The influence of wall temperature on NO2 removal and HONO levels released by indoor photocatalytic paints

Abstract Photocatalytic paints represent a promising remediation technology that has potential to be applied in mechanically ventilated buildings to improve indoor air quality. The photocatalytic paints are typically used to eliminate the gas-phase pollutants, like nitrogen oxides (NO x ) and volatile organic compounds (VOCs). Here, we demonstrate that indoor photocatalytic paints which contain TiO 2 nanoparticles can substantially reduce the concentrations of nitrogen dioxide (NO 2 ) at higher surface temperature of the indoor walls. We show that the efficiency of nitrogen dioxide (NO 2 ) removal increases linearly with the temperature in the range 290–305 K. The geometric uptake coefficients increase from 5.1 × 10 −6 at 290 K to 1.5 × 10 −5 at 305 K. In the temperature range between 305 and 313 K the removal efficiency of NO 2 remains the same with an average NO 2 uptake coefficient of 1.4 × 10 −5 . On the other hand, during the reactions of NO 2 with all the paints (0, 3.5, 5.25 and 7% of TiO 2 ) a harmful indoor air pollutant, nitrous acid (HONO) is formed, in temperature range between 303 K and 315 K. A maximum HONO value of 6 × 10 10 molecules cm −2  s −1 is released by a photocatalytic paint with 7% of TiO 2 (temperature of the walls is 313 K). A dynamic mass balance model applied to typical indoor environment predicts a steady state mixing ratio between 0 and 4.1 ppb at 296 K and between 2.6 and 10.3 ppb at 305 released upon surface reaction of adsorbed NO 2 with a photocatalytic paint (0, 3.5, 5.25 and 7% of TiO 2 ) and considering the photolysis process as the most important loss of HONO. The temperature of the indoor walls is of crucial importance with respect to NO 2 remediation, but at the same time has a strong impact on the formation of harmful intermediates like HONO, which is also a precursor of the OH radicals upon its photolysis. The photocatalytic paint (7% TiO 2 ) may contribute up to 57% to the total OH production rate in indoor air, via photolysis of HONO that is released by the paint upon the irradiation, at wall temperature of 305 K.