Secondary organics and atmospheric cloud condensation nuclei production

The influence of secondary organics on atmospheric cloud condensation nucle i (CCN) production was investigated using a zero-dimensional box model that simulates the production of secondary organics in the gas phase, the trans portation of these organics from gas to the particulate phase, and the resu lting growth of the particles. Model simulations demonstrated that the grow th of nanometer-size nuclei to a CCN size requires the presence of organics of extremely low volatility. These "nonvolatile" organics need to have sat uration vapors pressures of the order of 0.01-0.1 parts per trillion or low er and, in order to induce sufficient nuclei growth, must have gas phase pr oduction rates of the order of 0.3-1 mu g m(-3) d(-1) under conditions typi cal For continental background areas, As the nuclei grow in size, they star t to uptake volatile organics more efficiently. As a result, organic matter in both the nuclei grown into a CCN size and in the preexisting accumulati on mode particles is expected to be dominated by "low-volatile" organics ra ther than organics that actually are responsible for the nuclei growth. The modeling results suggest that the monoterpene oxidation products identifie d so far in field or laboratory experiments, although important contributor s Co secondary particulate: matter, are unlikely to be the ones that grow n uclei to a CCN size. In Field experiments, positive identification of organ ics producing new CCN would require information on the chemical composition of particles smaller than about 0.1 mu m in diameter, which is the size ra nge where nonvolatile organics are likely to be enriched compared with othe r secondary or primary organics. Since the gas phase production rate of non volatile organics needs not to be very large in order to induce significant nuclei growth, more attention should also be paid to reaction products tha t have minor yields in smog chamber experiments.