On the interpretation of the loading correction of the aethalometer

Abstract

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 ω 0 so that k decreased with increasing ω 0 . For the green light (λ = 520 nm) in the ω 0 range 0.870 ± 0.005, the average (± standard deviation) k ≈ 0.0047 ± 0.006 and in the ω 0 range 0.960 ± 0.005, k ≈ 0.0028 ± 0.0007. This difference was larger for the nearinfrared light (λ = 880 nm): in the ω 0 range 0.860 ± 0.005, k ≈ 0.0055 ± 0.0023 and in the ω 0 range 0.960 ± 0.005, k ≈ 0.0019 ± 0.0011. The negative dependence of k on ω 0 was also shown with a simple theoretical analysis.

On Aethalometer measurement uncertainties and an instrument correction factor for the Arctic

“…Also shown in Fig. 8 are aerosol back-scatter fractions (b) which are related to the size of the aerosol and have been shown to affect Aethalometers (Virkkula et al, 2015). The site that resembles Summit the most, considering α sp , b, and ω 0 is Pallas.…”

“…As the filter gets loaded with aerosol particles, loading effects come into play. These loading effects change between the filter spots depending on the optical properties of the aerosol that is being deposited on that particular spot (Virkkula et al, 2015) and even during sampling on the same spot (Drinovec et al, 2015).…”

“…8 are the slopes (k) of the ATN dependence. The slopes were calculated by linear regression to match the equation σ ap /σ 0 = 1 + k· ATN that follows the notation of previous work (Virkkula et al, 2007(Virkkula et al, , 2015. It is well known that light scattering by particles affects filterbased absorption photometers, which wrongly gets interpreted as light absorption, termed apparent absorption.…”

“…The term k describes the magnitude of the ATN dependence (Virkkula et al, 2015). Consider a time series with a constant light absorption coefficient of 2 Mm −1 .…”

Light absorption of brown carbon in eastern China based on 3-year multi-wavelength aerosol optical property observations and an improved absorption Ångström exponent segregation method

“…The wavelength of 370 nm was used because studies have found that BrC shows strong light absorption in the near-UV wavelength range (Andreae and Gelencsér, 2006;Hansen and Schnell, 2005). Measurement data at the wavelength of 880 nm were chosen because light absorption at this wavelength normally represents the BC absorption (Virkkula et al, 2015). Regarding the wavelength dependence analysis of BC and BrC, the wavelength near 880 nm is better for the calculation of the AAE of BC because BC is the dominant absorption components in that wavelength range.…”