The Farmer group studies how anthropogenic activity perturbs atmospheric chemistry - and the resulting implications on regional air quality, forest ecology and climate change. In order to understand this chemistry, we need sensitive instrumentation that is capable of making fast and selective measurements in field environments. To this end, we use a combination of commercial and home-built instrumentation. Our instrument development focuses on chemical ionization sources for use with time-of-flight mass spectrometry. We use both laboratory and field measurements to study the complex oxidation chemistry in both the gas and aerosol phases, and to investigate the interactions between the atmosphere and ecological processes.