Relative humidity (RH) history and chemical composition determine whether atmospheric particles are liquid or solid. Particle physical state affects many climatic and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we report the crystallization RH (CRH) at 293 K of particles throughout the entire sulfate-nitrate-ammonium composition space, and the new laboratory data are expressed as an empirical polynomial. Particles with compositions close to ammonium sulfate crystallize near 30% RH. Such particles are common in the boundary layer, including the eastern USA and East Asia though not Europe. Our measurements show the formation of several solids within the same particle for compositions enriched in nitrate or somewhat acidic, although the CRH is under 30%. The CRH for homogeneous nucleation is 40 to 55% below the deliquescence RH (DRH) for most chemical compositions. Our new results provide essential data for the incorporation of particle physical state in global models of chemistry and climate.