Secondary organics and atmospheric cloud condensation nuclei production

Abstract

The influence of secondary organics on atmospheric cloud condensation nuclei (CCN) production was investigated using a zero‐dimensional box model that simulates the production of secondary organics in the gas phase, the transportation of these organics from gas to the particulate phase, and the resulting growth of the particles. Model simulations demonstrated that the growth of nanometer‐size nuclei to a CCN size requires the presence of organics of extremely low volatility. These “nonvolatile” organics need to have saturation vapors pressures of the order of 0.01–0.1 parts per trillion or lower and, in order to induce sufficient nuclei growth, must have gas phase production rates of the order of 0.3–1 μg m−3 d−1 under conditions typical for continental background areas. As the nuclei grow in size, they start to uptake volatile organics more efficiently. As a result, organic matter in both the nuclei grown into a CCN size and in the preexisting accumulation mode particles is expected to be dominated by “low‐volatile” organics rather than organics that actually are responsible for the nuclei growth. The modeling results suggest that the monoterpene oxidation products identified so far in field or laboratory experiments, although important contributors to secondary particulate matter, are unlikely to be the ones that grow nuclei to a CCN size. In field experiments, positive identification of organics producing new CCN would require information on the chemical composition of particles smaller than about 0.1 μm in diameter, which is the size range where nonvolatile organics are likely to be enriched compared with other secondary or primary organics. Since the gas phase production rate of nonvolatile organics needs not to be very large in order to induce significant nuclei growth, more attention should also be paid to reaction products that have minor yields in smog chamber experiments.