Particulate matter

CAPS pursues research into all aspects of air quality and atmospheric chemistry with a focus on airborne particulate matter. Particulate matter is either emitted directly into the atmosphere or formed there as a result of the atmospheric oxidation of precursor species. Once formed, atmospheric particles (also known as aerosols): are detrimental to human health and play a pivotal role in clouds, precipitation, climate, and visibility. We are highly collaborative, pursuing this research through a combination of laboratory experiments, field measurements, theory, and modeling. We strive to balance fundamental and applied research and to make our findings policy-relevant.

Motivation and goals


image of wildfires

Source: College of Engineering

Anthropogenic aerosols cool the earth's climate by reflecting sunlight back to space and by serving as nuclei for cloud droplet and ice crystal formation. Their net effect has been to partially offset global warming from greenhouse gases, but uncertainty in the magnitude of this effect has complicated the assessment and forecasting of climate change.

Research in this area focuses on improving the representation of aerosols in global climate, chemistry, and transport models by incorporating size-resolved aerosol microphysics and thermodynamics and testing these improved aerosol models against observations from ground networks, intensive field campaigns, and satellites.

Laboratory experiments that investigate the liquid droplet and ice crystal nucleation properties of specific types of aerosol particles, and how atmospheric processing alters these cloud nucleation properties, are also conducted.

Finally, we participate in field campaigns to determine the contributions of different particle sources and particle formation/growth processes to the cloud precursor particle population. The interactions between aerosols and actual clouds are investigated using in situ cloud measurements from research aircraft.

Environmental health

map of NO2 in Pittsburgh

Source: College of Engineering

Particulate matter has been associated by epidemiologists with millions of premature deaths per year worldwide. It has been shown to increase the rates of heart disease, cancer, respiratory disease, and stroke.

We seek to understand the sources and properties of these particles, to improve our predictive ability of particulate matter formation, to quantify the exposure of communities and human populations to particulate matter and other air toxics, and to find cost-effective strategies for reducing their ambient concentrations.

Methods and approaches


CAPS lab

Source: College of Engineering

A wide variety of laboratory experiments that simulate important atmospheric processes under well-controlled conditions are conducted in our Air Quality Laboratory. We use these experiments to investigate the sources and evolution of particulate matter and improve their representation in atmospheric models, with the goal of better predictions and understanding of their impacts on climate and human health.

Large smog chamber reactors allow us to observe the evolution of a simulated atmosphere over many hours as the selected particles and gases added to the chamber are subjected to physical and chemical processes, such as photochemical oxidation. We have an extensive suite of state-of-the-art instrumentation for real-time analysis of the evolution of the physical and chemical properties of the gas and particle-phase constituents as the chamber’s atmosphere evolves.

Instruments that simulate warm cloud droplet and frozen ice crystal nucleation are used to assess the cloud nucleation properties of the chamber aerosol. The kinetics and mechanisms of gas-phase and gas-particle (heterogeneous) chemical reactions are determined using flow tube reactors. A unique optical tweezers system allows us to probe the response of individual particles to changes in its gas-phase environment, temperature, and humidity levels over timescales of hours.

Numerous specialized mass spectrometers are the primary analytical methods used to measure changes in gas and particle-phase chemical composition. We develop and use five different single-particle instruments to perform real-time analysis of the size and composition of individual aerosol particles. Other measurements used include FT-IR and Raman spectroscopy, aerosol optical properties, and the analysis of particles collected on filters by traditional analytical chemistry methods. 

Ambient measurements

air quality van

Source: College of Engineering

Ambient measurements of particulate matter and other air pollutants underpin the laboratory experiments and atmospheric modeling conducted in CAPS. These measurements are used to ground-truth atmospheric models, to verify the findings of laboratory experiments, to assess population exposures to air pollutant species, and to investigate the impacts of specific industries or pollutant sources. In CAPS we conduct ambient measurements that span multiple spatial and temporal scales.

Characterizing near-roadway particle microphysical evolution requires detailed measurements of particle size and composition within a few hundred meters of a rapidly changing roadway environment; understanding population exposures focuses on annual average concentrations at around 1 km resolution. We achieve this spatial and temporal coverage by combining stationary and mobile measurements, and state-of-the-art instrumental analysis with lower cost distributed sensors.

Atmospheric modeling 

illustration of formation of secondary aerosol mass in the atmosphere

Source: College of Engineering

Global and regional climate and chemical transport models (CTMs), such as the Unified Model, PMCAMx, and GEOS-CHEM, are some of the tools that we bring to bear on our research. Our models are three-dimensional representations of the atmosphere. Based on knowledge of the rates of emissions, chemical reactions, transport by wind, and removal from the atmosphere, CTMs predict the concentrations of the most important atmospheric species as a function of time and location, while weather and climate models also predict clouds, wind, and rain.

We use these tools to relate emissions to concentrations, test hypotheses, and quantify the effects of processes studied in the laboratory or in the field. Contributing to the development of global climate, weather, and air quality models is one way in which CAPS members help inform Intergovernmental Panel on Climate Change (IPCC) climate assessments and policy set by, for example, the US Environmental Protection Agency (EPA). CAPS operates a high-performance cluster consisting of 192 cores with 48 TB of RAID storage. The three 4-CPU Dell PowerEdge R820 servers are excellent modeling platforms. We also use the Bridges machine at the Pittsburgh Supercomputing Center.


CAPS actively collaborates with other interdisciplinary centers at Carnegie Mellon University in the areas of climate and energy policy, and several of the CAPS faculty hold joint appointments in CMU’s Engineering and Public Policy Department. Some of the policy-relevant research we have conducted includes the air quality impacts of changes in electric power generation technology, natural gas development, agricultural activities, carbon capture and sequestration (CCS), and short-lived climate forcers such as black carbon aerosol.

Related centers

More information on research in CAPS and in related research areas at CMU is available at the following sites.