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https://doi.org/10.5194/acp-2020-1
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-1
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 04 Mar 2020

Submitted as: research article | 04 Mar 2020

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A revised version of this preprint is currently under review for the journal ACP.

Apparent dust size discrepancy in aerosol reanalysis in north African dust after long-range transport

Samantha J. Kramer1, Claudia Alvarez1, Anne Barkley1, Peter R. Colarco2, Lillian Custals1, Rodrigo Delgadillo1, Cassandra Gaston1, Ravi Govindaraju2, and Paquita Zuidema1 Samantha J. Kramer et al.
  • 1University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, FL, USA
  • 2NASA Goddard Space Flight Center, Greenbelt, MD, USA

Abstract. North African dust reaches the southeast United States every summer. Measurements taken in Miami, Florida indicate that more than one-half of the surface dust mass concentrations reside in particles with diameters less than 2.1 μm, while vertical profiles of micropulse lidar depolarization ratios show dust reaching above four km during pronounced events. These observations are compared to the representation of dust in the MERRA-2 aerosol reanalysis and closely-related GEOS-5 Forward Processing (FP) aerosol product, both of which assimilate satellite-derived aerosol optical depths using a similar protocol and inputs. These capture the day-to-day variability in aerosol optical depth well, in a comparison to an independent sun-photometer-derived aerosol optical depth dataset. Measured near-surface dust mass concentrations slightly exceed model values, with most of the modeled dust mass in diameters between 2–6 μm. Modeled-specified mass extinction efficiencies equate light extinction with approximately three times as much aerosol mass, in this size range, compared to the measured dust sizes. GEOS-5 FP surface-layer sea salt mass concentrations greatly exceed observed values, despite realistic winds and relative humidities. In combination, these observations help explain, why, despite realistic total aerosol optical depths, 1) free-tropospheric model volume extinction coefficients are lower than those retrieved from the micro-pulse lidar, suggesting too low model dust loadings, and 2) model dust mass concentrations near the surface are higher than those measured. The modeled vertical distribution of dust, when captured, is reasonable. Large, aspherical particles exceeding the modeled dust sizes are also occasionally present, but dust particles with diameters exceeding ten μm contribute little to the measured total dust mass concentrations after such long-range transport. A further integrated assessment is needed to confirm this study's interpretations.

Samantha J. Kramer et al.

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Samantha J. Kramer et al.

Samantha J. Kramer et al.

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Latest update: 05 Jul 2020
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Short summary
Comparisons of sea salt and size-resolved dust mass concentration measurements over southeast Florida to those from the MERRA2/GEOS-5 FP aerosol reanalysis indicate the reanalysis depicts excessive sea salt and puts too much dust in larger intermediate sizes, than do the measurements. The dust vertical profile is approximately correct. The incorrect reanalysis aerosol speciation and dust sizes has implications for the modeling of their transport, deposition, and radiative impact.
Comparisons of sea salt and size-resolved dust mass concentration measurements over southeast...
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