Cappa, Christopher

Near Tromso

Understanding how the properties of aerosols in the Earth’s atmosphere vary in both space and time is of great importance owing to their influence on global climate, precipitation, visibility and human health. Research in the Cappa group focuses on understanding and characterizing the processes that drive variations in the chemical, physical and optical properties of atmospheric aerosols, both through laboratory experiments and field observations. A selection of some of our activities is below, and more information can be found at Field & Laboratory Campaigns and CAICE. You can also find the lab members involved.

Linking Composition and Volatility of Organic Aerosol

  • The sensitivity of atmospheric aerosols to changes in temperature can affect the observed abundance of the aerosol by influencing the gas-particle partitioning of semi-volatile compounds. Additionally, measurements of particle volatility can provide information on the chemical nature of the aerosol, especially when coupled with real-time composition measurements. The Cappa Group is working to understand how measurements of temperature-dependent composition and total particle mass can be used to provide quantitative information about the nature of particles sampled in different environments.
  • In addition, we are developing and utilizing a kinetic, microphysical model of aerosol evaporation to quantitatively characterize the volatility of ambient aerosol.

Oxidation of Organic Aerosol

  • Chemical composition plays an important role in determining the lifetime and climate impacts of atmospheric aerosols. The Cappa Group is working in collaboration with Kevin Wilson and others at the Advanced Light Source at Lawrence Berkeley National Laboratory to characterize how heterogeneous oxidation of organic aerosols by OH radicals affects particle composition, volatility, hygroscopicity and optical properties. The Cappa Group uses the world’s only VUV aerosol mass spectrometer (on beamline 9.0.2.) in order to determine the timescales and aerosol chemical evolution associated with the OH oxidation.
  • We have also developed a new model to describe the formation of secondary organic aerosol (SOA).

Water Uptake, Light Scattering and Clouds

  • The ability of particles to take up water is intimately linked to their composition. By changing particle size, water uptake has a strong influence on the ability of particles to scatter sunlight. In addition. all cloud droplets grow from particle seeds; thus, variability in the composition of particles can strongly influence their ability to grow into cloud drops. We are working to understand and characterize the relationship between particle composition and water uptake, especially the influence of organic molecules.

Light Absorption by Atmospheric Particles

  • Soot produced from combustion processes is the dominant source of particles that absorb light in the atmosphere. Soot (or “black carbon” has a complex morphology where a soot particle is generally composed of many small spherules which stick together. As such, prediction of the optical properties of soot from theory are difficult. It is also known that the presence of non-absorbing coatings on light absorbing particle cores can lead to an enhancement of the total light absorption. However, theoretical methods used to calculate this enhancement by necessity use a spherical core-shell model, which is most certainly not the case for soot. The Cappa Group is interested in constraining the theoretical predictions through direct measurement of the light absorption enhancement for soot to determine to what extent the “core-shell” model can be accurately used in global climate models.
  • Beyond black carbon, organic carbon can absorb sunlight. However, the properties of this so-called “brown carbon” are highly variable, depending on the source and changing as particles age in the atmosphere. We are working to characterize brown carbon in the lab and atmosphere, and to understand how its properties change as it is transported through the atmosphere.

Chemistry and Aerosol Optics

  • It is well known that the composition of particles is intimately tied to their observed optical properties (i.e. ability to absorb and scatter light). The Cappa Group is exploring this relationship in detail through laboratory studies, with a particular focus on understanding how chemical complexity of the organic aerosol fraction and heterogeneous reactions affect the aerosol optics. This work has potential implications for understanding how region-to-region variability in organic aerosol emission sources and long-term processing influences the local radiative balance. This work has been supported by NSF.

Expiratory Aerosol Emission and Indoor Air

  • People exhale (emit) small aerosols when breathing, speaking, shouting, or singing, in addition to when coughing or sneezing. These aerosols are produced primarily in the respiratory system and can carry viruses, contributing to disease transmission. We have interest in understanding the factors that determine what causes people to emit these respiratory aerosols at different rates, and how the use of face coverings can help mitigate their emission. We also have interest in improving the ways in which we manage and understand our indoor world to create healthier environments for everyone.