Athanasios Nenes

Associate Professor & First Blanchard-Milliken Young Faculty Chair

School of Chemical & Biomolecular Engineering

311 Ferst Drive, Atlanta, GA 30332

Phone: (404) 894-9225

FAX: (404) 894- 5638

Description of Research

The effect of human activities on climate is being recognized as one of the most important issues facing society. Humans influence climate in numerous ways; the effect of some is to cool the planet, and of others, to heat it. The significance of some components (such as the warming effect of carbon dioxide) is well understood and quantified; other components are subject to high uncertainty. Aerosols (airborne particulate matter) belong to the latter. The consensus in the scientific community is that aerosols have an overall cooling effect (comparable to the warming from greenhouse gases), but quantitative estimates of their effect are still highly uncertain. A large amount of this uncertainty originates from their effect on clouds (the aerosol "indirect effect"). Clouds have a strong influence on the Earth's radiative balance, but are poorly represented in current climate models. Since cloud droplets and ice crystals form on preexisting aerosol particles (thus having a strong effect on the resulting cloud properties), it is easy to see why quantitative estimates of the aerosol effect are so uncertain.

The greatest challenge in first-principle estimates of aerosol-cloud interactions is addressing the wide range of length scales involved. The size of a typical global model grid cell is on the order of a hundred kilometers (or a few degrees), and can only resolve the largest of cloud systems. Ideally, one needs to explicitly resolve processes taking place on meter scale. Global models are far from being able to achieve this resolution, and thus heavily rely on parameterizations to account for the sub-grid processes of cloud formation and aerosol-cloud interactions. Our research aims at improving (or developing new) parameterizations of aerosol-cloud processes.

Theory and modeling rely on observations. The quality of predictions can only be as good as the quality of the measurements used for testing models. Measuring the cloud droplet formation potential of aerosols is essential for evaluating models of aerosol-cloud interactions. Such measurements involve the generation of a controlled water vapor supersaturation of atmospheric relevance (0.01 to a few %). Aerosol is exposed to the supersaturation and monitored for its potential to form a droplet. Our research aims to understand and improve current instrumentation, as well as using observations of CCN activity to constrain aerosol-cloud interaction parameterizations. Currently we are focusing on the importance of organic species on cloud droplet formation.

Another area of research in the group is the development of computationally efficient and rigorous models of aerosol thermodynamics for usage in regional and global aerosol models. The thermodynamic model (called ISORROPIA) has been widely used in air quality and global aerosol modeling studies, and is currently being extended to incorporate a larger number of aerosol species and interactions.

Current research directions of the Nenes group include:

- Modeling of aerosol-cloud-climate interactions on a global scale.

- Modeling and parameterization of cloud microphysical processes.

- Thermodynamic modeling of tropospheric aerosols.

- Instrumentation and techniques for characterizing organic-water interactions, hygroscopicitiy and CCN activity of aerosols.

- Laboratory and field studies on CCN activity and aerosol-cloud interactions.

- New particle formation and its impact on CCN concentrations.

- Effect of pollution on marine ecosystem productivity and carbon cycle.

- Impact of marine ecosystem productivity on clouds.