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 ≈40ngm^-3 assuming an efficiency for light absorption of 10m²g^-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 ≈80, a resonance frequency of ≈500Hz, and a photoacoustic coefficient of 12.8Pa (Wm^-1)^-1. The equivalent noise bandwidth of the resonator is ≈5Hz. 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 ≈7.5mHz. This was achieved by vector time averaging the microphone signal for ≈8min. 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 532nm and 685nm, and corresponding average powers were 60 and 87nW. Some examples are provided for light absorption measurements using the photoacoustic instrument and a nearby aethalometer.