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The size distribution of mineral dust aerosols and implications for dust climate forcing

The size distribution of mineral dust aerosols and implications for dust climate forcing. Jasper F. Kok NESL Advanced Study Program. The NESL Mission is : To advance understanding of weather, climate, atmospheric composition and processes;

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The size distribution of mineral dust aerosols and implications for dust climate forcing

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  1. The size distribution of mineral dust aerosols and implications for dust climate forcing Jasper F. Kok NESL Advanced Study Program The NESL Mission is: To advance understanding of weather, climate, atmospheric composition and processes; To provide facility support to the wider community; and, To apply the results to benefit society. NCAR is funded by the National Science Foundation

  2. View from space

  3. The emitted dust size distribution • Dust effect on climate & weather depends on emitted dust size distribution • Emitted dust size distribution poorly understood • Models overestimate small particle fraction • What determines dust size distribution? Measurements: Gillette et al. (1972, 1974), Gillette (1974), Sow et al. (2009) NESL_NSF Review 09-12 May 2011

  4. Macrophysics of dust emission: Saltation • Dust aerosols (~0.1-10 mm) are emitted by saltation, the wind-driven hopping motion of sand grains (~200 mm) NESL_NSF Review 09-12 May 2011

  5. Microscopic physics of dust emission:Fragmentation of dust aggregates ? • Small particles (< ~20 mm) in desert soils form aggregates • Upon impact, energy is transferredfrom impactor to aggregate • What is final state of aggregate? Does it fragment? Into what particle sizes? impactenergy + Frames from Beladjine et al., 2007 + + + NESL_NSF Review 09-12 May 2011

  6. Analog: fragmentation of brittle materials • Dust aggregate fragmentation is very complex problem • Closest analog is fragmentation of brittle materials (e.g., glass) • Measurements show brittle size distribution is scale-invariant (a power law) • Resulting size distribution: Analog: brittle fragmentation Dust aggregate fragmentation: Brittle material fragmentation NESL_NSF Review 09-12 May 2011

  7. Theory is in agreement with measurements • Derived simple equation: • N = number of aerosols; Dd= aerosol size • Dsoil and σsoil describe soil size distribution • Theory in excellent agreement with available measurements • Resolves discrepancy between models and measurements • Models overestimate fraction of emitted clay aerosols (Dd < 2 μm) by× 2 – 8! Kok, J.F. (2011), PNAS, 108, 1016 Cumulative soil fraction (= correction for discrete particles) Scale invariance Emitted size distribution NESL_NSF Review 09-12 May 2011

  8. Implications for dust climate forcing • GCMs predict global dust radiative forcing of ~ -0.5 W/m2at top of atmosphere (TOA) • “Corrected” GCM estimates with theoretical dust size distribution • Yields lower TOA forcing than GCM estimates for given dust emission rate • Overestimation of clay (Dd < 2 μm) fraction in GCMs causes • Overestimate of dust radiative cooling, AND/OR • Underestimate of global dust emission rate Kok, J.F. (2011), PNAS, 108, 1016. NESL_NSF Review 09-12 May 2011

  9. Conclusions • Main point: dust aerosol emission similar to breaking glass! • Used analogy to develop expression of emitted dust size distribution • Matches measurements • Fixes overestimation of emitted small particle fraction by models • Implemented into several models • GCMs overestimate clay (< 2 mm diameter) fraction • GCMs overestimate dust radiative forcing AND/OR underestimate global dust emission rate Analog: brittle fragmentation Dust aggregate fragmentation: NESL_NSF Review 09-12 May 2011

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