The crystals formed at 293 K by aerosol particles composed Of SO42-, NO3-, NH4+, and H+ are determined by aerosol flow tube infrared spectroscopy. An innovative experimental protocol is employed to restore water content to the aerosol particles and thus remove the ambiguity of their physical state after exposure to low relative humidity. The six crystals formed include (NH4)(2)SO4, (NH4)(3)H(SO4)(2), NH4HSO4, NH4NO3, 2NH4NO3.(NH4)(2)SO4, and 3NH(4)NO(3).(NH4)(2)SO4. The dependence of which crystals form on aqueous chemical composition is reported. The infrared signatures of these crystals are determined. The infrared spectra of 2NH4NO3.(NH4)(2)SO4, and 3NH(4)NO(3).(NH4)(2)SO4 and their formation in aerosol particles are reported for the first time. The formation of NH4HSO4 and NH4NO3 in initially homogeneous aerosol particles is also reported for the first time: crystallization occurs only after another crystal has already formed, indicating that heterogeneous nucleation is necessary for their formation. For some chemical compositions, in a fraction of the aerosol particles, metastable crystals that form at low relative humidity reconstruct to thermodynamically stable crystals at higher relative humidity. An externally mixed aerosol results. Contact ion pairs are apparent in the infrared spectra of aerosol particles that do not crystallize even at 1% relative humidity. Taken together, our findings suggest a more diverse array and more frequent occurrence of crystalline SO42--NO3--NH4+H+ aerosol particles in the troposphere than currently considered in the literature.