Using Chemistry to Improve Understanding of Cloud Microphysics

Abstract

Aerosols are minute particles that are suspended in the air. Despite their small size, aerosols play a disproportionately large role in Earth’s climate by influencing the amount of radiative energy that enters and exits Earth’s atmosphere in the form of shortwave and longwave radiation, respectively. Moreover, aerosols directly and indirectly influence the amount of shortwave radiation (i.e., sunlight) entering Earth’s atmosphere. The direct effect consists of aerosols either scattering or absorbing sunlight, thus cooling or heating the Earth, respectively. The indirect effect consists of a fraction of aerosols (called cloud condensation nuclei, CCN) that seed cloud droplets and thus form clouds; the clouds, in turn, reflect sunlight back into space and cool the Earth. The interactions between aerosols and clouds are complex and are the largest source of uncertainty in quantifying the anthropogenic contribution to climate change. Of particular importance are stratocumulus (Sc) clouds, as they cover more area on Earth than any other type of cloud and hence significantly affect to Earth’s energy budget. There are several factors responsible for the uncertainty in aerosol-cloud interactions. One factor is the difficulty in assessing how aerosols affect cloud microphysical properties, namely, cloud droplet number concentration (Nd) and cloud droplet effective radius (re). Another factor is the convoluted nature of a variety of cloud processes such as entrainment, scavenging, and the aqueous chemistry within droplets. Studying the chemical composition of clouds offers a potential path to examine these two factors, i.e., the relation between aerosols and cloud microphysical properties, and the convoluted cloud processes. The justification of using a chemical approach rests on two principles: (1) When water vapor condenses onto a CCN to form a cloud droplet, the CCN dissolves in the droplet. Thus, the chemical composition of the droplet can reveal information about the CCN, including an approximation of the mass concentration and chemical composition of the CCN. (2) Once a droplet is formed, it can incorporate into it the surrounding gases and aerosols in the air, in addition to serving as a chemical vessel in which aqueous chemical reactions can take place. Thus, the chemical composition of the droplet can also reveal information about the air that has been in contact with the droplet and the chemical reactions that occurred inside it; both pieces of information can be used to deduce the processes the cloud has undergone. This dissertation presents two studies that use the chemical composition of cloud water to analyze aerosol-cloud interactions. The first study examines the vertical profiles of cloud water chemical composition to identify characteristic vertical profiles of composition and how these affect the rate of the removal of species through precipitation. The second study uses cloud water chemical composition to predict Nd and examine which and how many species best predict Nd, in addition to how environmental factors affect this prediction. Both studies use the same airborne data set collected throughout four summer campaigns in the Sc cloud deck off the California coast.