Estimate of scattering truncation in the cavity attenuated phase shift PMSSA monitor using radiative transfer theory.

The recently developed cavity attenuated phase shift particulate matter single scattering albedo (CAPS PM_SSA) monitor has been shown to be fairly accurate and robust for real-time aerosol optical properties measurements. The scattering component of the measurement undergoes a truncation error due to the loss of scattered light from the sample tube in both the forward and backward directions. Previous studies estimated the loss of scattered light typically using the Mie theory for spherical particles, assuming particles are present only on the sampling tube centerline, and without accounting for the effects of sampling tube surface reflection. This study overcomes these limitations by solving the radiative transfer equation in an axisymmetric absorbing and scattering medium using the discrete-ordinates method to estimate the scattering truncation error. The effects of absorption coefficient, scattering coefficient, asymmetry parameter of the scattering phase function, and the reflection coefficient at the sampling tube inner surface were investigated. Under typical conditions of CAPS PM_SSA operation of low extinction coefficients below about 5000 Mm⁻¹, the scattering loss remains independent of the absorption and scattering coefficients but is dependent on the scattering phase function and the reflection coefficient of the sampling glass tube inner surface. The proposed method was used to investigate the effects of asymmetry parameter and surface reflection coefficient on truncation for absorbing aerosol particles whose scattering phase function can be well represented by the Henyey-Greenstein approximation. The scattering loss increases with increasing the asymmetry parameter and the surface reflection coefficient. Copyright © 2018 National Research Council Canada [ABSTRACT FROM AUTHOR]