Laboratory Generation of HO Radicals and Their Effects on Vehicle Emissions in a TiO2 Coated Setting

This study focuses on the methodology of generating and measuring HONO to study its potential formation from self-cleaning surfaces such as TiO₂.   A HONO-generating system was built and optimized by reacting a humid nitrogen stream containing gaseous HCl with solid sodium nitrate and passing the flow through a set of denuders to remove HNO₃. The HONO produced was quantified by measuring its presence as NO_x using a TECO Model 42 chemiluminescence NO-NO₂-NO_x analyzer containing a molybdenum catalyst. NO and NO₂ are commonly classified as NO_x. However, the TECO measures NO_x as NO, NO₂, HONO and HNO₃. The optimum arrangement produced 84.7 ppb of NO_x of which 16.2 ppbv was HONO and 68.5 ppbv was HNO₃ in a 685 mL/min flow diluted to 5 L/min.   In a second set of experiments, a 150-L Teflon chamber was filled with HONO and also HONO and toluene and exposed to blacklight illumination. HONO photolyzed to produce the HO radical that oxidizes toluene. A proton-transfer-reactor mass spectrometer was used to measure the decay of toluene and generation of oxidation products. HONO generation from TiO₂ coated concrete was also measured.