Atmospheric chemistry impacts global climate, regional air pollution, and human health. Air-surface interactions are important components of global atmospheric and biogeochemical cycles. Chemical reactions occur in the environment between trace gases, within aqueous solutions (e.g. cloud droplets), within particles, and on surfaces (e.g. snow, ice, atmospheric particles, vegetation). Atmospheric particles, often ~3 nm to 10 microns in diameter, are constantly evolving, complex mixtures that interact with radiation, form cloud droplet and ice crystals, and deposit onto surfaces. Primary atmospheric particles are emitted from a variety of natural and anthropogenic sources (e.g., dust, bacteria, sea spray, soot) and undergo heterogeneous and particle-phase reactions during transport. Secondary organic aerosol, formed primarily from the oxidation of trace level volatile organic compounds and subsequent gas-particle partitioning, is a dynamic chemical mixture, wherein each particle can contain thousands of organic compounds, the vast majority of which remain unidentified. Given the complexity and evolving nature of atmospheric particles, their measurement represents an analytical challenge and has resulted in significant uncertainties in our understanding of particle formation, evolution, and environmental impacts and fate.
The Pratt group focuses on studies of interactions between atmospheric trace gases, particles, and clouds using novel mass spectrometry techniques. In particular, we tackle the major uncertainties that exist in our understanding of atmospheric aerosols with respect to interactions between urban pollution and trace gas emissions from forests, as well as feedbacks associated with sea ice loss in the Arctic. We use a field-portable single-particle mass spectrometer to measure the size and chemistry of individual particles in real-time. Trace gas composition is measured using a field-portable on-line chemical ionization mass spectrometer, as well as gas chromatography coupled with mass spectrometry. To study the molecular composition of high molecular weight organics, electrospray ionization coupled with high-resolution mass spectrometry is used to probe ambient samples. These measurements of trace gases, aerosols, and cloud water are complemented by one-dimensional modeling of chemical reaction mechanisms and kinetics to examine and predict chemistry versus altitude and time. These studies will improve our understanding of the feedbacks between natural aerosol sources, anthropogenic emissions, and the Earth system for future climate and air quality predictions.
Pratt, K.A., K. D. Custard, P.B. Shepson, T.A. Douglas, D. Pohler, S. General, J. Zielcke, W.R. Simpson, U. Platt, D.J. Tanner, L.G. Huey, M. Carlsen and B.H. Stirm. 2013. Photochemical production of molecular bromine in Arctic surface snowpacks. Nature Geoscience. 6: 351-356.
Pratt, K.A., L.H. Mielke, P.B. Shepson, A.M. Bryan, A.L. Steiner, J. Ortega, R. Daly, D. Helmig, C.S. Vogel, S. Griffith, S. Dusanter, P.S. Stevens and M. Alaghmand. 2012. A one-dimensional model study of individual reactive biogenic volatile organic compounds and their contributions to organic nitrates above a mixed forest. Atmospheric Chemistry & Physics. 12: 10125-10143.
Pratt, K.A. and K.A. Prather. 2012. Mass spectrometry of atmospheric aerosols - Recent developments & applications. Part II: On-line mass spectrometry techniques. Mass Spectrometry Reviews. 31: 17-48.
Pratt, K.A., C.H. Twohy, S.M. Murphy, R.C. Moffet, A.J. Heymsfield, C.J. Gaston, P.J. DeMott, P.R. Field, T.R. Henn, D.C. Rogers, M.K. Gilles, J.H. Seinfeld and K.A. Prather. 2010. Observation of playa salts as nuclei in orographic wave clouds. Journal of Geophysical Research. 115: doi:10.1029/2009JD013606.
Pratt, K.A., P.J. DeMott, J.R. French, Z. Wang, D.L. Westphal, A.J. Heymsfield, C.H. Twohy, A.J. Prenni and K.A. Prather. 2009. In-situ detection of biological particles in cloud ice crystals. Nature Geoscience. 2: 398-401.
Pratt, K.A., J.E. Mayer, J.C. Holecek, R.C. Moffet, R.O. Sanchez, T. Rebotier, H. Furutani, M. Gonin, K. Fuhrer, Y. Su, S. Guazzotti and K.A. Prather. 2009. Development and characterization of an aircraft aerosol time-of-flight mass spectrometer. Analytical Chemistry. 81: 1792-1800.