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:
Aerosols are introduced into the atmosphere by direct emission (e.g. soot, biomass burning smoke, wind-blown dust, and sea spray) and new particle formation from anthropogenic and biogenic gas-phase compounds (e.g. sulfur dioxide from combustion of sulfur-containing fuels and naturally-emitted volatile organic compounds from forested regions). Once in the atmosphere, particles grow by condensation of ambient gases and coagulation. As a result, the chemical character of the particles change, including their hygroscopicity and ability to serve as cloud condensation nuclei (CCN). Although some particles are removed from the atmosphere by deposition at the Earth's surface, most of those in the size range that affect radiation and air quality eventually undergo interaction with clouds and are ultimately removed by precipitation. All of these processes constitute the aerosol life cycle. Aerosol aging refers to the change in the composition, size, and mixing state (the extent to which an individual particle contains a complete mixture of all different chemical components of the aerosol population) of the aerosol. The mixing state of the aerosol and how the aerosol ages in the atmosphere strongly impact the radiative properties of the aerosol and its ability to serve as CCN.
The organic portion of the atmospheric aerosol that results from the condensation of oxidation products of volatile organic compounds is referred to Secondary Organic Aerosol (SOA). SOA is the dominant fraction of the organic aerosol. The chemistry of SOA formation is complex, and laboratory chamber studies, first initiated at Caltech, serve as the basis for developing a basic understanding of SOA formation. The Caltech laboratory chambers have been the source of much of the world's data on the formation of SOA from both anthropogenic and biogenic volatile organic compounds. Our research involves carefully designed laboratory experiments to characterize particle formation and to define the organic gases that react to form new organic aerosol. The dependence of SOA formation on the level of nitrogen oxides, type and amount of seed aerosol, relative humidity, and temperature is key. Data from our chambers are used to guide the development of comprehensive chemical mechanisms for SOA formation. The data are also parameterized for representation of SOA formation in atmospheric models, providing a strong connection with those members of the research group involved principally in atmospheric modeling.
2 Teflon chambers, 28 m3 each
Scanning differential mobility analyzer (DMA): particle size distribution and volume
Aerodyne High Resolution Aerosol Mass Spectrometer (HR-AMS): particle mass, composition
Particle into Liquid Sampler (PILS): aerosol phase concentrations of inorganic ions
GC - FID
Gas Chromatography Time-of-Flight Mass Spectrometer (GC-ToF MS)
DASH: provides rapid, size-resolved hygroscopicity measurements at multiple relative humidities. Its optical sizing technique quantifies aerosol growth factor on timescales of several seconds for particles between 135 nm and 1 micron.
Chemical Ionization Mass Spectrometer (CIMS): gas-phase composition
Filter samples: off-line chemical analysis
O3, NOx, RH, T
In the climate system, aerosols, clouds, precipitation, and radiation are a tightly coupled interacting system governed by thermodynamics and atmospheric dynamics. As a consequence, it is of great value to study these processes as a continuum. The ultimate goal of such study is to increase the accuracy of treatment of these processes in global climate models. About 20% of incoming solar radiation is reflected back to space by clouds. Thus, uncertainties in the representation of cloud processes in global climate models have the potential to introduce large uncertainties in computed climate properties. Clouds form when rising air cools and becomes supersaturated with water vapor. The water vapor condenses on aerosols in the air, forming cloud droplets. If the Earth's atmosphere were totally devoid of particles, there would be no clouds on Earth, and the planet would be vastly different. Atmospheric dynamics is key in cloud formation and evolution through vertical air motions and turbulent mixing of ambient air with cloudy air. The cloud droplet size distribution strongly determines a cloud's evolution and interaction with radiation. Aerosols and clouds are inextricably coupled. The climatic effect of changes in aerosol amounts or properties on cloud formation, persistence, and precipitation and ultimately radiative properties is referred to as the Aerosol Indirect Effect.
Because of the complexity of aerosol-cloud interactions, observations of clouds provide essential information to evaluate models of cloud formation and evolution in atmospheric models. For example, aerosol-induced changes in the number and size distribution of cloud droplets affect the development of precipitation, the amount of precipitation that actually reaches the ground, and the persistence of the cloud itself. As aerosol concentrations increase, cloud droplets become both more numerous and smaller, suppressing the formation of precipitation. One effect of this is that clouds may persist longer and increase in spatial coverage. However, in shallow boundary layer clouds, even though precipitation is suppressed, droplet evaporation can lead to increased entrainment and cloud thinning. Aerosol effects on deeper cumulus clouds can involve a tradeoff between warm and cold (ice) droplet formation.
Aerosols can be measured both in situ and remotely; in situ measurements can be made at the Earth's surface or by aircraft, while remote measurements can be made from the Earth's surface and from satellite. In situ aerosol measurements can define chemical, microphysical, and optical properties, which allow local closure of measurement and theory for hygroscopic, CCN, and radiative properties. In situ cloud measurements can reveal cloud droplet size distribution, drizzle drop size and amount, and vertical air motions. Such in situ measurements allow comparisons with detailed cloud models to assess the extent to which theory and observation agree.
Our group has utilized the CIRPAS Twin Otter aircraft. CIRPAS is a research arm of the Naval Postgraduate School in Monterey, CA (http://www.cirpas.org/index.html), located in a large, fully-equipped hangar at the Marina Municipal Airport, a few miles north of Monterey. John Seinfeld has been the Principal Investigator on a contract from the Navy to operate the Twin Otter for atmospheric science. The Caltech group, led by Professors Seinfeld and Flagan, has participated in a number of large field experiments using the Twin Otter. A typical instrumentation payload for an aerosol-cloud experiment is shown below. The Caltech group carries out, on average, one mission per year aimed at aerosol-cloud interactions or aerosol characterization.
Modeling of atmospheric phenomena has always been a foundation of the Seinfeld group. Earliest studies were directed to development of three-dimensional models of Los Angeles air pollution. Atmospheric modeling efforts now address the global effect of aerosols on climate, the coupling between atmospheric chemistry, aerosols, and climate, and the microphysics of aerosol-cloud interactions. A major goal of much of the work worldwide in global aerosol modeling is to narrow the uncertainties in predicting the effects of aerosols on climate. Physical and chemical representations of the aerosol-related processes are developed on the basis of detailed microscale models, or laboratory data, and implemented in large-scale models. The enormous difference in scale between a global model (100's of km in horizontal resolution) and that needed to resolve all the relevant physical processes (e.g. less than 1 km for cloud processes) leads to significant challenges. When important processes occur at scales too fine to be explicitly resolved in a large-scale model, these must be approximated or parameterized. Single cloud models include large eddy simulation (~100 m horizontal resolution) and cloud-resolving models (1-10 km horizontal resolution). Such models reveal basic understanding of cloud physics, but are too computationally intensive to be imbedded in a global model. A means to effectively transfer results from these models to large-scale models has not yet been surmounted. Models are continually being updated to reflect the latest understanding on gas-phase chemistry, aerosol microphysics, and cloud processes. Research on aerosol-cloud interaction modeling in our group is also tied with observations made with the Twin Otter aircraft during field missions.
Detailed chemical modeling of SOA formation and evolution is also being carried out, in conjunction with the experimental studies of SOA formation in the Caltech chambers. SOA formation results from the gas-particle partitioning of low volatility oxidation products of atmospheric hydrocarbons. The extent of partitioning depends critically on the volatility of these oxidation products and on the amount of pre-existing aerosol into which these species may partition. Since equilibrium gas-particle partitioning is governed by the thermodynamics of the condensed phase, we have long been interested in methods to predict the thermodynamic properties of inorganic-organic-aqueous mixtures characteristic of atmospheric aerosols. Key questions include: (1) What role does aerosol-phase water play in the partitioning of organic compounds? (2) Can multiple condensed phases exist in a single particle, and, if so, what effect might phase separation have on overall gas-particle partitioning of organics? (3) Detailed mass spectrometric data on SOA generated in chamber studies indicate the existence of high-molecular weight compounds (often oligomers) that likely formed in the aerosol phase after partitioning of low volatility oxidation products. Can one develop chemical mechanisms that describe the formation of these compounds and what effect do aerosol-phase reactions have on the overall formation of SOA? (4) Mass spectrometric data indicate the presence of both organo-sulfur and organo-nitrogen aerosol species. Can we identify the routes by which such species form? How important are such compounds in atmospheric aerosols?