Growth of nucleation mode particles: Source rates of condensable vapour in a smog chamber

We investigated the formation and subsequent growth of organic aerosol particles in laboratory conditions by irradiating a mixture of xylene and nitrogen oxides (NOx) with UV light in a 6 m3 Teflon chamber. We used different initial hydrocarbon (HC) and NOx concentrations, with mixing ratios of 3.5–30 (28–240 ppmC/ppm), and monitored the changes in particle size distribution in the range of 7–330 nm. We applied the concept of condensation sink to the measured size distribution data in order to estimate the concentration and source rate of condensable vapours in the chamber. We observed the nucleation and Aitken mode growth rate to be 10.6–18.6 nm h−1, which corresponds to a vapour concentration of 1.5–2.4×108 cm−3, from which our deduced estimation for the source rate was 0.2–2.5×107 cm−3 s−1. These vapour source rates are up to four orders of magnitude higher than the atmospheric values observed in Antarctica and other background stations, up to two orders of magnitude higher than at urban areas of Athens and Marseille, up to 13 times higher than at a coastal site and of the same magnitude, within a factor of five, as in New Delhi, a heavily polluted urban area. In the chamber experiments we observed a strong dependence of the source rate of condensable vapours on the initial NOx concentration. This indicates that oxidation processes play an important role in particle formation and subsequent growth.