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(3)(NO)) together with measurements of nitric oxide (NO), nitrogen dioxide (NO2), ozone (O-3) 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 O-3 ranged from <2-28 ppbv and those of NO2 from 4.5-27 ppbv. The rapid ventilation of the room (air change rates of similar to 4 h(-1)) meant that indoor mixing ratios mirrored the variability in NO2 and O-3 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(3)(NO) 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.