Measurement of Light Absorption and Elemental Carbon in Atmospheric Aerosol Samples from Remote Locations

Abstract

A new double beam, single detector integrating sphere photometer has been developed for light measurements on aerosol samples. The light measurements are completely digital (photon counting), controlled by a desk computer. The resolution of the digital photometer is such that it can detect a 10⁻⁵ difference between the sample and the reference signals. Taking a typical background aerosol sample (10⁵ m length of air column swept by the filter), this results in a detection limit of 10⁻¹⁰ m⁻¹ for the light absorption coefficient of the aerosol particles. This sensitivity is sufficient for the detection of elemental carbon in size segregated samples at ground level background locations or in stratospheric air.

Two independent light absorption parameters can be determined by the instrument:

• a. the conventional absorption coefficient.

• b. the pure light absorption of the particles independent of the filter they are sampled on and independent of their scattering properties.

In the second case the supporting filter is dissolved and the particles are analyzed as a hydrosol in the integrating sphere. By means of a calibration with hydrosols of known amounts of soot with known composition, the results can be expressed in terms of equivalent amounts of elemental carbon in the atmospheric samples.

The first results from size segregated aerosol samples inArctic haze on Spitsbergen show light absorption coefficients similar to those measured in Northern Alaska. The dominating part of the elemental carbon in Artic haze is located in particles smaller than 0.1 µm radius.