MAGEE Scientific Aethalometer AE33 User Manual page 132

Aethalometer
Model AE33
concentration of BC in urban, suburban, regional, rural and remote locations; source testing;
and laboratory-based research.
'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 7 illustrates two datasets from urban locations with loading
effects either present or not.
____________________________________________________________________________________________________________
User's manual – Ver. 1.55 July 2017 132/146