Influence of clouds on aerosol particle number concentrations in the upper troposphere

This work investigates statistically the influence of clouds on aerosol particles in the upper troposphere, from the tropics to midlatitudes. More than 7500 particle concentration values, collected with the Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container, were analyzed for the occurrence of cloud contacts using backward trajectories and satellite images from the International Satellite Cloud Climatology Project. The majority of high-altitude clouds over the Arabian Sea most likely contributed to the formation of new particles. At the contact, most clouds act as sink for Aitken and accumulation mode particles. However, as new particles grow these clouds act as indirect source for Aitken mode particles. A similar behavior was observed for clouds over the Caribbean. In contrast, no clear influence on nucleation mode particles was observed for most cloud contacts over the Middle East, southeastern Europe, and the midlatitude North Atlantic Ocean. However, in all three regions, the clouds act on average as sink for Aitken and accumulation mode particles. An analysis investigating the influence of the cloud contact duration showed that cloud contacts lasting 4 h or longer resulted in significantly higher number concentrations of nucleation and Aitken/accumulation mode particles than did short cloud contacts (1 h). This effect was observed over the Caribbean and southeastern Europe. Over the Arabian Sea, the Middle East, and the midlatitude North Atlantic Ocean, however, the duration of the cloud contact had no effect; it was only important whether a cloud contact had occurred.