US EPA

20650 

Journal Article 

Photoacoustic spectrometer for measuring light absorption by aerosol; instrument description 

Arnott, WP; Moosmuller, H; Rogers, CF; Jin, T; Bruch, R 

1999 

1 

Atmospheric Environment
ISSN: 1352-2310
EISSN: 1873-2844 

33 

17 

2845-2852 

English 

WOS:000080975600024 

US Environmental Protection Agency; Desert Research Institute. #A photoacoustic spectrometer has been developed to measure in situ light absorption by aerosol. The measured quantity is the sound pressure produced in an acoustic resonator caused by light absorption. The current lower detection limit for light absorption is 0.4 Mm-1 which corresponds to an elemental carbon mass density of approximately 40 ng m-3 assuming an efficiency for light absorption of 10 m2 g-1. Calibration is performed using simple theory for the instrument along with use of a calibrated microphone and laser. The acoustic resonator is operated in the plane wave mode, which has a quality factor of approximately 80, a resonance frequency of approximately 500 Hz, and a photoacoustic coefficient of 12.8 Pa (Wm-1)-1. The equivalent noise bandwidth of the resonator is approximately 5 Hz. Coherent acoustic noise was supressed through the use of acoustic notch filters and laser beam ports at pressure nodes of the resonator. The relatively low-quality factor made it possible to use phase-sensitive detection having an equivalent noise bandwidth of approximately 7.5 mHz. This was achieved by vector time averaging the microphone signal for approximately 8 min. Two compact, efficient lasers were used during instrument evaluation performed in the Northern Front Range Air Quality Study (Colorado, 1996/97). One was a laser diode pumped, frequency doubled, solid state laser, and the other was a laser diode. Laser wavelengths were 532 nm and 685 nm, and corresponding average powers were 60 and 87 nW. Some examples are provided for light absorption measurements using the photoacoustic instrument and a nearby aethalometer. 

light absorption; photoacoustic spectrometer; aethalometer; aerosol; elemental carbon; instrumentation;INSITU MEASUREMENTS; OPTICAL-ABSORPTION; REAL-TIME; SPECTROSCOPY 

6th International Conference on Carbonaceous Particles in the Atmosphere 

SEP 22-24, 1997