Abstract: Atmospheric aerosol contributes the largest uncertainty to predictions of future climate and has implications for human and ecosystem health. My research uses in-situ measurements of aerosol to investigate the behavior of different kinds of aerosol in the global atmosphere including their impacts on radiation, clouds and precipitation as well as interactions between aerosol and biological systems. In this talk I will discuss two recent avenues of my work; the first involves black carbon, which represents an important positive radiative forcing; the second involves primary biological aerosols, consisting of airborne bacteria, spores and pollen, which are an area of growing interest in atmospheric science due to their potential importance to precipitation processes. In each case I will briefly describe the methods used and the characterization work required to ensure robust measurements. I will then present case studies highlighting the utility of in-situ measurements in developing a mechanistic understanding of aerosol sources, sinks and processing and in improving model representations of atmospheric aerosol. Specifically I will discuss observations of black carbon hygroscopicity in wildfire plumes, measured with a newly developed instrument to detect water uptake by black carbon aerosol as well as recent bioaerosol observations in a variety of locations and their relationship to modeled loadings.