Aerosol microphysics from a polar-integrating nephelometer: shutter angles, information content and angular redundancy

Abstract. A polar-integrating nephelometer enables measurements of aerosol light scattering at various shutter angles. There is currently no generally accepted methodology for selecting optimal angular configurations based on maximizing the instrument’s sensitivity to specific microphysical particle parameters. A shutter angle of ∼60° is found to provide maximum discrimination between absorbing and non-absorbing aerosols, while small angles (between 0° and 20°) and maximum angle (90°) are preferable for distinguishing non-absorbing aerosol types. This conclusion is obtained by performing a comprehensive derivative-based global sensitivity (DGSM) analysis, degrees of freedom for signal (DFS) assessment, and variance-ratio analysis for aerosol type discrimination to determine optimal measurement conditions, quantify retrievable parameters, and establish the limits of applicability of nephelometer data. A forward model of the polar-integrating nephelometer is studied within the GRASP analytical framework. The analysis includes global sensitivity assessment, estimation of minimum detectable concentration changes, DFS analysis via singular value decomposition, and calculation of the variance-ratio metric for ranking shutter positions. The results demonstrate that maximum sensitivity is observed for fine-mode particles with radii of 113–439 nm (for concentration), 86–335 nm (for the real part of the refractive index), and 148–576 nm (for the imaginary part). To detect a 3 % change in the signal, concentration changes of less than 1 µm³ cm⁻³ are sufficient for particles with radii below 1 µm, whereas for particles in the 3–5 µm range, changes of 1–10 µm³ cm⁻³ are required, and for particles larger than 5 µm, changes exceeding 10 µm³ cm⁻³ are necessary, which makes the coarse mode non-retrievable. The DFS analysis indicates that for bimodal size distributions, the total number of degrees of freedom for signal is 3 for all investigated aerosol types. Based on the Fisher information matrix analysis, three retrievable parameter combinations are identified (in order of decreasing information content): total aerosol concentration, fine-to-coarse mode partitioning, and effective particle radius. The results show that the limited information content of polar-integrating nephelometer measurements is a direct consequence of the compact and smoothing nature of the scattering operator, which restricts the effective dimensionality of the measurement space despite the availability of multiple angular observations.