Quantification of graphitic carbon in atmospheric aerosol particles by Raman spectroscopy and first application for the determination of mass absorption efficiencies

The determination of the content of graphitic carbon (GC) in atmospheric aerosol samples deposited on glass fiber filters (Pallflex E70-2075W) is accomplished by means of a new Raman spectroscopic method. Raman spectra of atmospheric GC contain two main bands located at about 1600 and 1300 cm−1. The GC mass loading of the filter samples is obtained by integrating the band at 1600 cm−1, which scales with the amount of graphitic structures. The method is calibrated with the carbon black Monarch 71 (M71), because the location and shape of the Raman bands of M71 and atmospheric GC are very similar. Due to the specific sensitivity of the Raman method for GC a high precision of 2% is achieved (detection limit of 0.08 μg cm−2). The special filter type was chosen for the Raman method, because it is employed inside the particle soot absorption photometer (PSAP), which measures the particle absorption coefficient σap by a change in light transmission during aerosol collection. This offers the opportunity to relate σap to the subsequent Raman measurement of the GC mass concentration, mGC, on one and the same aerosol sample in order to determine the GC mass absorption efficiency δGC. In a first attempt this approach is applied to atmospheric aerosol particle measurements carried out on a mountain site in central Germany yielding δGC values between 10 and 18 m2 g−1 with an average of (14.7±2.8) m2 g−1. These results are interpreted by published model calculations where mass absorption efficiencies of light absorbing carbon inside internally mixed particles are computed. From the comparison it can be concluded that GC constitutes the dominant part of light absorbing carbon and is mainly located in the accumulation mode of aged continental aerosol particles prevailing at the observational site.