Assessing indoor gas phase oxidation capacity through real-time measurements of HONO and NOx in Guangzhou, China

Abstract

The hydroxyl radical (OH) is one of the most important oxidants controlling the oxidation capacity of the indoor atmosphere. One of the main OH sources indoors is the photolysis of nitrous acid (HONO). In this study, real-time measurements of HONO, nitrogen oxides (NO_x) and ozone (O₃) in an indoor environment in Guangzhou, China, were performed under two different conditions: (1) in the absence of any human activity and (2) in the presence of cooking. The maximum NO_x and HONO levels drastically increased from 15 and 4 ppb in the absence of human activity to 135 and 40 ppb during the cooking event, respectively. The photon flux was determined for the sunlit room, which has a closed south-east oriented window. The photon flux was used to estimate the photolysis rate constants of NO₂, J(NO₂), and HONO, J(HONO), which span the range between 8 × 10⁻⁵ and 1.5 × 10⁻⁵ s⁻¹ in the morning from 9:30 to 11:45, and 8.5 × 10⁻⁴ and 1.5 × 10⁻⁴ s⁻¹ at noon, respectively. The OH concentrations calculated by photostationary state (PSS) approach, observed around noon, are very similar, i.e., 2.4 × 10⁶ and 3.1 × 10⁶ cm⁻³ in the absence of human activity and during cooking, respectively. These results suggest that under “high NO_x” conditions (NO_x higher than a few ppb) and with direct sunlight in the room, the NO_x and HONO chemistry would be similar, independent of the geographic location of the indoor environment, which facilitates future modeling studies focused on indoor gas phase oxidation capacity.