Indoor measurements of the sum of the nitrate radical, NO3, and nitrogen pentoxide, N2O5 in Denmark

There is a need for indoor measurements of nitrate radicals (NO3) and nitrogen pentoxide (N2O5) to better understand removal and transformation of volatile organic compounds in indoor environments, and to evaluate the possible health effects from exposure to nitrated reaction products. NO3 and NO2 react to form N2O5 in the presence of a third molecule, and the fast equilibrium necessitates measurements of both NO3 and N2O5 in the evaluation of indoor NO3 chemistry. The sum of these two species, NO3∗, was quantified in an office building in Denmark by measuring an oxidation product of the cyclohexene/NO3 reaction in a flow-tube set-up. NO3∗ concentrations ranged from 1 to 58 ppt, where N2O5 was estimated to account for more than 68%. The concentrations of the precursors, NO2 and O3, and the photolysis of NO3 were parameters, which clearly influenced NO3∗ apparent from the different precursor concentrations, lighting and daylight versus dark samples in this study. Also indoor air pollutants, in particular alkenes such as limonene and α-pinene, can significantly reduce NO3∗. These first indoor measurements of NO3∗, warrant further high time resolution measurements of NO3, N2O5, and organic nitrates indoors.