Recent Studies on the Sources, Prevalence and Optical Importance of Brown Carbon

Components of the ambient fine organic aerosol selectively absorb light at short wavelengths. Little is known about the prevalence or optical importance of this so-called brown carbon, especially throughout the tropospheric column. Aerosol optical light absorption measurements quantify brown carbon by a difference from black carbon, and so have difficulty identifying brown carbon, except in cases of high loadings. This may be why brown carbon has generally been overlooked, yet recent work shows it might significantly contribute to the visible light absorption budget, particularly at shorter wavelengths. In contrast to optical measurements of all light-absorbing materials, direct spectrophotometric measurements of aerosol particle extract solutions show the ubiquitous presence of brown carbon. In the past few years we have used a variety of techniques to investigate the sources, prevalence and potential optical importance of brown carbon. Measurements have been made at a number of surface sites, including rural, representative urban, and next to major highways during various seasons. Brown carbon has also been measured throughout the tropospheric column. This talk will focus on recent studies investigating brown carbon's optical importance relative to total light absorption. Size distributions of brown carbon from surface measurements, along with Mie theory, are used to estimate brown carbon optical properties from bulk solution measurements. When applied to solution measurements from the wide range of sites studied, the optical and potential climatic importance can be estimated from direct measurements of brown carbon and compared to black carbon measured with traditional optical instrumentation.