The Effect of Aerosol Humic-like Substances (HULIS) on Aerosol Absorption

Nancy Marley University of Arkansas, Little Rock
Kristi Kelley University of Arkansas, Little Rock
Amrita Sarkar University of Arkansas, Little Rock
Gail Bridges University of Arkansas, Little Rock

Category: Aerosol Properties

Working Group: Aerosol Life Cycle

Image Thumbnail
The peak areas of the HULIS carboxylate (1620, 1420 cm-1) and polysaccharide (1020 cm-1) IR bands correlated with the enhanced aerosol AAEs and with the fraction of modern carbon as measured by 14C, indicating that the HULIS are of biogenic origin and are the source of the enhanced aerosol UV absorption.

In past work, black carbon (BC) produced from incomplete combustion has been assumed to be the only major absorbing species in atmospheric aerosols. BC absorption follows a broadband spectral profile with an inverse wavelength dependence (λ-1) from the UV to the near IR. This wavelength dependence is described by the Ångstrom absorption exponent (AAEs), which is 1 for BC. Recent work has identified other absorbing aerosol species that can add to the absorption of BC, resulting in enhanced absorption primarily at shorter wavelengths yielding AAEs greater than 1. The most important absorbing aerosol species other than BC is the water soluble humic-like substances, or HULIS. These aerosol species can become internally mixed with black carbon as the combustion aerosols age, resulting in larger AAEs, enhanced hygroscopicity, and removal of the aerosols through cloud formation and rainout. By using surface reflection spectroscopy, the absorption spectra of atmospheric aerosols were obtained in the UV-visible for accurate determination of aerosol AAEs. These results are compared to total aerosol carbon content, carbon isotopic analysis, and aerosol HULIS content measured by diffuse reflectance FTIR spectroscopy. These results indicate that the aerosol HULIS content increases as the aerosols age and that this is primarily responsible for the observed enhanced UV absorption yielding overall aerosol AEAs from 1–2. In addition, carbon isotopic measurements show that the aerosol HULIS are biogenic in origin, arising from biomass burning and/or SOA formation from biogenic precursors.

This poster will be displayed at ASR Science Team Meeting.

POSTER in PDF: for proper viewing, it should be viewed with Adobe Acrobat Reader. Download the latest version from the Adobe Reader website.