A single-pass RGB differential photoacoustic spectrometer (RGB-DPAS) for aerosol absorption measurement at 473, 532, and 671 nm

Abstract. Light-absorbing organic aerosol, or brown carbon (BrC), has significant but poorly constrained effects on climate; for example, oxidation in the atmosphere may alter its optical properties, leading to absorption enhancement or bleaching. Here, we investigate for the first time the effects of heterogeneous OH oxidation on the optical properties of a laboratory surrogate of aqueous, secondary BrC in a series of photo-oxidation chamber experiments. The BrC surrogate was generated from aqueous resorcinol, or 1,3-dihydroxybenzene, and H2O2 exposed to >300 nm radiation that is atomized, passed through trace gas denuders, and injected into the chamber, which was conditioned to either 15 % or 60 % relative humidity (RH). Aerosol absorption and scattering coefficients and single scattering albedo (SSA) at 405 nm were measured using a photoacoustic spectrometer. At 60 % RH, upon OH exposure, absorption first increased, and the SSA decreased sharply. Subsequently, absorption decreased faster than scattering, and SSA increased gradually. Comparisons to the modelled trend in SSA, based on Mie theory calculations, confirm that the observed trend is due to chemical evolution, rather than slight changes in particle size. The initial absorption enhancement is likely due to molecular functionalization and/or oligomerization and the bleaching to fragmentation. By contrast, at 15 % RH, slow absorption enhancement was observed without appreciable bleaching. A multi-layer kinetics model, consisting of two surface reactions in series, was constructed to provide further insights regarding the RH dependence of the optical evolution. Candidate parameters suggest that the oxidation is efficient, with uptake coefficients on the order of unity. The parameters also suggest that, as RH decreases, reactivity decreases and aerosol viscosity increases, such that particles are well-mixed at 60 % RH but not at 15 % RH. These results further the current understanding of the complex processing of BrC that may occur in the atmosphere.

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Aerosol Absorption Measurement using Photoacoustic Spectroscopy: Sensitivity, Calibration, and Uncertainty Developments

Aerosol Science and Technology ◽

10.1080/02786820600803917 ◽

2006 ◽

Vol 40 (9) ◽

pp. 697-708 ◽

Cited By ~ 177

Author(s):

Daniel A. Lack ◽

Edward R. Lovejoy ◽

Tahllee Baynard ◽

Anders Pettersson ◽

A. R. Ravishankara

Keyword(s):

Photoacoustic Spectroscopy ◽

Absorption Measurement ◽

Aerosol Absorption

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Optical-chemical relationships for carbonaceous aerosols observed at Jeju Island, Korea with a 3-laser photoacoustic spectrometer

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-9369-2010 ◽

2010 ◽

Vol 10 (4) ◽

pp. 9369-9389 ◽

Cited By ~ 3

Author(s):

B. A. Flowers ◽

M. K. Dubey ◽

C. Mazzoleni ◽

E. A. Stone ◽

J. J.. Schauer ◽

...

Keyword(s):

Optical Properties ◽

Chemical Composition ◽

Radiative Forcing ◽

Optical Model ◽

Index Of Refraction ◽

Enhanced Absorption ◽

Aerosol Absorption ◽

Photoacoustic Spectrometer ◽

Wide Range ◽

Pollution Plumes

Abstract. Transport of aerosols in pollution plumes from the mainland Asian continent was observed in situ at Jeju, South Korea during the Cheju Asian Brown Cloud Plume-Asian Monsoon Experiment (CAPMEX) field campaign throughout August and September 2008 using a 3-laser photoacoustic spectrometer. Transport of mixed sulfate, carbonaceous, and nitrate aerosols from various Asian pollution plumes to Jeju accounted for 76% of the deployment days, showing large variations in their measured chemical and optical properties. Our analysis of eight distinct episodes, spanning a wide range of chemical composition, optical properties, and source regions, reveals that at episodes with higher OC/SO2−4 and NO−3/SO2−4 composition ratios exhibit lower single scatter albedo at shorter wavelengths (ω405); significantly lower [ω405meas = 0.79±0.06, ω405calc = 0.86±0.01] than predicted by an optical model that assumes constant complex index of refraction with wavelength (an optical model of soot). We attribute this discrepancy to enhanced absorption by organic material. Organic carbon absorption accounts for up to 50% of the measured aerosol absorption at 405 nm for the high OC/SO2−4 episode. Coatings of elemental carbon aerosol cores are hypothesized to increase absorption by factors up to 6 at visible wavelengths. Carbonaceous aerosol absorption can alter global radiative forcing estimates substantially, underscoring the need to understand and predict chemical composition effects on optical properties.

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