Connecting Chemistry and Climate through Aerosol Particles: Laboratory and Field Studies of Cloud Condensation Nuclei

The influence of aerosol particles on the Earth's climate is a major driver of scientific uncertainty in assessing future conditions. The importance of aerosols in their role as cloud condensation nuclei (CCN) and ice nuclei (IN), known as the Aerosol Indirect Effect, is most poorly understood. The number concentration of CCN available to nucleate droplets can have important influences on cloud albedo, lifetime, and propensity to form precipitation. Natural sources are of particular importance, since the absolute influence of aerosols on cloud properties is highly sensitive to background concentrations of CCN before anthropogenic emissions. Chemical studies of sea spray aerosol (SSA) particles, the second most abundant type of natural aerosol globally, were conducted to better understand the influence of marine organic matter on CCN activity. While direct chemical measurements of aerosol particles with diameter (d) > 500 nm indicated that the production mechanism of SSA controls particle composition, especially with respect to the amount of organic matter transferred across the air-sea interface. CCN activity studies, on the other hand, showed a weak dependence on seawater organic matter concentration. The extent to which organic matter and sea salt were externally mixed for particles with d < 100 nm more strongly affected CCN activity than the overall volume fraction of organic matter in the population. Secondary aerosol formation, which was observed to be associated specifically with phytoplankton senescence in laboratory experiments, could be an additional means of influencing marine clouds. Secondary aerosol were less CCN-active than SSA particles, but growth of secondary particles led to their contribution to CCN concentrations, and could potentially affect the formation of marine stratus clouds. Orographic clouds also form in pristine marine air masses over the Sierra Nevada Mountains. Below the marine clouds, highly CCN-active aerosols were redistributed by a barrier jet during winter storms and could influence rainfall in some regions of California. Overall, these studies show that chemistry is closely linked to climate through cloud droplet nucleation, and that studies of fundamental chemistry stemming from the complex systems described in this work could yield marked advances in scientific understanding of the indirect effect.