Teledyne Advanced Pollution Instrumentation | Model 633 Aethalometer® Black Carbon Monitor User Manual |
The ‘spot loading effect’ is a variable phenomenon which appears as a gradual reduction of instrumental response as the aerosol deposit density of the filter tape increases from zero to the predetermined limit of ‘Maximum Attenuation’ (Gundel 1984, Weingartner 2003, Arnott 2005, Virkkula 2007, Kanaya 2008). When the filter tape advances to a fresh spot, the data undergoes a discontinuous jump from its previous lower value, calculated when the spot was heavily loaded; to a higher value, calculated from collection on a fresh spot at zero loading. In the Aethalometer the reduction of data at increasing loadings is well described by a linear function of attenuation, but its magnitude cannot be predicted: some aerosols in some locations in some seasons may show a small or zero ‘loading effect’; while under other conditions, the effect may be larger and noticeable. Empirically, it is found that fresher aerosols closer to their combustion sources will show a larger ‘spot loading effect’; while well‐ aged aerosols under atmospheric conditions of high chemical activity and oxidative processing may show an almost zero effect. The effect is revealed statistically by processing data collected over a large number of tape advances, representing many data points collected at loadings (‘ATN values’) ranging from zero to the preset maximum. The data is collected into bins according to loading (attenuation, ATN). If there is a systematic reduction of the calculated result as a function of loading, the data will show a clear negative slope, with the intercept representing the ‘zero loading’ value. Figure 2 illustrates two datasets from urban locations with loading effects either present or not.
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