During the Atlanta Supersite Experiment (ASSE) in August 1999, a three-channel particle composition monitor (PCM) was used to measure PM2.5 mass and composition as well as gas-phase NH3, HCl, HNO3, HONO, SO2, and acetic, formic, and oxalic acids over integrating periods of 10–24 hours. The period was characterized by stagnant periods associated with high temperatures, relative humidities, and UV radiation, underlying parameters causing photochemical activity with intense ozone, and PM2.5 formation. Analysis of diurnal measurements points to photochemical sources of HNO3, HONO, acetic, and formic acids. NO2 denuder artifact reactions are assessed, and it is found that the secondary heterogeneous O3 reaction potentially overcorrects the nitrite to nitrate oxidation step on the denuder walls, underestimating [HNO3] and correspondingly overestimating [HONO]. The high ambient humidity caused gravimetric mass to be systematically high by about 20 ± 8%, which was interpreted as hydrate artifact and corrected for. Despite this correction, and inclusion of other organic elements (OOE) (OOE = 0.6 * organic carbon (OC)), an average percent fraction unidentified mass of 13 ± 10% relative to the total reported mass concentration still remained. However, if semivolatile OC collected on an XAD-coated quartz backup filter placed downstream of an XAD-denuded quartz front filter are included, an almost complete closure in the mass balance is achieved. The possible use of different OOE factors for OC and semivolatile OC (here 0.6 and 0.4, respectively) is also investigated, taking into account the different physical properties of carbon species collected from quartz front and XAD-coated quartz backup filters. It can be speculated that photochemically well-aged and well-mixed air masses contain organic compounds with more highly oxygenated and less volatile functional groups, whereas under more stagnant conditions, particle-phase organics might be less oxygenated and therefore more volatile. Sulfate and all organic compounds (including particle bound light organic acids, organic carbon, and OOE) are the main identified contributors to overall PM2.5 mass with 32 ± 8 and 39 ± 12%, respectively. Acidity calculations based on the particle-phase SO42−/NO3−/NH4+ system show slightly acidic conditions.