, whereas errors using the Bond correction (Bond et al., 1999) are up to 100%. The correction scheme was tested using data from an independent experiment. The tests confirm the modeled performance of the correction scheme when comparing the CTS method to other established correction methods.
Highlights
Absorption of solar radiation by particles plays an important role in the Earth’s radiative balance
In this chapter we show results of a re-evaluation of data from the Reno Aerosol Optics Study (RAOS) experiment (Sheridan et al, 2005), in which absorption coefficients were measured using a Particle Soot Absorption Photometer (PSAP) (Virkkula et al, 2005) and scattering and backscattering were measured with a nephelometer
In the present paper the constrained two-stream (CTS) method is introduced for PSAPs
Summary
Absorption of solar radiation by particles plays an important role in the Earth’s radiative balance. Pros and cons for two categories of methods, filter-based and in situ, are discussed In this context, in situ denotes methods where the analysis is made while the particles are suspended in air, e.g., photoacoustic photometers, extinction-minus-scattering measurements, and cavity ring-down techniques. T. Müller et al.: Constrained two-stream PSAP correction methods measure transmittance or reflectance, or even both, of the particle-filter system. The measurement principle to relate the transmittance to the particle absorption and not to the particle extinction was used for filter-based absorption photometers, since fiber filters act as angular integrating media through multiple scattering of light inside the filter. Since particles are embedded in a multiple-scattering medium, corrections have to be developed to derive the particle absorption from measurements of transmittance. An error analysis of the new correction scheme and comparison to existing correction methods are given in Sects. 5 and 6, respectively
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