Understanding the Relationship Between Aerosols and Clouds

EPA Grant Number: FP916321
Title: Understanding the Relationship Between Aerosols and Clouds
Investigators: Rissman, Tracey A.
Institution: California Institute of Technology
EPA Project Officer: Michaud, Jayne
Project Period: January 1, 2004 through December 31, 2006
Project Amount: $111,688
RFA: STAR Graduate Fellowships (2004) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Academic Fellowships , Fellowship - Atmospheric Sciences


Clouds form by water condensing on small particles in the air (aerosols), and human emissions are increasing levels of aerosols in our atmosphere. How these increasing levels of particles will affect Earth's clouds and its hydrologic cycle represents one of the key problems in the science of climate. The effects of atmospheric aerosols on the formation, characteristics, and lifetime of clouds are essential to climatic studies because clouds themselves are an extremely important part of Earth's climate system. The objective of my research is to examine the formation of cloud droplets from atmospheric aerosols. The ability of atmospheric aerosols to activate into cloud droplets depends on the size and chemical composition of the particles, as well as meteorological conditions. Atmospheric aerosol particles that are capable of activating into cloud droplets are referred to as cloud condensation nuclei (CCN). The work that is required to further our understanding of cloud/aerosol processes will involve three aspects of scientific research-theoretical development, field measurements, and laboratory studies-with each aspect feeding back into the others.


Theoretical development in aerosol/CCN relationships consists primarily of modifications to Köhler theory to expand its predictive ability by including different aerosol properties, such as surface tension and solute effects of chemical aerosol species. By comparing field measurements of aerosol chemical composition, aerosol size distribution, and CCN concentration to theoretical predictions, current theories of aerosol activation can be tested and expanded to include new chemical species and phenomena. This type of study, called an aerosol/CCN closure study, can be very complicated, especially when the atmospheric aerosols are influenced by anthropogenic/continental sources. A combination of modifications to Köhler theory and numerical models of aerosol activation will be used to improve the aerosol/CCN closure to give insight into the processes that are most important in aerosol activation. These studies also will be enhanced by measurements of the CCN properties of various organic aerosols, as well as aerosols of mixed organic and inorganic material. The laboratory component of the research will involve creating aerosols of known chemical composition in a laboratory setting and measuring the CCN properties of these aerosols using a three column (three operating supersaturations) CCN instrument that was built last year and used in two field campaigns.

Supplemental Keywords:

fellowship, aerosols, atmospheric aerosols, cloud formation, cloud condensation nuclei, human emissions, particles, Köhler theory, climate,, RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Ecosystem/Assessment/Indicators, Ecosystem Protection, Ecological Effects - Environmental Exposure & Risk, Atmospheric Sciences, Ecology and Ecosystems, Ecological Indicators, ecosystem assessment, meteorology, human impacts, hydrological, human emissions, cloud condensation nuclei, ecological impacts, atmospheric aerosols

Relevant Websites:

2004 STAR Graduate Fellowship Conference Poster (PDF, 1p., 182KB, about PDF)

Progress and Final Reports:

  • 2004
  • 2005
  • Final