Chemical Composition and Mixing States of Individual Particles in Indoor and Outdoor Atmospheres

Understanding the chemical composition and mixing states of individual particles in indoor/outdoor environments is important for assessing daily human exposure. In this study, the chemical composition and mixing states of micron-sized individual particles in university classrooms, dwellings, and corresponding outdoor atmospheres collected between November 2024 and January 2025 were analyzed using micro-Raman spectroscopy. Inorganics and carbonaceous matter were identified in the individual particles; inorganics included CaCO3, CaMg(CO3)2, Ca(NO3)2, CaSO4, CaSO4•2H2O, Mg(NO3)2, Na2SO4, SiO2, NH4NO3, and (NH4)2SO4, and carbonaceous matter included soot and organics. This study found significant differences in the chemical composition of indoor and outdoor particles. For example, the percentage of particles containing CaSO4 was higher in university classrooms than in corresponding outdoor atmospheres, which may be related to the use of chalk. Particles containing organics in the dwelling accounted for more than 80% of the total, which was significantly higher than those found in the corresponding outdoor atmospheres. This may be due to indoor cooking and cleaning activities. Internally mixed CaSO4/NH4NO3 particles and internally mixed CaSO4•2H2O/NH4NO3/(NH4)2SO4 particles were identified in the indoor atmospheres, indicating the complexity of indoor particle formation. In addition, soot and organics were primarily internally mixed with inorganics in individual particles in both indoor and outdoor atmospheres. This study offers new insights for understanding the formation mechanisms and sources of individual atmospheric particles.