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Discussion papers
https://doi.org/10.5194/acp-2018-459
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-459
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Jul 2018

Research article | 19 Jul 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Interpretation of Measured Aerosol Mass Scattering Efficiency Over North America Using a Chemical Transport Model

Robyn N. C. Latimer1 and Randall V. Martin1,2 Robyn N. C. Latimer and Randall V. Martin
  • 1Department of Physics and Atmospheric Science, Dalhousie University, Halifax, B3H 4R2
  • 2Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA

Abstract. Aerosol mass scattering efficiency affects climate forcing calculations, atmospheric visibility, and the interpretation of satellite observations of aerosol optical depth. We evaluated the representation of aerosol mass scattering efficiency (αsp) in the GEOS-Chem chemical transport model over North America using collocated measurements of aerosol scatter and mass from IMPROVE network sites between 2000–2015. We found a positive bias in mass scattering efficiency given current assumptions of aerosol size distributions and particle hygroscopicity in the model. We found that overestimation of mass scattering efficiency was most significant in dry (RH<35%) and midrange humidity (35%<RH<65%) conditions, with biases of 87% and 38%, respectively. To address these biases, we investigated assumptions surrounding the two largest contributors to fine aerosol mass, organic (OA) and secondary inorganic aerosols (SIA). Inhibiting hygroscopic growth of SIA below 35% RH and decreasing the dry geometric mean radius, from 0.069μm for SIA and 0.073μm for OA to 0.058μm for both aerosol types, significantly decreased the bias observed in dry conditions from 87% to 13%. Implementation of a widely used alternative representation of hygroscopic growth following k-Kohler theory for secondary inorganic (hygroscopicity parameter k=0.58) and organic (k=0.10) aerosols eliminated the remaining overall bias in αsp. Simulated average αsp over North America increased by 12%, with larger increases of 20% to 40% in northern regions with high RH and hygroscopic aerosol fractions, and decreases in αsp up to 15% in the southwestern U.S. where RH is low.

Robyn N. C. Latimer and Randall V. Martin
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Robyn N. C. Latimer and Randall V. Martin
Robyn N. C. Latimer and Randall V. Martin
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Short summary
Long-term aerosol measurements from the IMPROVE network were used to investigate the simulation of mass scattering efficiency in the GEOS-Chem chemical transport model. The simulation of mass scattering efficiency was developed to better represent observations by refining the representation of aerosol size and hygroscopicity. Simulated average mass scattering efficiency over North America increased by 12 %, with larger increases in northern regions and reductions in the southwest.
Long-term aerosol measurements from the IMPROVE network were used to investigate the simulation...
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