The Reno Aerosol Optics Study: An Evaluation of Aerosol Absorption Measurement Methods

Abstract

The Reno Aerosol Optics Study (RAOS) was designed and conducted to compare the performance of many existing and new instruments for the in situ measurement of aerosol optical properties with a focus on the determination of aerosol light absorption. For this study, simple test aerosols of black and white particles were generated and combined in external mixtures under low relative humidity conditions and delivered to each measurement system. The aerosol mixing and delivery system was constantly monitored using particle counters and nephelometers to ensure that the same aerosol number concentration and amount reached the different instruments. The aerosol light-scattering measurements of four different nephelometers were compared, while the measurements of seven light-absorption instruments (5 filter based, 2 photoacoustic) were evaluated. Four methods for determining the aerosol lightextinction coefficient (3 cavity ring-down instruments and 1 foldedpath optical extinction cell) were also included in the comparisons. An emphasis was placed on determining the representativeness of the filter-based light absorption methods, since these are used Address correspondence to Patrick J. Sheridan, Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, CO 80305, USA. E-mail: patrick.sheridan@noaa.gov widely and because major corrections to the raw attenuation measurements are known to be required. The extinction measurement from the optical extinction cell was compared with the scattering measurement from a high-sensitivity integrating nephelometer on fine, nonabsorbing ammonium sulfate aerosols, and the two were found to agree closely (within 1% for blue and green wavelengths and 2% for red). The wavelength dependence of light absorption for small kerosene and diesel soot particles was found to be very near λ −1 , the theoretical small-particle limit. Larger, irregularly shaped graphite particles showed widely variable wavelength dependencies over several graphite runs. The light-absorption efficiency at a wavelength of 530 nm for pure kerosene soot with a number size distribution peak near 0.3 µm diameter was found to be 7.5 ± 1.2 m 2 g −1 . The two most fundamental independent absorption methods used in this study were photoacoustic absorption and the difference between suspended-state light extinction and scattering, and these showed excellent agreement (typically within a few percent) on mixed black/white aerosols, with the photoacoustic measurement generally slightly lower. Excellent agreement was also observed between some filter-based light-absorption measurements and the RAOS reference absorption method. For atmospherically relevant levels of the aerosol light-absorption coefficient (<25 Mm −1 ), the particle soot absorption photometer (PSAP) absorption measurement at mid-visible wavelengths agreed with the reference absorption measurement to within ∼11% for experiment tests on externally mixed kerosene soot and ammonium sulfate. At higher absorption ...

Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

“…For atmospheric applications, the only instrument that can measure light absorption directly on an airborne aerosol is the photoacoustic spectrometer (PAS) (Truex and Anderson, 1979;Yasa et al, 1979;Petzold and Niessner, 1995;Arnott et al, 1999). In principle, light absorption by airborne aerosols can also be determined by the so-called "difference method", where absorption is determined from the difference between extinction and scattering as measured by an extinction cell and a nephelometer, respectively (Schnaiter et al, 2005b;Sheridan et al, 2005). Comparison experiments with the PAS and optical extinction cells show excellent agreement for a wide variety of aerosols including diesel soot, biomass smoke particles and ambient aerosol Schnaiter et al, 2005b).…”

“…Comparison experiments with the PAS and optical extinction cells show excellent agreement for a wide variety of aerosols including diesel soot, biomass smoke particles and ambient aerosol Schnaiter et al, 2005b). While this method is useful for the validation of absorption techniques with high concentrations of relatively "dark" aerosols (single scattering albedo at 550 nm less than ∼0.8), it results in unacceptably large errors for typical atmospheric conditions, since it determines σ abs from a fairly small difference of two large numbers (extinction and scattering coefficient) (Schnaiter et al, 2005b;Sheridan et al, 2005). Historically, absorption measurements have most frequently been performed with on-line "integrating plate" techniques (e.g., aethalometer and particle soot absorption photometer, PSAP), which deposit aerosol onto a filter and measure the resulting change in light transmittance through the filter.…”

“…On the other hand, laboratory comparisons between the PAS and difference method suggest that the PAS can determine aerosol absorption with an accuracy <10% (Moosmüller et al, 1998;Arnott et al, 1999;Arnott et al, 2003;Sheridan et al, 2005).…”

“…Comparisons of simultaneous measurements made using PAS and filter-based techniques (PSAP, aethalometer, integrating plate) show considerable differences (up to a factor of two) under some circumstances, especially when the relative humidity is variable (Moosmüller et al, 1998;Arnott et al, 1999;Arnott et al, 2003;Sheridan et al, 2005). The most reliable filter-based instrument for absorption measurements at this time is the multi-angle absorption photometer (MAAP), which simultaneously measures transmittance and reflectance of the filter at multiple angles and uses a twostream radiative transfer model to determine the filter and aerosol scattering corrections for the absorption measurement (Petzold and Schönlinner, 2004).…”

Vertical distributions of aerosol optical properties during the spring 2016 ARIAs airborne campaign in the North China Plain

“…Its observation principle and uncertainties have been well documented (Bond et al, 1999;Sheridan et al, 2005;Virkkula et al, 2005;Chaudhry et al, 2007). To ensure a steady sample flow during flights, we monitored the total flow and PSAP flow rates to enhance the accuracy of measurements.…”