| Atmospheric aerosol particles have adverse effects on human health, climate, and ecosystems, that are poorly understood. These effects are dependent on particle size and composition, and therefore on nucleation and growth. Nucleation is a significant source of atmospheric aerosol particles. It is the process whereby semi-volatile vapor condenses together to form a stable cluster of molecules. These small clusters then grow by condensation.; The nucleation rate can not be directly determined, because these particles are too small to be detected by current technology. Predictions of the nucleation rate from theory deviate substantially from measurements. A new analytical procedure has been developed to determine nucleation and growth rates from measurements of consecutive aerosol size distributions. This provides an opportunity to obtain empirical information about nucleation and growth rates independent of theory.; The particle growth rate is determined from the change in size distribution over time, taking into account the effects of other processes, such as coagulation, deposition, and mixing. Knowing the growth rate as a function of time (and size), the measured particles are tracked backwards in time to their time of formation, where the diameter of the particle is assumed to be 1 nm. The number of nucleated particles is obtained by integrating the particle losses from time of formation to time of measurement. Knowing the number of nucleated particles and the time interval in which they formed gives the nucleation rate.; The procedure is applied to a SO2 induced atmospheric nucleation event, measured at Hastings, Ontario, and to measurements made in Calspan's 590 m3 smog chamber, following SO2 and alpha-pinene oxidation. The empirically determined nucleation rates are higher than predicted by theory. The maximum nucleation rate occurs simultaneously with the maximum in model calculated vapor concentration. The aerosol yield resulting from alpha-pinene oxidation is found to be 6.5%, while the (effective) saturation vapor pressure of the condensable product(s) is estimated to be 1 ppt. The time dependence of the yield is at least partially due to the time needed for condensation and to particles not having reached the minimum detectable size.; This newly developed program is demonstrated to be a versatile tool to investigate aerosol dynamics in general, and particle nucleation and growth in particular. |
