Aerosols can have a significant influence on air quality, visibility, health, and regional and global climate. As is evident from the Intergovernmental Panel on Climate Change report [2001], the effect of aerosols on climate are the most uncertain, could be positive or negative depending on the chemical composition, can have secondary effects that are poorly understood, and could be as large as that of the greenhouse gases. However, aerosol properties are difficult to measure because there is so much variability in size, composition, and other properties. The more local effects, such as air quality, visibility, and health, are also difficult to determine because aerosols are strongly influenced by local conditions as well as long-range transport. Currently, there are no instruments capable of measuring all aerosol properties, either in-situ or remotely. Most measurements are restricted to a certain size range, to only one property, or multiple properties (e.g., an impactor with size and chemical composition) with low time resolution. In 1999, the Aerosol Mass Spectrometer (AMS) developed by Aerodyne Research, Inc., was deployed for the first time in the Atlanta Supersites Project. The AMS provides real time in-situ quantitative chemical composition and size data of submicron non-refractory aerosols. The compositional information is obtained with standard quadrupole mass spectrometry and the size data by particle time of flight across a vacuum chamber. Particles are first vaporized, typically at a temperature of approximately 600 °C, and then ionized by electron impact before being analyzed by the quadrupole mass spectrometer. It is this separation of vaporization and ionization that allows the AMS to produce quantitative data. During the last four years, the AMS has progressed from a pilot instrument with several unresolved issues to a self-sufficient, tested, instrument that provides more complete information about fine aerosols than most other instruments. This thesis presents the operation and development of the Aerodyne Research Aerosol Mass Spectrometer (AMS) and results from several field campaigns. The main focus will be two large field campaigns at which the AMS was deployed, PROPHET 2001 at the University of Michigan Biological Station in northern Michigan and CELTIC 2003 at Duke Forest in North Carolina, are both located in forested areas. The PROPHET site is truly a remote site with no large population centers nearby while CELTIC is located just outside Raleigh, North Carolina, and has a much larger urban influence. Aerosol properties at the PROPHET site are clearly dependent on wind direction, with large concentrations of aged, processed aerosols dominated by sulfate arriving from the south and low concentrations dominated by organic species arriving from the less populated region to the north. Aerosols at the CELTIC site are more of a mixture due to the variety of nearby sources, although large changes in air masses are also clearly apparent. These measurements characterize the aerosols at a particular location, indicate the level of processing the aerosols have undergone and provide some information about sources.