Aerosol composition of the tropical upper troposphere 1NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO, USA
2Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO, USA
3Department of Mechanical and Materials Engineering, University of Denver, Denver, CO, USA
4NASA Ames Research Center, Moffett Field, CA, USA
5Goddard Earth Sciences and Technology Center, University of Maryland Baltimore County, Baltimore, MD, USA
Received: 25 March 2009 – Published in Atmos. Chem. Phys. Discuss.: 09 April 2009 Abstract. Aerosol composition was measured by the NOAA single-particle mass
spectrometer (PALMS) aboard the NASA WB-57 high altitude aircraft platform
during two Aura Validation Experiment (AVE) campaigns based in Costa Rica in
2004 and 2006. These studies yielded the most complete set of aerosol
composition measurements to date throughout the tropical tropopause layer
(TTL) and tropical lower stratosphere. We describe the aerosol properties of
the tropical atmosphere and use composition tracers to examine particle
sources, the role of recent convection, and cirrus-forming potential in the
TTL. Tropical dynamics and regional air sources played principal roles in
dictating tropospheric aerosol properties. There was a sharp change in
aerosol chemical composition at about 12 km altitude coincident with a
change in convective influence. Below this level, maritime convection lofted
condensable material that generated acidic, sulfate-rich aerosol. These
particles contained significant amounts of methanesulfonic acid (MSA) and
showed evidence of cloud processes. In contrast, continental convection
injected particles and precursors directly into the TTL, yielding a
population of neutralized, organic-rich aerosol. The organics were often
highly oxidized and particles with oxidized organics also contained nitrate.
Above the tropopause, chemical composition gradually changed toward sulfuric
acid particles but neutralized particles were still abundant 2 km above the
tropopause. Deep continental convection, though sporadic and geographically
localized, may strongly influence TTL aerosol properties on a global scale.
The abundance of organic-rich aerosol may inhibit ice nucleation and
formation of tropopause level cirrus.
Revised: 25 June 2009 – Accepted: 26 June 2009 – Published: 07 July 2009
Citation: Froyd, K. D., Murphy, D. M., Sanford, T. J., Thomson, D. S., Wilson, J. C., Pfister, L., and Lait, L.: Aerosol composition of the tropical upper troposphere, Atmos. Chem. Phys., 9, 4363-4385, doi:10.5194/acp-9-4363-2009, 2009.