Development and Evaluation of a Near-infrared (1047 nm) Photoacoustic-Nephelometer Spectrometer for Detection and Optical Characterization of Black Carbon Aerosol
Date of Award
Master of Science (MS)
Black carbon (BC) aerosol are aggregates of small carbon spherules of <10 nm to approximately 50 nm in diameter. They are characterized by their strong visible light absorption property with a mass absorption cross-section (MAC) value above 5 m2 g −1 at a wavelength λ = 550 nm, which increases inversely with wavelengths from near-infrared (≈1 µm) to ultraviolet with a power law of one. The absorbing nature of BC aerosol has been implicated in regional atmospheric warming, changing of monsoon patterns, and accelerated melting of the glaciers. The BC radiative effects over earth is currently estimated within a factor of four, resulting in one of the largest uncertainties in climate modeling. This uncertainty results from our inadequate knowledge about regional BC emission rates, and associated aerosol microphysical properties. Conventional methods for quantifying BC mass concentration in the atmosphere are filter-based techniques, which have limitations with respect to dynamic range, lack of a common definition of BC, and scattering aerosol interferences during measurement. Contact-free measurement techniques such as Photoacoustic spectroscopy and nephelometry overcome these deficiencies associated with filter-based sampling methods.
This thesis discusses the design and development of a novel, integrated photoacoustic-nephelometer (IPN) spectrometer operating at 1047 nm wavelength for accurate detection and measurement of BC aerosol. Choice of this wavelength was made because of minimal gaseous interference and availability of a high-power laser source that allowed for direct electronic modulation of the power at the resonator acoustic frequency. Simultaneous measurement of light scattering by reciprocal nephelometry within the photoacoustic resonator cell facilitates measurement of aerosol absorption (βabs) and scattering (βsca) coefficients at 1 Hz frequency. The principles behind instrument operation is described in detail. Also described and implemented is a robust method of instrument calibration and stability analysis using the Allan deviation method. The Allan deviations for βabs and βsca were estimated to be 0.14 and 3.42 Mm-1, respectively. The 2σ (two standard deviation) values, which are considered to be the detection limit for a 10-min averaged βabs and βsca measurements, were 1.15 and 40.14 Mm-1, respectively. The potential performance of this instrument at lower pressure environments (such as on aircrafts) was also evaluated. Finally, this instrument was used to measure the MAC and mass scattering cross-sections (MSC) values of BC aerosols. The measurement results were in good agreement with theoretical predictions.
Rajan K. Chakrabarty
Dr. Pratim Biswas Dr. Richard L. Axelbaum
Permanent URL: https://doi.org/10.7936/K74T6GNB