Absorption Enhancement of Coated Absorbing Aerosols: Validation of the Photo-Acoustic Technique for Measuring the Enhancement |
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| Authors: | Daniel A Lack Christopher D Cappa Eben S Cross Paola Massoli Adam T Ahern Paul Davidovits |
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| Affiliation: | 1. NOAA Earth System Research Laboratory, Chemical Sciences Division , Boulder, Colorado, USA;2. Cooperative Institute for Research in Environmental Sciences, University of Colorado , Boulder, Colorado, USA;3. Department of Civil and Environmental Engineering , University of California , Davis, California, USA;4. Chemistry Department , Boston College , Chestnut Hill, Massachusetts, USA;5. Chemistry Department , Boston College , Chestnut Hill, Massachusetts, USA;6. Aerodyne Research, Inc. Billerica , Massachusetts, USA |
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| Abstract: | A photo-acoustic absorption spectrometer (PAS) and a cavity ring down aerosol extinction spectrometer (CRD-AES) were used, in conjunction with Mie Theory, to measure the refractive index (RI) of absorbing polystyrene spheres (APSS). The PAS and CRD-AES were also used to measure the absorption and extinction enhancement after these APSS were coated in oleic acid. The experimental enhancements were then compared to predictions from coated-sphere Mie Theory. The measured absorption and extinction enhancements both agreed with modeled enhancements to within an average of 5%. A filter-based absorption technique (particle soot absorption photometer, PSAP) was also used to measure the absorption by the APSS and showed a significant size-dependent bias, as evidenced by the filter-based method measuring significantly lower absorption for both uncoated and coated APSS compared to the PAS. These results suggest the validity of applying photo-acoustics to measure the absorption enhancement created by semi-volatile atmospheric species coating absorbing particles. |
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