NO3 reactivity measurements in an indoor environment: a pilot study†

We present the first direct indoor measurements of VOC-induced nitrate radical (NO₃) reactivity (k^NO₃) together with measurements of nitric oxide (NO), nitrogen dioxide (NO₂), ozone (O₃) and dinitrogen pentoxide (N₂O₅) inside a laboratory during a four-day period in October 2021 in a suburban area (Mainz, Germany). Indoor mixing ratios of O₃ ranged from <2–28 ppbv and those of NO₂ from 4.5–27 ppbv. The rapid ventilation of the room (air change rates of ∼4 h⁻¹) meant that indoor mixing ratios mirrored the variability in NO₂ and O₃ outdoors. NO₃ production rates were between <0.02 and 0.12 pptv s⁻¹ with indoor N₂O₅ mixing ratios increasing to 4–29 pptv during five NO-depleted day- or nighttime periods when k^NO₃ was between 0.04 and 0.2 s⁻¹. Steady-state calculations resulted in a peak NO₃ mixing ratio of 6 pptv. A comparison of measured N₂O₅ mixing ratios to those derived from steady-state calculations and the equilibrium coefficient for the NO₂, NO₃, N₂O₅ system showed very good agreement, indicating that heterogeneous reactions do not contribute significantly to the overall NO₃ loss rate (L_NO3). During these five periods, NO₃ was mostly lost to NO and VOCs, the latter contributing on average 65% to L_NO3. This pilot study underlines the necessity of further indoor NO₃ reactivity measurements and that the nitrate radical can be a significant indoor oxidizing agent when the room is sufficiently ventilated during episodes of moderate outdoor air pollution.