Aerosols influence climate by altering the global energy balance via scattering and absorbing solar radiation (direct effects), and by their effect on the reflectance of clouds and occurrence of precipitation (indirect effects). As noted in the IPCC Fourth Assessment Report (2007), aerosol effects exhibit the greatest uncertainty in our understanding of all global radiative forcing components. Aerosols also influence biogeochemical cycles, lead to diminished visibility, and harm human health. There is an urgent need to understand the character of atmospheric aerosols, including their sources, transformations, and fates. Observational methods such as ground-based and airborne sampling are central to this effort.  In-situ observations are used to evaluate atmospheric models; since discrepancies invariably exist between predictions and measurements, models are constantly being improved as aerosol instrumentation becomes more advanced.

This talk will address the development and characterization of two instruments that measure aerosol composition and hygroscopicity. The first instrument is the particle-into-liquid sampler (PILS), which quantifies water-soluble aerosol composition. The second instrument is the differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP), which measures size-resolved aerosol hygroscopic growth and refractive index. Special attention is given to measurements of hygroscopic growth for various inorganic and organic acid salts, and how they compare to thermodynamic predictions.

Airborne measurements from four separate field campaigns are discussed.  A few topics of investigation include the following: 1) secondary organic aerosol formation in clouds; 2) an aerosol composition – hygroscopicity closure study using measurements downwind of a large container ship operating on heavy fuel oil; and 3) characterization of aerosol downwind of a massive bovine source.