turbidity, τ in light scattering, in catalysis

https://dx.doi.org/10.1351/goldbook.T06533
The apparent absorbance of the incident radiation due to scattering. For small particles, direct proportionality exists between turbidity and the Rayleigh ratio.
Source:
See also:
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2218