Daniel Knopf, Associate Professor
Ph.D., 2003, Swiss Federal Institute of Technology, Switzerland
My research focuses on the physical and chemical properties of aerosol particles and their interaction with the atmosphere. Aerosol particles play a key role in air pollution, cloud formation, and global warming. Phase transitions of aerosol particles e.g. the formation of ice and its corresponding impact on the global radiation budget is one of the least understood processes in the atmosphere. Aerosol particles also provide a medium for gas-to-particle reactions, so-called heterogeneous reactions, which can significantly alter atmospheric chemistry. The Antarctic ozone hole is the most prominent example for efficient heterogeneous chemistry. Heterogeneous reactions can change the physical and chemical properties of particles, thus influencing air quality, source apportionment, cloud formation, and climate.
The growth, crystallization, nucleation, and freezing of aerosol particles are studied in the laboratory under atmospherically relevant conditions. The understanding of these particle phase transitions is crucial to accurately predict the effects of aerosol particles in the atmosphere. The particles studied here range from aqueous inorganic solution droplets and field-collected particles to marine bio-aerosol particles. In the laboratory phase transitions of aerosol particles are investigated using an aerosol nucleation cell coupled to an optical microscope. We use 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) to infer chemical composition and morphological features of the particles. Continuous development of these experimental techniques is performed to improve our understanding of the microphysical and chemical processes that govern atmospheric aerosol particles.
Heterogeneous oxidation of particles by atmospheric trace gases such as O3, NO2, N2O5, NO3, and OH are investigated in the laboratory using a flow reactor coupled to a chemical ionization mass spectrometer (CIMS). This approach allows to infer heterogeneous kinetics and products. Additionally, a high-resolution proton-transfer time-of-flight mass spectrometer (HR-PTR-ToF-MS) is applied to determine volatile organic reaction products. Our focus is to study the chemical transformation of biomass burning aerosol particles and its effect on particle water uptake and ice nucleation. Chemical transformation during atmospheric transport of bio-molecular markers representative of biomass burning aerosol can significantly affect aerosol source apportionment. Another research focus lies on the implementation of detailed heterogeneous chemistry in atmospheric models. This allows to assess the importance of various heterogeneous reaction pathways for condensed and gas-phase chemical composition.
Knopf, D. A., Alpert, P. A., Wang, B., O’Brien, R. E., Kelly, S. T., Laskin, A., Gilles, M. K., Moffet, R. C., Micro-Spectroscopic Imaging and Characterization of Individually Identified Ice Nucleating Particles from a Case Field Study, J. Geophys. Res., 119, 17, 10365–10381, 2014.
Slade, J. H. and Knopf, D. A., Multiphase OH oxidation kinetics of organic aerosol: The role of particle phase state and relative humidity, Geophys. Res. Lett., 41, 14, 5297–5306, 2014.
Forrester, S. M., Knopf, D. A., Photosensitised heterogeneous oxidation kinetics of biomass burning aerosol surrogates by ozone using an irradiated rectangular channel flow reactor, Atmos. Chem. Phys., 13, 6507–6522, 2013.
Rigg, Y. J., Alpert, P. A., Knopf, D. A., Immersion freezing of water and aqueous ammonium sulphate droplets initiated by Humic Like Substances as a function of water activity, Atmos. Chem. Phys., 13, 6603–6622, 2013.
Knopf, D. A., Alpert, P. A., A Water Activity Based Model of Heterogeneous Ice Nucleation Kinetics for Freezing of Water and Aqueous Solution Droplets, Farad. Disc., 165, 513-534, 2013.
Slade, J. H., Knopf, D. A., Heterogeneous OH oxidation of biomass burning organic aerosol surrogate compounds: Assessment of volatilisation products and the role of OH concentration on the reactive uptake kinetics, Phys. Chem. Chem. Phys. , 15 (16), 5898–5915, 2013.
Wang, B., Laskin, A., Roedel, T., Gilles, M. K., Moffet, R. C., Tivanski, A. V., Knopf, D. A., Heterogeneous ice nucleation and water uptake by field-collected atmospheric particles below 273 K, J. Geophys. Res. , 117, D00V19, 2012.
Wang, B., Lambe, A. T., Massoli, P., Onasch, T. B., Davidovits, P., Worsnop, D. R., Knopf, D. A., The deposition ice nucleation and immersion freezing potential of amorphous secondary organic aerosol: Pathways for ice and mixed-phase cloud formation, J. Geophys. Res. , 117, D16, D16209, 2012.
Shiraiwa, M., Pöschl, U., Knopf, D. A., Multiphase Chemical Kinetics of NO3 Radicals Reacting with Organic Aerosol Components from Biomass Burning. Environ. Sci. Technol. , 46, 6630–6636, 2012.
Alpert, P. A., Aller, J. Y., Knopf, D. A., Initiation of the Ice Phase by Marine Biogenic Surfaces in Supersaturated Gas and Supercooled Aqueous Phases. Special issue “Physics and Chemistry of Water and Ice” of Phys. Chem. Chem. Phys. , 13, 19882-19894, 2011.
Knopf, D. A., Forrester, S. M., Slade, J. H., Heterogeneous oxidation kinetics of organic biomass burning aerosol surrogates by O 3, NO 2, N 2O 5, and NO 3. Phys. Chem. Chem. Phys. , 13, 21050-21062, 2011.
Alpert, P. A., Aller, J. Y., Knopf, D. A., Ice nucleation from aqueous NaCl droplets with and without marine diatoms, Atmos. Chem. Phys., 11, 5539–5555, 2011.
Knopf, D. A., Forrester, S. M., Freezing of Water and Aqueous NaCl Droplets Coated by Organic Monolayers as a Function of Surfactant Properties and Water Activity, J. Phys. Chem. A, 115, 5579–5591, 2011.
Kaiser, J. C., Riemer, N., Knopf, D. A., Detailed heterogeneous oxidation of soot surfaces in a particle-resolved aerosol model, Atmos. Chem. Phys. , 11, 4505–4520, 2011.
Knopf, D. A., Alpert, P. A., Wang, B., Aller, J. Y., Stimulation of Ice Nucleation by Marine Diatoms, Nat. Geosci. , 4, 88–90, 2011.
Wang, B., Knopf, D. A., Heterogeneous ice nucleation on particles composed of humic-like substances impacted by O 3, J. Geophys. Res. , 116, D03205, 2011.
Knopf, D. A., Rigg, Y. J., Homogeneous Ice Nucleation From Aqueous Inorganic Organic Particles Representative of Biomass Burning: Water Activity, Freezing Temperatures, Nucleation Rates, J. Phys. Chem. A, 115, 762–773, 2011.
Knopf, D. A., Wang, B., Laskin, A., Moffet, R. C., Gilles, M. K., Heterogeneous nucleation of ice on anthropogenic organic particles collected in Mexico City, Geophys. Res. Lett. , 37, L11803, 2010.
Springmann, M., Knopf, D. A., Riemer, N., Detailed heterogeneous chemistry in an urban plume box model: Reversible co-adsorption of O 3, NO 2, and H 2O on Soot Coated with Benzo[a]pyrene. Atmos. Chem. Phys. , 9, 7461–7479, 2009.
Knopf, D. A., Lopez, M. D. Homogeneous Ice Freezing Temperatures and Ice Nucleation Rates of Aqueous Ammonium Sulfate and Aqueous Levoglucosan Particles for Relevant Atmospheric Conditions. Special issue “Physical Chemistry of Aerosols” of Phys. Chem. Chem. Phys., 11, 8056–8068, 2009.
Cosman, L. M., Knopf, D. A., Bertram, A. K., N 2O 5 reactive uptake on aqueous sulfuric acid solutions coated with branched and straight-chain insoluble organic surfactants, J. Phys. Chem. A, 112, 2386–2396, 2008.
Knopf, D. A., Cosman, L. M., Mousavi, P., Mokamati, S., Bertram A. K., A Novel Flow Reactor for Studying Reactions on Liquid Surfaces Coated by Organic Monolayers: Methods, Validation, and Initial Results, J. Phys. Chem. A, 111, 11021–11032, 2007.
Knopf, D. A., Reply to “Comment “Do NAD and NAT form in liquid stratospheric aerosols by pseudo-heterogeneous nucleation?””, J. Phys. Chem. A, 111, 1376–1377, 2007.
Knopf, D. A., Mak, J., Gross, S., Bertram, A. K.: Does Atmospheric Processing of Saturated Hydrocarbon Surfaces by NO 3 Lead to Volatilization?, Geophys. Res. Lett., 33 (17), L17816, 2006.
Knopf, D. A., Koop, T., Heterogeneous Nucleation of Ice on Surrogates of Mineral Dust, J. Geophys. Res., 111 (D12), D12201, 2006.
Knopf, D. A., Do NAD and NAT form in liquid stratospheric aerosols by pseudoheterogeneous nucleation?, J. Phys. Chem. A, 110, 5745–5750, 2006.
Dymarska, M., Murray, B. J., Sun, L., Eastwood, M., Knopf, D. A., Bertram, A. K., Ice nucleation on soot at temperatures relevant for the lower troposphere, J. Geophys. Res., 111 (D4), D04204, 2006.
Knopf, D. A., Anthony, L. M., Bertram*, A. K., Reactive Uptake of O 3 by Multicomponent and Multiphase Mixtures Containing Oleic Acid, J. Phys. Chem. A, 109, 5579–5589, 2005.
Murray, B. J., Knopf, D. A., Bertram, A. K., The formation of cubic ice under conditions relevant to Earth’s atmosphere, Nature, 434, 202–205, 2005.
Knopf, D. A., Luo B. P., Krieger, U. K., Koop, T., Reply to “Comment on ‘Thermodynamic Dissociation Constant of the Bisulfate Ion from Raman and Ion Interaction Modeling Studies of Aqueous Sulfuric Acid at Low Temperature”’, J. Phys. Chem. A, 109, 2707–2709, 2005.
Parsons, M. T., Knopf, D. A., Bertram, A. K., Deliquescence and Crystallization of Ammonium Sulfate Particles Internally Mixed with Water-Soluble Organic Compounds, J. Phys. Chem. A, 108, 11600–11608, 2004.
Knopf, D. A., Luo B. P., Krieger, U. K., Koop, T., Thermodynamic Dissociation Constant of the Bisulfate Ion from Raman and Ion Interaction Modeling Studies of Aqueous Sulfuric Acid at Low Temperatures, J. Phys. Chem. A, 107, 4322–4332, 2003.
Knopf, D. A., Koop, T., Luo B. P., Weers, U. G., Peter, T., Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification, Atmos. Chem. Phys. , 2, 207–214, 2002.
Zink, P., Knopf, D. A., Schreiner, J., Mauersberger, K., Möhler, O., Saathof, H., Seifert, M., Tiede, R., Schurath, U., Cryo-chamber simulation of stratospheric H2SO4/H2O particles: Composition analysis and model comparison, Geophys. Res. Lett. , 11 (29), 1551, 2002.
Schreiner, J., Voigt, C., Zink, P., Kohlmann, A., Knopf, D., Weisser, C., Budz, P., Mauersberger, K., A Mass Spectrometer System for Analysis of Polar Stratospheric Aerosols, Rev. Sci. Inst. , 73, 446–452, 2002.
Knopf, D. A., Zink, P., Schreiner J., Mauersberger, K., Calibration of an Aerosol Composition Mass Spectrometer with Sulfuric Acid Water Aerosol, Aerosol Sci. Technol. , 35, 924–928, 2001.