Emerging investigator series: an instrument to measure and speciate the total reactive nitrogen budget indoors: description and field measurements.

Reactive nitrogen species (N _r ), defined here as all N-containing compounds except N ₂ and N ₂ O, have been shown to be important drivers for indoor air quality. Key N _r species include NO _x (NO + NO ₂ ), HONO and NH ₃ , which are known to have detrimental health effects. In addition, other N _r species that are not traditionally measured may be important chemical actors for indoor transformations ( e.g. amines). Cooking and cleaning are significant sources of N _r , whose emission will vary depending on the type of activity and materials used. Here we present a novel instrument that measures the total gas-phase reactive nitrogen (tN _r ) budget and key species NO _x , HONO, and NH ₃ to demonstrate its suitability for indoor air quality applications. The tN _r levels were measured using a custom-built heated platinum (Pt) catalytic furnace to convert all N _r species to NO _x , called the tN _r oven. The measurement approach was validated through a series of control experiments, such that quantitative measurement and speciation of the total N _r budget are demonstrated. The optimum operating conditions of the tN _r oven were found to be 800 °C with a sampling flow rate of 630 cubic centimetres per minute (ccm). Oxidized nitrogen species are known to be quantitatively converted under these conditions. Here, the efficiency of the tN _r oven to convert reduced N _r species to NO _x was found to reach a maximum at 800 °C, with 103 ± 13% conversion for NH ₃ and 79-106% for selected relevant amines. The observed variability in the conversion efficiency of reduced N _r species demonstrates the importance of catalyst temperature characterization for the tN _r oven. The instrument was deployed successfully in a commercial kitchen, a complex indoor environment with periods of rapidly changing levels, and shown to be able to reliably measure the tN _r budget during periods of longer-lived oscillations (>20 min), typical of indoor spaces. The measured NO _x , HONO and basic N _r (NH ₃ and amines) were unable to account for all the measured tN _r , pointing to a substantial missing fraction (on average 18%) in the kitchen. Overall, the tN _r instrument will allow for detailed survey(s) of the key gaseous N _r species across multiple locations and may also identify missing N _r fractions, making this platform capable of stimulating more in-depth analysis in indoor atmospheres.