CARNEGIE MELLON UNIVERSITY
We will acquire two instruments in order to probe the complex interactions between gas-phase organic oxidation chemistry and the evolution of organic particulate matter (PM). One instrument is a Chemical Ionization Mass Spectrometer (CIMS) designed for flexible, highly sensitive measurement of critical radicals (OH, HO2, and ??RO2), sulfuric acid vapor, or ammonia. The radical levels control the rate as well as the mechanisms for oxidation of organic compounds in the atmosphere; ammonia and sulfate, along with organics, are critical players in new-particle formation and grown in the atmosphere. The other instrument is a High-Resolution Time of Flight Aerosol Mass Spectrometer (HR-AMS), with a mass resolution of 5000 (?0.02 amu). The HR-AMS will measure the size-dependent non-refractory fraction of particle composition between 50 and 700 nm, explicitly resolving C:H:O:N:S in the particles. With a light-scattering module to detect individual particles, the HR-AMS will be able to detect single particles, enabling experiments addressing the evolving mixing state of aerosols from different sources.
Taken together, these instruments will permit research at the forefront of atmospheric chemistry. Organic aerosols are a dynamic system, combining phase partitioning between the gas and particulate phases with oxidation chemistry, which causes the volatility distribution of organic material associated with atmospheric particles to evolve continuously over their roughly week-long residence time in the atmosphere. Our understanding of this system relies heavily on chamber experiments focused on organic-aerosol formation, but the large majority of these experiments have not included sufficient measurements to understand the gas- phase chemistry that controls the oxidation mechanisms. The CIMS instrument will provide a significant new constraint to those experiments. Concurrently, the state-of-the-art in condensed-phase measurements is evolving rapidly, so that it is now possible to measure the molecular formula of most fragments observed in an organic mass spectrum. The HR-AMS will thus enable measurements of the oxidation state of organic particulate matter as it evolves due to gas-phase oxidation reactions.
Additional research projects using these instruments will focus on the chemistry behind new-particle formation and growth in the atmosphere, which is a critical contributor to the total particle number budget and thus to the formation of cloud condensation nuclei. Large uncertainties remain regarding the identity and sources of constituents forming the “critical cluster” for nucleation, and the CIMS instrument will provide measurements of several key species. Also, the HR-AMS will support intensive field campaigns designed to understand PM sources, atmospheric transformations, and contributions to health effects.