How much can the vertical distribution of black carbon affect its global direct radiative forcing?

Colin M. Zarzycki, Tami C. Bond

Research output: Contribution to journalArticlepeer-review


Black carbon (BC) has an increased forcing per unit mass when it is located above reflective clouds. To explore sensitivity of forcing to aerosol vertical location, we used a column radiative transfer model to produce globally-averaged values of normalized direct radiative forcing (NDRF) for BC over and under different types of clouds. We developed a simple column-weighting scheme based on the mass fractions of BC that are over and under clouds in measured vertical profiles. The resulting NDRF is in good agreement with global 3-D model estimates, supporting the column-weighted model as a tool for exploring uncertainties due to diversity in vertical distribution. BC above low clouds accounts for about 20% of the global burden but 50% of the forcing. We estimate maximum-minimum spread in NDRF due to modeled profiles as about 40% and uncertainty as about 25%. Because models overestimate BC in the upper troposphere compared with measurements, modeled NDRF might need to be reduced by about 15%. Redistributing BC within the lowest 4 km of the atmosphere affects modeled NDRF by only about 5% and cannot account for very high forcing estimates.

Original languageEnglish (US)
Article numberL20807
JournalGeophysical Research Letters
Issue number20
StatePublished - Oct 1 2010

ASJC Scopus subject areas

  • Geophysics
  • Earth and Planetary Sciences(all)

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