The Pratt group focuses on studies of interactions between atmospheric trace gases, particles, and clouds using novel mass spectrometry techniques. In particular, we tackle the major uncertainties that exist in our understanding of atmospheric aerosols with respect to interactions between urban pollution and trace gas emissions from forests, as well as feedbacks associated with sea ice loss in the Arctic. We use a field-portable single-particle mass spectrometer to measure the size and chemistry of individual particles in real-time. Trace gas composition is measured using a field-portable on-line chemical ionization mass spectrometer. To study the molecular composition of high molecular weight organics, electrospray ionization coupled with high-resolution mass spectrometry is used to probe ambient samples. These measurements of trace gases, aerosols, and cloud water are complemented by one-dimensional modeling of chemical reaction mechanisms and kinetics to examine and predict chemistry versus altitude and time. These studies will improve our understanding of the feedbacks between natural aerosol sources, anthropogenic emissions, and the Earth system for future climate and air quality predictions.