On the interpretation of the loading correction of the aethalometer

Aerosol optical properties were measured with a seven-wavelength aethalometer and a three-wavelength nephelometer at the suburban site SORPES in Nanjing, China, in September 2013-January 2015. The aethalometer compensation parameter k, calculated with the Virkkula et al. (2007) method depended on the backscatter fraction, measured with an independent method, the integrating nephelometer. At lambda = 660 nm the daily averaged compensation parameter k approximate to 0.0017 +/- 0.0002 and 0.0042 +/- 0.0013 when backscatter fraction at lambda = 635 nm was in the ranges of 0.100 +/- 0.005 and 0.160 +/- 0.005, respectively. Also, the wavelength dependency of the compensation parameter depended on the backscatter fraction: when b(lambda = 525 nm) was less than approximately 0.13 the compensation parameter decreased with wavelength and at larger b it increased with wavelength. This dependency has not been considered in any of the algorithms that are currently used for processing aethalometer data. The compensation parameter also depended on the single-scattering albedo omega(0) so that k decreased with increasing omega(0). For the green light (lambda = 520 nm) in the omega(0) range 0.870 +/- 0.005, the average (+/- standard deviation) k approximate to 0.0047 +/- 0.006 and in the omega(0) range 0.960 +/- 0.005, k approximate to 0.0028 +/- 0.0007. This difference was larger for the near-infrared light (lambda = 880 nm): in the omega(0) range 0.860 +/- 0.005, k approximate to 0.0055 +/- 0.0023 and in the omega(0) range 0.960 +/- 0.005, k approximate to 0.0019 +/- 0.0011. The negative dependence of k on omega(0) was also shown with a simple theoretical analysis.