Motivation and Introduction:

Measuring Aerosol Absorption using a UV-Visible Photoacoustic Spectrophotometer Joe Wiegand, Dalila Mathews, Geoff Smith

The University of Georgia, Department of Chemistry, Athens, Ga

A multitude of techniques have been developed to measure the absorption of light by aerosols, but until recently filter-based instruments, such as the aethalometer, and techniques, such as solvent extraction, were the sole source of aerosol UV absorption measurements. There are well documented issues with filter-based instruments, including multiple scattering within the filter, and loading-dependent correction factors, while filter solvent extraction can takes up to hours or even days to make a measurement. These limitations call for new aerosol absorption measurement techniques to be developed. Photoacoustic spectrometry (PAS) provides an in-situ direct measurement of the absorption of light by aerosol particles. A novel PAS system was developed with the ability to make absorption measurements ranging from 300-700 nm. Light from a Hg arc lamp is filtered to eight separate wavelength bands centered at 301 nm, 314 nm, 364 nm, 405 nm, 436 nm, 546 nm, 578 nm, 687 nm. Once calibrated, this instrument shows the potential of making ambient aerosol measurements.

Aerosol Validation: Nigrosin

Lamp Spectrum: Eight Wavelength Bands:

• Using a Hg arc lamp as a light source in lieu of lasers provides the ability to measure absorption eight wavelength bands ranging from 300 nm-700 nm.

• One light source, one cell, and one microphone allows for a simple calibration

• Acetone and nigrosin measurements show the instrument’s ability to make quantitative measurements of gases and aerosol particles

• The instrument shows the potential to make ambient aerosol measurements

All eight wavelengths coming from a single arc light allowed for minimal complications while aligning. Light from an Hg arc lamp is tightly focused by a 40 mm focal length lens through an optical chopper which modulates it. The light then travels through a 1 cm aperture and is loosely focused by a 2nd 40 mm f.l. lens. The light passes through one of the bandpass filters contained in a motorized filter wheel. Then the light enters the PAS cell where the signal is detected by an optoacoustic microphone.

Apparatus:

Summary:

The graph above shows a typical calibration plot . The observed agreement at all three wavelengths shows the instrument's ability to use a calibration from a single wavelength to make measurements at all other wavelengths, a unique feature of a single-cell PAS instrument. This graph also shows the instrument’s ability to make absorption measurements at the same levels as ambient aerosols.

Gas Validation: Acetone Results:

Acknowledgements:

Acetone absorption measured at 301 nm and 314 nm. The dashed lines correspond to the weighed absorption cross-sections and the solid lines represent the linear least-square fits of the data. It is not surprising these measurements are low compared to the literature values since acetone photodissociates resulting in lower PA signal. Thus, the higher the quantum yield the greater the loss of PA signal, which is a trend seen here. These results show the instrument’s ability to accurately measure absorption by gases.

This figure shows nigrosin aerosol absorption cross-section measured with PAS (symbols) and calculated using Mie theory (lines) vs the particle diameter.

The largest discrepancy is only 20% for 450 nm particles, a size for which there were relatively few particles. At all other sizes, the measurements agreed to within 7% of the cross-sections calculated by Mie theory. These measurements prove the instruments ability to accurately measure aerosol absorption.

Wavelength Quantum Yield

Cross-Section Percent

Difference nm % 301 0.17 9 314 0.05 3

When modulated light is absorbed by a gas-phase species or a particle, the energy is translated into a transient heating of the surrounding gas creating pressure waves. These acoustic waves can be detected with a sensitive microphone and the amplitude of the signal is proportional to the strength of absorption. When the light is modulated at a resonant frequency of the cavity, a standing wave is created and the sound is amplified.

Description of Photoacoustic Spectroscopy:

𝑆𝑆𝑆𝑆𝑆 𝐴𝐴𝐴𝐴𝐴𝐴𝑆𝑆𝐴 𝜶 𝜎𝑎𝑎𝑎 ∗ 𝐿𝐴𝐿𝐿𝐴 𝐼𝑆𝐴𝐴𝑆𝐼𝐴𝐴𝐼

Calibration: NO2

This research is supported by the National Science Foundation under grant

AGS-1241621.

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