Description of Particle Soot Absorption Photometer (PSAP)

The PSAP is a filter-based method for determining light absorption.

PSAP Instrument
PSAP readout box
PSAP filter holder
Top view of inside of PSAP instrument

General Information

The Particle Soot/Absorption Photometer (PSAP) is used to measure in near real time the light absorption coefficient. The method is based on the integrating plate technique in which the change in optical transmission of a filter caused by particle deposition on the filter is related to the light absorption coefficient of the deposited particles using Beers Law. Older versions operated at 1 wavelength (nominally 565 nm) while new versions have 3 wavelengths (467, 530 and 660 nm). The description below is for the 1 wavelength version, but the principle is the same for the 3 wavelength instrument.

Beers Law is given by:



Calculation of absorption coefficient is based on the change in filter transmission (I/Io) for a given volume of sample air. Without correcting for filter type and loading, the absorption coefficient is:

    bap = (area/volume)ln(Io/I)


With correction for filter nonlinearity (loading+filter characteristics?) the corrected absorption coefficient is:

    bap,corr = bap f(Tr)

where f(Tr) is the transfer function based on filter loading (Tr) for Pallflex filters. The transfer function is incorporated in the software.

In order to ensure that the observed change in filter transmittance is not due to changes in the intensity of the LED light source, a second filter is used as a reference. This reference filter is adjacent to the primary filter.

The particle laden airstream first passes through the primary filter which removes the particles and then the clean airstream passes through the reference filter.

Corrections to PSAP measurements

We make several corrections to the PSAP measurements to obtain our final value for absorption coefficient. These corrections are described in detail Bond et al., (1999) but are briefly mentioned here. An update to these corrections for three-wavelength absorption measurements is provided in Ogren, 2010

Measurement Uncertainties

These values are from Bond et al. (1999), and are reported at 95% confidence