Ambient aerosol particle were sampled continuously with a time resolution of ~4 hours for a period of 8 days at the Mauna Loa Observatory, Hawaii, in August 1986. The samples were analyzed on-site for their chemical composition by attenuated total internal reflection infrared spectroscopy. The infrared absorption spectra of the samples also provided data on aerosol light absorbance characteristics at 9.1 and 10.6 μm-wavelengths of interest in determining aerosol backscatter coefficients for CO2 lidars. The chemical species in the ambient aerosol varied considerably during this 8-day period. The aerosol was acidic <(NH4)3H(SO4)2, NH4HSO4, or H2SO4> rather than neutral <(NH4)2SO4> for a major fraction of the sampling time. The samples generally showed much higher absorbance 9.1 μm than at 10.6 μm. Changes in the chemical composition between (NH4)2SO4 and the more acidic forms were accompanied by substantial changes in the sample's absorbance at 9.1 μm but lesser changes in the absorbance at 10.6 μm. These variations could have a profound effect on backscatter coefficients of atmospheric aerosol particles at CO2 wavelengths. ¿American Geophysical Union 1991