John H. Seinfeld

Dual Affiliation with Division of Engineering and Applied Science

Assistant: Matt Buga

Increases in the levels of greenhouse gases, such as carbon dioxide, and airborne particulate matter, aerosols, are impacting the Earth's climate. Understanding the chemical and physical processes that govern the dynamics and distribution of gases and aerosols in the atmosphere and their effects on climate and air quality represents one of the grand challenges of science in the 21st Century. That the increase of greenhouse gases attributable to human activities is causing a steady rise in the Earth's global mean temperature is unequivocal.

The ability to forecast future climate based on scenarios of energy consumption and other activities is hampered by uncertainties in two major climate factors. Aerosols, on the whole, partially offset the global warming due to the increase of greenhouse gases, but the complex life cycle of aerosols in the atmosphere is still incompletely understood. This uncertainty translates into an uncertainty of the effect of aerosols on climate. Second, clouds are a large source of uncertainty in climate models, and the most uncertain aspect is the extent to which changes in aerosol levels have influenced cloud amounts and precipitation and will do so in the future.

Our research is broadly aimed at improving our understanding of the physics and chemistry of atmospheric aerosols, at scales ranging from the urban to the global atmosphere. This improved understanding will lead to more accurate representations of these processes in urban, regional, and global atmospheric models. We focus on the fundamental processes of aerosol formation and growth in the atmosphere. Of these, both the most important and the most uncertain are those involving the organic fraction of the atmospheric aerosol, which can be as large as 90% in some regions. Aerosol formation and evolution processes involve detailed gas-phase atmospheric chemistry and gas-particle interactions. The interaction of aerosols with atmospheric water is key to much of their behavior. We also focus on developing and evaluating the representation of aerosol-cloud-precipitation interactions in atmospheric models. Our research is broadly divided into three strongly overlapping areas:

• Laboratory chamber studies of the formation and evolution of atmospheric organic aerosols.
• Airborne field measurement of atmospheric aerosols and clouds.
• Urban, regional, and global modeling of air quality and climate.