title: Composition and physical properties of the Asian Tropopause Aerosol Layer and the North American Tropospheric Aerosol Layer.
abstract: Recent studies revealed layers of enhanced aerosol scattering in the upper troposphere and lower stratosphere over Asia (Asian Tropopause Aerosol Layer (ATAL)) and North America (North American Tropospheric Aerosol Layer (NATAL)). We use a sectional aerosol model (Community Aerosol and Radiation Model for Atmospheres (CARMA)) coupled with the Community Earth System Model version 1 (CESM1) to explore the composition and optical properties of these aerosol layers. The observed aerosol extinction enhancement is reproduced by CESM1/CARMA. Both model and observations indicate a strong gradient of the sulfur-to-carbon ratio from Europe to the Asia on constant pressure surfaces. We found that the ATAL is mostly composed of sulfates, surface-emitted organics, and secondary organics; the NATAL is mostly composed of sulfates and secondary organics. The model also suggests that emission increases in Asia between 2000 and 2010 led to an increase of aerosol optical depth of the ATAL by 0.002 on average which is consistent with observations.
  
  
  CONCLUSIONS
  The Asian Tropopause Aerosol Layer is composed of sulfate, primary organics, and secondary organics The North American Tropospheric Aerosol Layer is mostly composed of sulfate and secondary organics Aerosol Optical Depth of Asian Tropopause Aerosol Layer increases by 0.002 from 2000 to 2010.
authors.affiliation: Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder Boulder, Colorado, USA ; Laboratory for Atmospheric and Space Physics, University of Colorado Boulder Boulder, Colorado, USA.
authors.forename: Pengfei
authors.initials: P
authors.lastname: Yu
authors.affiliation: Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder Boulder, Colorado, USA ; Laboratory for Atmospheric and Space Physics, University of Colorado Boulder Boulder, Colorado, USA.
authors.forename: Owen B
authors.initials: OB
authors.lastname: Toon
authors.affiliation: National Centre for Atmospheric Science and the Institute of Climate and Atmospheric Science, University of Leeds Leeds, UK ; Atmospheric Chemistry Division, National Center for Atmospheric Research Boulder, Colorado, USA ; CIRES, University of Colorado Boulder Boulder, Colorado, USA.
authors.forename: Ryan R
authors.initials: RR
authors.lastname: Neely
authors.affiliation: Division of Nuclear Physics, Lund University Lund, Sweden.
authors.forename: Bengt G
authors.initials: BG
authors.lastname: Martinsson
authors.affiliation: Air Chemistry Division, Max Planck Institute for Chemistry (Otto Hahn Institute) Mainz, Germany.
authors.forename: Carl A M
authors.initials: CA
authors.lastname: Brenninkmeijer
keywords: ATAL; NATAL