Abstract
Validation of remote sensing retrievals of aerosol microphysical and optical properties requires in situ measurements of the same properties. We present here an improved imaging nephelometer for measuring the directionality and polarization of light (i.e. polarimetry) scattered at two wavelengths (405 nm and 660 nm) with high temporal resolution. The instrument was designed for airborne deployment and is capable of ground-based measurements as well. The Laser Imaging Nephelometer (LiNeph) uses two orthogonal detectors with wide-angle lenses and linearly polarized light sources to measure both the phase function, P11(θ), and degree of linear polarization, -P12/P11(θ). In this work, we will describe the instrument function and calibration, as well as data acquisition and reduction. The instrument was first deployed aboard the NASA DC-8 during the 2019 FIREX-AQ campaign. Here, we present field measurements of smoke plumes that show that the LiNeph has sufficient resolution for 0.24 Hz polarimetric measurements at two wavelengths, 405 and 660 nm, at integrated scattering coefficients ranging from 50–80,000 Mm−1.
Highlights
Greenhouse gases are a dominant climate forcer, tropospheric particles have large and under-constrained effects on the Earth’s radiative budget
Future work will explore the relationship between the asymmetry parameter and the hemispheric backscatter fraction, both of which can be derived from the phase function directly
We investigated the effect of truncation on 395 the asymmetry parameter using simulated phase functions calculated from measured particle size distributions during FIREX-AQ
Summary
Greenhouse gases are a dominant climate forcer, tropospheric particles have large and under-constrained effects on the Earth’s radiative budget. The Polarized Imaging Nephelometer, PI-Neph, was developed as an aircraft instrument for measuring the directionality and polarization of light scattering (Dolgos and Martins, 2014) It uses a wide-angle lens and a folded laser path. The scattered light is imaged using a cooled charge-coupled detector (CCD) which provides excellent sensitivity This sensitivity means that the instrument is capable of measuring scattering from submicron particles like biomass burning aerosol, and is sensitive to stray light in the instrument sample volume. This original LiNeph, described in Manfred et al (2018), was designed to investigate the optical properties at near-ultraviolet wavelengths, equipped with lasers at 375 nm and 405 nm This instrument uses circularly polarized light, and only measures the directionality of the scattered light, with no information regarding changes in polarity. We use two wide-angle lenses and cooled CCDs to collect images of light scattered perpendicular and parallel to the lasers’ polarization, allowing us to measure both the directionality and the polarization of light scattered by the sample
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