Aerosols can indirectly effect the radiative balance of the environment by acting as Cloud Condensation Nuclei (CCN) and modifying the microphysical properties of clouds by forming droplets which can either scatter solar radiation back into space, or allow it to pass through and reach the surface of the earth. The potential to activate as CCN and form cloud droplets in the atmosphere is primarily a function of chemical composition and size, and can vary with the aerosol source (i.e. combustion, sea salt, etc.). However, information in regards to the CCN activity of different primary and secondary aerosol sources is limited and not always well defined due to the complexity of the particles. As a result, the aerosol-cloud-climate relationship is not well understood and is the largest source of uncertainty in radiative forcing estimates.

Unique methods to estimate the CCN potential of aerosols derived from different emission sources will be presented. First, a method of analysis to observe and predict the aerosol morphology in CCN activation data for various aerosol compositions representative of emissions sources was developed to investigate the effect of mixing states on aerosol hygrosocopicity. Next, a method of characterizing rapidly changing particle hygroscopicity generated by vehicles operating over transient drive cycles was developed and utilized to gain information related to the hygroscopicity of emissions derived from the use different biofuels in modern gasoline vehicles. This study was extended to investigate the effect of using biodiesel and renewable diesel fuels in modern diesel vehicles equipped with advanced emissions control technologies on aerosol hygroscopicity and CCN activity. Although modeling atmospheric interactions is highly dependent on source emissions data such as that provided in this work, aerosol emissions from vehicles may be subject to rapid physical and chemical changes during atmospheric aging which can modify the hygroscopic properties. Thus, to assess the effect of atmospheric interactions on chemical and physical CCN transformations a new mobile atmospheric chamber was developed to age and characterize fresh vehicle emissions derived from modern gasoline vehicles. By investigating both fresh and aged emissions, this work presented provides a comprehensive understanding of the aerosol hygroscopicity from the emissions source to its aged state in the atmosphere.