Polarized Imaging Nephelometer  Development and Applications on Aircraft
Creators: name: Dolgos, Gergely
Publisher: name: Maryland Shared Open Access Repository
Year: 2014
Subjects: Aerosols
Imaging
Laser
Nephelometer
Polarization
Scattering

description: Satellite remote sensing is the only method that is able to measure climate forcing atmospheric constituents on a global scale. Aerosols have large impacts on climate, through influencing radiation directly and through their effects on clouds. Remote sensing algorithms that deduce aerosol properties from satellite measurements of scattered sunlight rely on information about scattering patterns of aerosols. Chapter 1 places the work in context through a literature survey, and introduces and clarifies theoretical constructs necessary for understanding the remaining chapters. In order to improve our understanding of light scattering by aerosol particles, and to enable routine in situ airborne measurements of aerosol light scattering to calibrate and validate satellite algorithms, at LACO (Laboratory for Aerosols Clouds and Optics) we have developed an instrument called the Polarized Imaging Nephelometer (PI-Neph). We designed and built the portable PI-Neph instrument at UMBC (University of Maryland, Baltimore County); it directly measures the ambient volume scattering coefficient and the phase matrix elements P11, phase function, and -P12/P11, polarized phase function. Chapter 2 introduces the PI-Neph instrument, while chapter 3 lays out the calibration and data reduction algorithm. The PI-Neph employs illuminating lasers and polarization control, sampling of ambient air through an inlet, and wide field of view detection of scattered light in a scattering angle range of 3? to 176?. The instrument does not employ any moving parts, therefore it is robust and fast enough for airborne measurements. The PI-Neph first measured at a laser wavelength of 532nm, and was first deployed successfully in 2011 aboard the B200 aircraft of NASA Langley during the DEVOTE (Development and Evaluation of satellite ValidatiOn Tools by Experiments) project. In 2013, we upgraded the PI-Neph to measure at 473nm, 532nm, and 671nm nearly simultaneously. LACO has deployed the PI-Neph on a number of airborne field campaigns aboard three different NASA aircraft.  This dissertation describes the PI-Neph instrument, the measurement approach, algorithm, and calibrations. Chapter 4 is a validation and error analysis case study, which quantifies the exceptionally good agreement of artificial polystyrene sphere data with Mie theory. The PS sphere validation data was collected during the 2012 deployment for the Deep Convective Clouds and Chemistry (DC3) field campaign. Residuals between Mie fit and PS data (angular mean and population standard deviation) in case of the cross fits were the following. The differences of P11 data and P11 fit (predicted by fit to -P12/P11), relative to the P11 data, was -1.9% ? 5.5%. The differences of -P12/P11 data versus -P12/P11 fit (predicted by the fit to P11) was 0.028 ? 0.062. Chapter 4 also quantifies the P11 and -P12/P11 calibration errors through a Monte Carlo simulation. At an ambient aerosol scattering coefficient of 100 Mm-1, it is estimated that the PI-Neph will measure P11 (at individual scattering angles) with relative systematic error of approximately 2.3%, and -P12/P11 with absolute systematic error of approximately 0.033.  Chapter 5 presents highlights from field measurements. The DEVOTE project enabled the initial PI-Neph results, such as scattering coefficient validation via comparison to other measurements, comparison of polystyrene sphere data to Mie theory, a comparison to AERONET derived phase function, and retrieval of microphysical properties of aerosols in collaboration with Dr. Oleg Dubovik. Chapter 5 also provides an overview of DC3 data, an assessment of data quality, and quantifications of PI-Neph capabilities and noise levels. This dissertation demonstrates the capability of the PI-Neph to measure ambient levels of aerosols down to 10 Mm-1, by measurement examples from the DEVOTE and DC3 aircraft deployments. As a conclusion, an itemized list of accomplishments is given in chapter 6, along with promising future research directions.