Concept, absolute calibration and validation of a new, bench-top laser imaging polar nephelometer
Abstract. Polar nephelometers provide in situ measurements of aerosol angular light scattering and play an essential role in validating numerically calculated phase functions or inversion algorithms used in space-borne and land-based aerosol remote sensing. In this study, we present a prototype of a new polar nephelometer called uNeph. The instrument is designed to measure the phase function, F11, and polarized phase function, –F12/F11 over the scattering range of around 5° to 175° with an angular resolution of 1° at a wavelength of 532 nm. In this work, we present details of the data processing procedures and instrument calibration approaches. The uNeph was validated in a laboratory setting using mono-disperse polystyrene latex (PSL) and Di-Ethyl-Hexyl-Sebacate (DEHS) aerosol particles over a variety of sizes, ranging from 200 nm to 800 nm. An error model was developed and the level of agreement between uNeph measurements and Mie theory was found to be consistent within the uncertainties of the measurements and the uncertainties of the input parameters for the theoretical calculations. The estimated measurement errors were between 5 % to 10 % (relative) for F11 and smaller than ~0.1 (absolute) for –F12/F11. Additionally, by applying the Generalized Retrieval of Aerosol and Surface Properties (GRASP) inversion algorithm to the measurements conducted with broad unimodal DEHS aerosol particles, the volume concentration, size distribution and refractive index of the ensemble of aerosol particles were accurately retrieved. This paper demonstrates that the uNeph prototype can be used to conduct accurate measurements of aerosol phase function and polarized phase function and to retrieve aerosol properties through inversion algorithms.
Alireza Moallemi et al.
Status: open (until 30 Apr 2023)
- RC1: 'Comment on egusphere-2023-392', Anonymous Referee #1, 28 Mar 2023 reply
Alireza Moallemi et al.
Model code and software
Platform for GRASP open source code Get public access to the code, documentation and user assistance https://www.grasp-sas.com/
miepython: a pure Python module to calculate light scattering by non-absorbing, partially-absorbing, or perfectly conducting spheres https://github.com/scottprahl/miepython
Alireza Moallemi et al.
Viewed (geographical distribution)
This manuscript presents a newly developed polar nephelometer for aerosol phase function characterisation, along with comprehensive calibration discussion and quantitative error analysis. While other groups have demonstrated polar nephelometry with similar configurations, the novelty of this instrument is its reduced size. The authors provide an extremely detailed and rigorous discussion of the data analysis, including the main sources of uncertainty in the data inversion procedures. The performance of the nephelometer and predicted errors are demonstrated via laboratory experiments using monodisperse and unimodally distributed non-absorbing aerosol with known optical properties. While it would have been interesting to see an exploration of more novel materials or more significant instrumentation developments, the strength of this manuscript is in the thorough analysis. I especially appreciate all the detail discussed in the appendix and shown in Supplementary figures. As such, it is a valuable and excellent piece of research that is highly recommended for publication in AMT after a few minor revisions.
L95: One of the novel features of the uNeph, compared to other polar nephelometers, is its relatively small size. It would be worth reiterating this advantage elsewhere, e.g. the conclusion section.
L97-109: Details on the optical system are scant. It would be useful to provide more details on:
L111: Based on Figure 1, it would appear that the camera lens forms part of the chamber seal, and the rooftop reflector is outside a window – is that correct?
L117-121: Can you clarify the dimensions of the region of interest within the camera pixel array that was used, and whether pixels were binned at all? Despite the large pixel array indicated in Section 2.1, Figure S3 indicates a much smaller array.
L120: More details about the objective (lens) would be helpful, e.g. camera position relative to the focal length, the field-of-view…
L162-164: Depending on the CPC mode used, this would imply that the aerosol flow to the uNeph was around 3.5 or 4.7 L min-1 – can you clarify what you estimate the volume flow in the uNeph to be (and hence the estimated residence time).
L485; L721-22: I believe the N limit in the summations over i indices refers to the number of angles – is that correct? It’s unclear whether this is incremented by pixel angle or by absolute angle. How does the angular resolution of measurements vary across the full range of measurement?
L592: It would appear that the Vtot predicted by the retrieval systematically overestimate the volume when compared to the SMPS measurement. Could this be because the measured size distribution (Figure 8) appears by eye to deviate from an ideal lognormal distribution?
L618-619: It would be interesting and useful to test your calibration and data processing procedures for absorbing spherical particles in the future.
L797: This seems like a very conservative (i.e. erring on the side of large uncertainty) way of quantifying this error. No corrections needed, just an observation!
Table 1: Please clarify whether “q25” and “q75” refer to the 25th and 75th percentile quantile points, or other values.
Figure 2: No image appears in the manuscript
Figure 5: It is unclear the purpose for showing the line depicting “3% of uNeph air measurement” in the lefthand panels
Figure 6: Check units on stated number concentrations in caption
Figure 8: The information presented in (b,c) plots might be more clearly depicted in a table
General comments on Figures 6, 7, S2, S4, S8, S9, S11, S12, S13, S14, S15, S16, and S17 : The font size in the axes labels, legends, and/or schematics is far too small.