Atmospheric Chemistry

Dr. John Mak and Dr. Alex Guenther, NCAR senior scientist, taking air samples in the Gobi Desert, Mongolia;

Dr. John Mak and Dr. Alex Guenther, NCAR senior scientist, taking air samples in the Gobi Desert, Mongolia;

Atmospheric chemistry encompasses the interaction of gases and aerosol particles with each other and with the environment. The sum of these interactions determines, in large part, the composition of Earth’s atmosphere, which therefore also changes over time. Furthermore, aerosol particles govern cloud formation with subsequent important implications for the radiative budget of the atmosphere, water vapor distribution, and the hydrological cycle.

Our faculty study i) the origin of certain trace gases, with special emphasis on the large scale (hemispheric, or global) contribution from human activities, and how those contributions vary over time; ii) the potential of natural and anthropogenic aerosol particles to form ice clouds and how this can be parameterized; iii) the interaction of aerosol particles with atmospheric trace gas species to assess the impact of multiphase chemical kinetics on air quality and climate; iv) the global rate of removal of several reactive species and how this is affected by human or natural changes over time; v) the role of marine biological activity in sea spray aerosol production and its climatic impact.

Joseph Ruggieri working in the Mak lab;

Joseph Ruggieri working in the Mak lab;

To address these important questions ITPA maintains two well equipped atmospheric chemistry laboratories to conduct fundamental studies pertaining to the role of gases and particles in Earth’s atmosphere. These investigations require state-of-the-art analytical tools such as gas chromatography (for concentration measurements of trace gases), isotope ratio mass spectrometry (for isotopic analysis of trace gases), conventional mass spectrometry (for compound identification), chemical ionization mass spectrometer (for chemical kinetics study), high-resolution proton-transfer time-of-flight mass spectrometer (for detection of volatile organic compounds), ice nucleation cells (for aerosol phase transition observations for temperatures as low as 180 K), and other analytical approaches. Our faculty and students also use three-dimensional computer modeling of atmospheric chemistry to interpret the observations. In addition to these laboratory and modeling based approaches, investigators from ITPA have conducted field studies in various regions, including Antarctica, Rocky Mountains, Atlantic Ocean, Gobi Desert, French Guiana, and South Korea.

Atmospheric Composition – Isotopic Trace Gas Analysis (Mak)

Zhihui Wang (former lab member) working with the ice core analytical system.

Zhihui Wang (former lab member) working with the ice core analytical system.

Mak’s group is known for isotopic studies of trace gases, including, but not limited to, carbon dioxide, carbon monoxide, methane, isoprene, acetaldehyde, and acetone. Mak’s group has developed various techniques for compound specific gas isotope measurements (see Atmospheric Research Facilities). In 2010 Mak’ group measured, for the first time, the isotopic composition of atmospheric carbon monoxide in ice cores, and from these data concluded that biomass burning during the last 650 years was much more variable than previously supposed.  Mak’s group is currently funded by NSF to reconstruct paleo CO over the past 10,000 years from the newly drilled South Pole Ice Core Project. In addition, Mak is collaborating with scientists from the Laboratory for Glaciology and Geophysics  (LGGE< CNRS) in Grenoble, France on reconstructing CO from Dome Concordia and WAIS Divide. Complementary to this work, development of new analytical techniques are crucial such as the ability to measure 17O in CO from ice cores. C17O has been shown to be an effective proxy of OH abundance, thus measuring 17CO from ice cores will allow to constrain OH in the paleo atmosphere.

Quantification of Volatile Organic Compounds (Mak, Knopf)

Volatile organic compounds (VOCs) affect atmospheric composition, the loss of atmospheric radicals such as OH, and the formation of secondary organic aerosol (SOA). Of particular interest are the biogenic VOC emissions from, e.g., a forest, and how those impact atmospheric gas and aerosol chemistry. ITPA faculty have been working on the quantification of oxygenated VOCs for eddy covariance studies using the proton-transfer time-of-flight mass spectrometry (see Atmospheric Research Facilities).

Mak’s group collaborates with investigators to help determine the emissions of various trace gases, which require extensive field work to remote locations, including South America, Asia, and Africa.

In the summer of 2013, Mak’s group was funded by the EPA to quantify vertical profiles of reactive VOCs within the planetary boundary layer during the Southern Oxidant and Aerosol Study (SOAS). More than 25 research groups from around the world participated in this 2 month intensive in the forests of Alabama. Mak’s group utilized the Whole Air Sample Profiler (WASP) and UltraPure Air LLC’s research aircraft (see Atmospheric Research Facilities) to obtain, for the first time, high resolution snapshot vertical profiles of reactive VOCs within the planetary boundary layer. In addition, Mak’s group quantified the fluxes of reactive VOCs within the forest canopy via eddy covariance.

Mak’s group also carries out instrument development and has recently been working on analytical platforms for research aircraft.

 

Aerosol Phase Transition and Ice Cloud Microphysics (Knopf)

Ice formation initiated by sea spray aerosol particles as observed using the ice nucleation apparatus coupled to optical microscopy. The ice crystals grow (panels left to right) as relative humidity is increased.

Ice formation initiated by sea spray aerosol particles as observed using the ice nucleation apparatus coupled to optical microscopy. The ice crystals grow (panels left to right) as relative humidity is increased.

Atmospheric aerosol particles, 2 nm to 100 μm in diameter, exist in various phase states which are modulated by the environmental conditions such as temperature and relative humidity. The particle phase state will govern the role of the particle in the atmosphere including aerosol optical properties, interaction with gas phase species, and cloud formation ultimately impacting health related issues, air quality, and climate. The growth, crystallization, nucleation, and freezing of aerosol particles are studied in our laboratories under atmospherically relevant conditions. Of current particular interest is the ability of organic and biological particles to initiate ice crystal formation examined using custom-built aerosol nucleation cells coupled to optical microscopes. Single particle micro-spectroscopic analyses such as computer controlled scanning electron microscopy with energy dispersive analysis of X-rays (CCSEM/EDX) and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) are applied to assess the chemical composition and morphological features of the ice nucleating particles. Physical parameterizations of ice nucleation are developed for implementation in cloud and climate models. Continuous improvement of these experimental techniques is performed to advance our understanding of the microphysical and chemical processes that govern atmospheric aerosol phase transitions.

 

Multiphase Chemical Kinetics Between Aerosol and Reactive Trace Gases (Knopf)

Multiphase chemical kinetics experimental setup including rectangular channel flow reactor and chemical ionization mass spectrometer.

Multiphase chemical kinetics experimental setup including rectangular channel flow reactor and chemical ionization mass spectrometer.

During atmospheric transport aerosol particles undergo chemical transformation by reaction with atmospheric radicals and oxidants such as OH, O3, and NO3. These heterogeneous and multiphase reactions can impact the particle’s physicochemical characteristics with subsequent important consequences for the particle’s role in atmospheric processes such as air quality and climate. For example, oxidation of organic aerosol particles can lead to the volatilization of organic material or to the increased amount of condensed phase oxygenated material thereby enhancing particle hygroscopicity. The most popular effect of heterogeneous reactions is the occurrence of the stratospheric ozone hole. During winter, inert chlorine reservoir species are becoming activated on the surface of polar stratospheric cloud particles. As the sun rises, the active chlorine species can destroy the ozone layer. The chemical kinetics between particles and reactive gases are determined in the laboratory. A custom-built chemical ionization mass spectrometer coupled to different reaction chambers are used to quantitatively determine gas-to-particle kinetic reaction rates. Gas phase products are analyzed using proton-transfer time-of-flight mass spectrometer. The multiphase chemical kinetics are represented by physical models and implemented in atmospheric chemistry models to improve our predictive understanding of the atmospheric evolution of particulate matter.

 

Ocean-Atmosphere Interaction: Generation and Physicochemical Characteristics of Sea Spray Aerosol (Knopf, Aller)

1000 L mesocosm experiment to simulate an algal bloom and sea spray aerosol generation.

1000 L mesocosm experiment to simulate an algal bloom and sea spray aerosol generation.

The oceans cover the majority of our planet Earth. Sea spray aerosol emitted by the oceans through wave breaking and bubble bursting constitutes one of the most abundant sources of natural aerosol particles. In recent years, the scientific community has recognized that sea spray aerosol also consists of biogenic material which orignates from biological activity in ocean surface waters. The unique setting of ITPA within SoMAS allows to combine the expertise of atmospheric chemistry and marine microbiology and institutional infrastructure to study the formation of sea spray aerosol under controlled conditions in the laboratory and in the field. Of particular interest is, how different microbial species such as phytoplankton and bacteria impact ocean surface water composition with subsequent consequences for sea spray aerosol composition and morphology. The climatic impact of sea spray aerosol is assessed by examination of its physicochemical properties and ability to form clouds. To achieve these goals algal blooms are simulated in large scale mesocosm experiments using artificial sea water and phytoplankton cultures or water collected from the Atlantic Ocean using one of our research vessels. Furthermore, marine particles are sampled at Long Island’s south shore and during participation in research cruises covering the open oceans. These research activities aim to improve our understanding of the complex processes governing ocean-atmosphere interactions.