A Simple Procedure for Correcting Loading Effects of Aethalometer Data

Abstract

A simple method for correcting for the loading effects of aethalometer data is presented. The formula BC CORRECTED ϭ (1 ϩ k ⅐ ATN) ⅐ BC NONCORRECTED , where ATN is the attenuation and BC is black carbon, was used for correcting aethalometer data obtained from measurements at three different sites: a subway station in Helsinki, an urban background measurement station in Helsinki, and a rural station in Hyytiälä in central Finland. The BC data were compared with simultaneously measured aerosol volume concentrations (V). After the correction algorithm, the BC-to-V ratio remained relatively stable between consequent filter spots, which can be regarded as indirect evidence that the correction algorithm works. The k value calculated from the outdoor sites had a clear seasonal cycle that could be explained by darker aerosol in winter than in summer. When the contribution of BC to the total aerosol volume was high, the k factor was high and vice versa. In winter, the k values at all wavelengths were very close to that obtained from the subway station data. In summer, the k value was wavelength dependent and often negative. When the k value is negative, the noncorrected BC concentrations overestimated the true concentrations.

The "dual-spot" Aethalometer: an improved measurement of aerosol black carbon with real-time loading compensation

“…All filter-based optical methods exhibit loading effects (Bond et al, 1999;Weingartner et al, 2003;Virkkula, 2010;Virkkula et al, 2005Virkkula et al, , 2007Collaud Coen et al, 2010;Park et al, 2010;Hyvärinen et al, 2013). The relationship between attenuation and BC surface loading is linear for low attenuation values.…”

“…2a are not used for fitting the dependence of the instrumental sensitivity as the function of ATN. (Table 1) was calculated using f = 1.4; for the Virkkula et al (2007) approach (Table 1) and the proposed dual-spot compensation model (Eq. 6), k = 0.005 was used.…”

“…However, it has been shown that the loading effect differs between the seasons, depending on the aerosol properties (Virkkula et al, 2007). Weingartner et al (2003) have shown that fresh aerosols exhibit more pronounced loading effects than aged ones.…”

“…This has been performed in the past with an algorithm embedded in the filter photometer, assuming fixed compensation parameters (Bond et al, 1999;Virkkula, 2010;Virkkula et al, 2005Virkkula et al, , 2007 or with an attempt to compensate the data by performing an additional measurement of reflection (Petzold and Schönlinner, 2004) or scattering . Alternatively, a post-processing algorithm can be employed using either fixed parameters (Hyvärinen et al, 2013) or parameters obtained from the data themselves (Virkkula et al, 2007;Park et al, 2010;Hyvärinen et al, 2013). Weingartner et al (2003) and Virkkula et al (2007) models have been most frequently used for loading effect compensation of the Aethalometer data (Table 1).…”

“…Alternatively, a post-processing algorithm can be employed using either fixed parameters (Hyvärinen et al, 2013) or parameters obtained from the data themselves (Virkkula et al, 2007;Park et al, 2010;Hyvärinen et al, 2013). Weingartner et al (2003) and Virkkula et al (2007) models have been most frequently used for loading effect compensation of the Aethalometer data (Table 1). The non-compensated concentration, BC NC , reported by the instrument is compensated using a pre-determined compensation parameter (f or k depending on the model) and the attenuation values, as measured by the Aethalometer, to determine the compensated BC concentration.…”