The effect of nitrogen dioxide on particle formation during ozonolysis of two abundant monoterpenes indoors

The effect of the nitrogen dioxide (NO2) concentration on particle formation during ozonolysis of two abundant monoterpenes indoors, α-pinene and d-limonene, was studied in dry air in 1000 l Tedlar bags at 21±2 °C and ambient pressure. Particle size distributions were measured during 1 h after the reaction was initiated. In mixtures of 50 parts per billion volume (ppbv) of monoterpene and 50 ppbv of ozone (O3), d-limonene produced about five times as many particles (10–350 nm) as α-pinene after 60 min. The presence of NO2 introduced an additional loss term for O3, resulting in formation of the nitrate radical. This affected particle formation, since the nucleation potential of NO3 is much lower than O3. Modeling showed that the observed decrease in particle concentration from d-limonene/O3/NO2 mixtures was likely to be ascribed to the O3/NO2 reaction at NO2 concentrations <150 ppb, above which unknown mechanisms additionally reduced the particle formation. In similar experiments with α-pinene, the particle concentration and volume were substantially reduced in the presence of NO2, e.g. 162 ppbv NO2 reduced the particle number concentration by a factor of 10. In addition, the detection of particle formation was delayed as the NO2 concentration increased, but the additional loss of O3 in the O3/NO2 reaction could not explain the observation. The particle mode progressively increased with the NO2 concentration for both monoterpenes. Oxidation of d-limonene may be highly relevant for new particle formation in indoor air, whereas ozonolysis products of α-pinene seem less likely to nucleate in indoor environments.