Despite significant progress in our understanding of the global aerosol system over the past two decades, our knowledge of the sources and dynamics of atmospheric aerosols remains limited. Difficulties associated with measurement of the aerosol size/composition distribution, combined with the significant spatial and temporal variability of tropospheric aerosol, have resulted in only scattered knowledge of its global distribution. Aerosols in remote locations and in the middle and upper troposphere have received relatively little attention, and their size/composition distribution remains largely unexplored. Most of the existing measurements are ground based, and, consequently, there is a general lack of information on the vertical aerosol distribution. Moreover, few measurements of the chemical composition of the smallest atmospheric particles, e.g., smaller than 50 nm diameter, are available.

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