Radiative absorption enhancements due to the mixing state of atmospheric black carbon
Abstract
Atmospheric black carbon (BC) warms Earth's climate through absorption of solar radiation and its reduction has been targeted for near-term climate change mitigation. Additionally, absorption by BC above the Earth's surface can alter local atmospheric dynamics and the hydrologic cycle. Most models that include forcing by BC and that account for internal mixing with non-BC aerosol components assume that this internal mixing enhances BC absorption, some by a factor of ~2 or more; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of the influence of photochemical ageing on BC absorption enhancements (Eabs) and mixing state are reported for two California regions as observed during the CalNex and CARES field studies. The observed Eabs values were small, 6% on average at 532 nm, and increased only weakly with photochemical ageing despite substantial secondary production of and internal mixing with non-BC aerosol. The observed Eabs is less than predicted from observationally-constrained theoretical calculations, suggesting that many climate models may overestimate the direct effect of BC on the Earth's radiation budget. These ambient observations stand in contrast to laboratory measurements that show significant absorption enhancements that are in good agreement with Mie theory calculations for BC when internally mixed (coated) with dioctyl sebacate, a liquid organic compound. New laboratory experiments that aim to identify conditions under which Eabs deviates from theoretical predictions will also be discussed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A43I..05C
- Keywords:
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- 0345 ATMOSPHERIC COMPOSITION AND STRUCTURE / Pollution: urban and regional;
- 0365 ATMOSPHERIC COMPOSITION AND STRUCTURE / Troposphere: composition and chemistry;
- 1610 GLOBAL CHANGE / Atmosphere