Balloon-borne in situ measurements of aerosol size and concentration in the mid latitudes and tropics

National Science Foundation: $797,160, January 2005 - January 2010, PI: T. Deshler, Location: Laramie, Wyoming, Natal, Brazil.

    This proposal seeks to continue and extend vertical profile, surface to ~33 km, aerosol size and concentration measurements at Laramie, Wyoming, (41°N, 106°W) during the longest volcanically quiescent period in the modern stratospheric record. Analysis of these unique continuing measurements, utilizing well-developed and new technology to address gaps in our knowledge of stratospheric aerosol, forms the core of the intellectual merit of this proposal. The continuing measurements will consist of condensation nuclei (CN, r > 0.01 m) and aerosol with radius  0.15-2.0 µm, using University of Wyoming instruments. Extended measurements will add new size-resolved concentration measurements for aerosol between 0.03 and 0.15 m, and tropical measurements from Natal, Brazil (6S, 35W). The 0.03-0.15 m measurements will use an instrument in development with funding from an NSF Major Research Infrastructure grant. The balloon-borne measurements will be conducted at two month intervals from Laramie and annually from Natal over five years. The Natal measurements will be conducted in collaboration with Volker Kirchoff, Instituto Nacional de Pequisas Espaciais. A collaboration with Neil Harris and John Pyle, University of Cambridge, will permit exploratory measurements of OCS and SO2 in conjunction with some aerosol profiles. A collaboration with Niels Larsen, Danish Meteorological Institute, will add backscattering measurements and modeling expertise to the tropical work.

    Continuing stratospheric aerosol measurements at Laramie is necessary to help establish, unequivocally, stratospheric aerosol levels during volcanically-quiescent, "background", periods. The sulfur sources necessary to maintain background aerosol levels, and the extent of perturbations to stratospheric aerosol, are still uncertain. The Laramie record is the longest sustained record of stratospheric aerosol existent, and thus favorably positioned to address these questions. New measurements of smaller particles will address a persistent discrepancy between satellite and in situ estimates of aerosol surface area during background periods, when small particles control surface area. In the event of a large tropical volcanic eruption, the measurement focus will shift to the fresh volcanic plume. Thus these measurements can address two stratospheric states, still suffering from a dearth of measurements, background and fresh volcanic. In addition, new size distribution information, 0.03-0.15 m will be available for the troposphere.

    The annual tropical measurements are exploratory to address several questions. How well do mid and high latitude size distributions characterize aerosol in the tropics? The most fundamental measurement required to characterize the impact of particles on the atmosphere is size distribution, yet few instruments provide this directly. Converting more common, and widely distributed, extinction or backscatter measurements to aerosol surface area/volume requires assumptions about size distribution, yet these assumptions can be tested only for mid and high latitudes where size distributions are available. Do similar distributions apply in the tropics? The tropical measurements will also test earlier tropical stratospheric measurements of new particle formation and large particle layers, and whether models properly account for the stratospheric aerosol source gases OCS/SO2.

    Stratospheric aerosol have broad impacts on the atmosphere and thus on society. During periods of low aerosol loading, such as the present, aerosol affect global ozone through controls on the abundance of NOx. Modeling this requires accurate estimates of aerosol surface area. Major volcanic eruptions produce direct impacts on ozone through heterogeneous chemistry, involving chlorine, and on radiation leading to stratospheric warming and tropospheric cooling. Tropospheric aerosol have even more far reaching impacts on radiation both directly and through their impact on clouds. Thus maintaining and improving our understanding of stratospheric and tropospheric aerosol is required for global atmospheric modeling. Measurements proposed here will: 1) continue long term stratospheric aerosol measurements, establishing a baseline to test our understanding of the source gases required to maintain background stratospheric aerosol, 2) refine current estimates of stratospheric aerosol surface area, 3) add critical new size distribution information from a tropical site, 4) add new size distribution information for tropospheric aerosol in both the mid latitudes and tropics, and 5) possibly new measurements in a fresh volcanic plume. The project will add to our technical work force through the involvement of scientists, engineers, technicians, and students in completing and analyzing these measurements.