On the interpretation of the loading correction of the aethalometer
Abstract. 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 λ = 660 nm the daily averaged compensation parameter k ≈ 0.0017 ± 0.0002 and 0.0042 ± 0.0013 when backscatter fraction at λ = 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(λ = 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 ω0 so that k decreased with increasing ω0. For the green light (λ = 520 nm) in the ω0 range 0.870 ± 0.005, the average (± standard deviation) k ≈ 0.0047 ± 0.006 and in the ω0 range 0.960 ± 0.005, k ≈ 0.0028 ± 0.0007. This difference was larger for the near-infrared light (λ = 880 nm): in the ω0 range 0.860 ± 0.005, k ≈ 0.0055 ± 0.0023 and in the ω0 range 0.960 ± 0.005, k ≈ 0.0019 ± 0.0011. The negative dependence of k on ω0 was also shown with a simple theoretical analysis.
