MAGEE Scientific AE33 User Manual page 7

Aethalometer
Model AE33
'DualSpot™' technology
The Aethalometer Model AE33 uses the patented DualSpot™ method to compensate for the
'spot loading effect'; and also to provide a real-time output of the 'loading compensation'
parameter, which may provide additional information about the physical and chemical
properties of the aerosol.
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, Hyvärinen 2012). 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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User's manual – Ver. 1.53 July 2015 7/75