Portable instrumentation for UV-visible spectroscopy of ambient and laboratory-generated aerosols
Fischer, Donald Alphonse
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Aerosols directly affect Earth’s climate by scattering and absorbing solar radiation. Although they are ubiquitous in Earth’s atmosphere, direct, in-situ, wavelength-resolved measurements of aerosol optical properties remain elusive. As a result, the so-called aerosol direct effects are one of the largest uncertainties in predictions of Earth’s future climate, and new instrumentation is needed to provide measurements of the scattering and absorption of sunlight by atmospheric particles, especially in the UV. This work involved the development of three pieces of equipment to address the gap in instrumentation: (1) a UV-visible broadband cavity enhanced spectrometer for the measurement of wavelength-resolved extinction from 375–700 nm; (2) a four-wavelength, single-cell photoacoustic spectrometer for simultaneous measurement of aerosol absorption at 406, 532, 662, and 780 nm; and (3) a three-wavelength, single-cell UV photoacoustic spectrometer for measurements of absorption at 320, 377, and 445 nm. Extra effort has been made to make these instruments compact and robust while maintaining exceptional detection limits (< 1 1/Mm of absorption/extinction). Various versions of these instruments, and their earlier prototypes have been used in collaborations at other laboratories to make the first measurements of the Ångström exponent (the wavelength dependence to extinction/absorption) of flame-generated soot in the UV, to test traditional calibration methods for photoacoustic spectroscopy, to validate methods of measuring aerosol absorption, and in international intercomparisons of aerosol absorption measurements.