Aerosols vary widely in size, number concentration, chemical composition and, in spatial and temporal distribution. Typically ranging in diameter from 0.001 µm -100 µm, aerosols are classified as shown Figure . Nucleation mode particles include those freshly made from the clustering and uptake of trace vapors in the atmosphere (new particle formation, NPF). The small, thermodynamically stable clusters formed via NPF contribute to 10-90% of particles in the atmosphere. Aitken mode particles are formed by the accretion of vapors on nucleation mode particles. Accumulation mode particles are composed of those that were either formed by the growth of smaller particles via coagulation and condensation processes while coarse particles are made by mechanical processes such as wind and erosion or are directly emitted (primary particles such as dust, soot, and sea spray) into the atmosphere. Atmospheric particles are chemically complex and require powerful analytical techniques to be applied for their characterization. Field studies of atmospheric aerosols offer information on particle abundances, chemical composition, variation in particle size, and regional conditions that promote particle growth. Laboratory efforts to investigate aerosols can be broadly categorized as techniques that analyze aerosols obtained by direct sampling from the atmosphere or by their production in simulation chambers, sea spray chambers, or other validated sources of atmospherically-relevant particles. These efforts have identified sulfates, nitrates, organics, ammonia, sea salt, metal oxides, and iodine-containing compounds as some of the key chemical species in atmospheric particles, largely by the application of mass spectrometry (MS) methods. Efforts are ongoing to link particle composition and state to their roles in atmospheric processes such as cloud formation. The majority of MS studies have been performed on particles, which is to say that the goal of the studies is to measure the mass of the molecules composing the particle. More rarely is the mass of the particle measured, as for most particles, this information is largely redundant with increasing particle sizes.