Abstract
A method for the retrieval of aerosol optical and microphysical properties from in situ light-scattering measurements is presented and the results are compared with existing measurement techniques. The Generalized Retrieval of Aerosol and Surface Properties (GRASP) is applied to airborne and laboratory measurements made by a novel polar nephelometer. This instrument, the Polarized Imaging Nephelometer (PI-Neph), is capable of making high-accuracy field measurements of phase function and degree of linear polarization, at three visible wavelengths, over a wide angular range of 3 to 177°. The resulting retrieval produces particle size distributions (PSDs) that agree, within experimental error, with measurements made by commercial optical particle counters (OPCs). Additionally, the retrieved real part of the refractive index is generally found to be within the predicted error of 0.02 from the expected values for three species of humidified salt particles, with a refractive index that is well established. The airborne measurements used in this work were made aboard the NASA DC-8 aircraft during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) field campaign, and the inversion of this data represents the first aerosol retrievals of airborne polar nephelometer data. The results provide confidence in the real refractive index product, as well as in the retrieval’s ability to accurately determine PSD, without assumptions about refractive index that are required by the majority of OPCs.
Highlights
Aerosols, and their interaction with clouds, play a key role in the climate of our planet
The κ range used for ammonium nitrate are derived from measurements of cloud condensation nuclei (CCN) at supersaturations less than 1 %, and originate from Svenningsson et al (2006), with the spread representing an uncertainty of 1 standard deviation
The Polarized Imaging Nephelometer (PI-Neph)/Generalized Retrieval of Aerosol and Surface Properties (GRASP) inversion makes fewer assumptions regarding the shape of the recovered size distribution and particle sphericity than previous in situ light-scattering retrievals
Summary
Their interaction with clouds, play a key role in the climate of our planet. These instruments are among the most widespread and precise available, but the vast majority of optical particle counter (OPC) designs require significant assumptions about the aerosol being sampled These simplifications result from the limited information content present in typical OPC measurements, which frequently sample scattered light over a single angular range, often 4 to 22◦ (Pinnick et al, 2000) or roughly 30 to 150◦ (Cai et al, 2008) in so called wide angle. In this work we apply a complex inversion algorithm, the Generalized Retrieval of Aerosol and Surface Properties (GRASP), to airborne and laboratory measurements made with the Polarized Imaging Nephelometer (PINeph), a multiwavelength, multiangle light-scattering instrument. The ambient airborne measurements presented here were made in parallel to a large variety of independent instrumentation, allowing for very robust intercomparisons of the retrieved products
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