R.C. Moffet & K.A. Prather: In-situ measurements of the mixing state and optical properties of soot with implications for radiative forcing estimates

Proceedings of the National Academy of Sciences, published online before print July 6, 2009; doi: 10.1073/pnas.0900040106

In-situ measurements of the mixing state and optical properties of soot with implications for radiative forcing estimates

Ryan C. Moffet (Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, U.S.A.) and Kimberly A. Prather* (Department of Chemistry and Biochemistry, University of California, San Diego, and Scripps Institute of Oceanography, La Jolla, CA 92093, U.S.A.)

Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved May 29, 2009 (received for review January 10, 2009).

Abstract

Our ability to predict how global temperatures will change in the future is currently limited by the large uncertainties associated with aerosols. Soot aerosols represent a major research focus as they influence climate by absorbing incoming solar radiation resulting in a highly uncertain warming effect. The uncertainty stems from the fact that the actual amount soot warms our atmosphere strongly depends on the manner and degree in which it is mixed with other species, a property referred to as mixing state. In global models and inferences from atmospheric heating measurements, soot radiative forcing estimates currently differ by a factor of 6, ranging between 0.2–1.2 W/m², making soot second only to CO₂ in terms of global warming potential. This article reports coupled in situ measurements of the size-resolved mixing state, optical properties, and aging timescales for soot particles. Fresh fractal soot particles dominate the measured absorption during peak traffic periods (6–9 a.m. local time). Immediately after sunrise, soot particles begin to age by developing a coating of secondary species including sulfate, ammonium, organics, nitrate, and water. Based on these direct measurements, the core-shell arrangement results in a maximum absorption enhancement of 1.6× over fresh soot. These atmospheric observations help explain the larger values for soot forcing measured by others and will be used to obtain closure in optical property measurements to reduce one of the largest remaining uncertainties in climate change.

• *Correspondence e-mail: kprather@ucsd.edu

• Author contributions: R.C.M. and K.A.P. designed research, performed research, analyzed data, and wrote the paper. The authors declare no conflict of interest.

Link to abstract: http://www.pnas.org/content/early/2009/07/02/0900040106.abstract