Atmospheric particulate matter (PM) has significant implications for air quality and climate through direct or indirect effects and has detrimental impacts on human health. More importantly, PMs are subjected to multiphase chemical processes that can alter their chemical composition, physical properties (such as morphology), and toxicity. This thesis presents the results of laboratory investigations into photochemical processes of selected PM under ultra-violet (UV) wavelengths with environmental relevance.

Sucrose, a surrogate of viscous organic material, suppressed nitrate crystallization and afforded effective nitrate photolysis. Under irradiation at 30% RH, ammonium nitrate (AN) particles with molar fractions of sucrose (F_SU) of 0.5 and 0.33 showed morphological changes from droplets to the formation of inclusions and then hollow semisolid particles. The hindered diffusion of gaseous products (i.e., NO_x, NO_y) from nitrate photolysis is a likely reason for the morphological changes. Furthermore, the decay of glycine (GC, a model free amino acid (FAA)) triggered by nitrate photolysis was also examined. Under light illumination at 80% RH, AN+GC particles showed almost no spectral changes, while rapid decay of glycine and nitrate were observed in sodium nitrate (SN)+GC particles. The distinct phase transition behaviours and photochemistry between AN+GC and SN+GC deliquescent particles were attributable to the different molecular interactions between glycine and nitrate salts.

Photosensitized sulfate formation was found upon UVA (peak at 405 nm) illumination on fresh incense burning particles in a Go: PAM flow reactor in the presence of SO₂. Sulfate formation, as represented by the number fraction of sulfate-containing particles (FS), under dark was likely due to gaseous oxidants. FS increased with UV, mainly attributable to photosensitization reactions. Nitrate formation was observed in incense-burning particles under O₃ exposure and dark condition, mainly attributable to the ozonolysis of nitrogen-containing organic compounds. With UVC (254 nm) on, nitrate formation was significantly enhanced, likely due to HNO₃/HNO₂/NOx uptake triggered by OH chemistry, which is more effective than ozone oxidation. The extent of nitrate formation is insensitive to O₃ and OH exposure, which can be explained by the diffusional limitation on interfacial uptake.

Lastly, 222 nm Far-UVC illumination was used to investigate the photochemistry of bioaerosol particles. More rapid inactivation and increase in the total endotoxin activity ratio of the UV-irradiated cells to fresh cells (EAR) were found in Gram-Negative bacteria (GNB) bioaerosols illuminated by 222 nm than 254 nm, attributable to the different inactivation mechanisms. 222 nm illumination could excite the cell membrane components (e.g., proteins) more effectively than 254 nm, leading to more reactive oxygen species (ROS) formation and membrane damage, followed by exposing the lipid A to release the free-endotoxin into the aqueous environment of aerosols.