Radiative Absorption Enhancements Due to the Mixing State of Atmospheric Black Carbon

TLDR

Atmospheric black carbon warms the climate, and models assume internal mixing with other aerosols can roughly double its absorption, yet this enhancement has not been clearly validated by observations. The study reports direct in situ measurements of black carbon absorption enhancements and mixing state in two California regions. The authors measured BC absorption enhancements and mixing state directly in the atmosphere of these regions. Observed absorption enhancements were only about 6 % on average at 532 nm, slightly increasing with photochemical aging, and were lower than model predictions, implying that many climate models may overestimate black carbon warming, contrary to laboratory findings that show larger enhancements.

Abstract

Atmospheric black carbon (BC) warms Earth's climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E(abs)) and mixing state are reported for two California regions. The observed E(abs) is small-6% on average at 532 nm-and increases weakly with photochemical aging. The E(abs) is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E(abs) for BC are possible.