Alpha-pinene oxidation in the presence of seed aerosol: estimates of nucleation rates, growth rates, and yield.

A recently developed inverse-modeling procedure has been applied to a case study of particle nucleation and growth following alpha-pinene and SO2 oxidation in a smog chamber. With the use of only the measured aerosol size distributions as input, the condensational growth rate is obtained by regression analysis of the general dynamic equation, taking into account coagulation and wall losses. The growth rate provides an indirect measure of the concentration of the condensing species, offset by their vapor pressures. Assuming a particle density of 1.0 g cm(-3), an aerosol yield of 7 +/- 1% is obtained for an initial alpha-pinene concentration of 14 ppbv and a final organic aerosol mass of 4 microg m3. Using the estimated vapor concentration, we show that the time-dependence of the yield is at least partly due to the time needed for condensation. Such a kinetic limitation to secondary organic aerosol formation may have implications for our understanding of gas-particle partitioning. The measured size distributions are also used to determine the empirical nucleation rate; it appears to be enhanced by the presence of organics.