Marcelo I. Guzman is an Associate Professor of Chemistry and the Principal Investigator of the Environmental Chemistry Laboratory at the University of Kentucky, where he teaches analytical and environmental chemistry courses. In 2013, he received a NSF CAREER award. He earned his Ph.D. at the California Institute of Technology (Caltech, 2007) working on ice chemistry. In 2002, he was an Andrew W. Mellon Fellow at the Metropolitan Museum of Art (New York) working on Paper and Photograph Conservation in the Sherman Fairchild Center. For his postdoctoral experience he joined the Origins of Life Initiative at Harvard University as an Origins Fellow.
Appointed to the Editorial Board of Photocatalysis: Research and Potential 2022.
Appointed to the Distinguished Advisory Board of Sustainable Horizons 2021.
Appointed to the Editorial Advisory Board of Photochem 2021.
Committee of Visitors NSF Division of Chemistry 2020.
Outstanding Graduate Student Mentoring Award 2020.
Top Reviewer in the Global Peer Review Award (Web of Science). Top 1% in cross-field of reviewers in Publons between 9/1/2018 and 9/1/2019.
ACS Division of Environmental Chemistry Certificate of Appreciation for 15 years of service, 2019.
Appointed to the Photochemistry Section of the Editorial Board of Molecules 2019.
Appointed to the Editorial Advisory Board of the International Journal of Environmental Research and Public Health (IJERPH) 2019.
NSF CAREER Award 2013–2018.
Selected for the NSF/NASA RCN Origins Meeting at the Santa Fe Institute 2018.
Awarded an A&S international scholar program grant 2018–2019.
Awarded an ISARRA grant for the 2018 Conference.
Appointed to the Editorial Board of Environments 2018.
Awarded an International Global Atmospheric Chemistry (IGAC) Grant by the Surface Ocean Lower Atmospheric Study (SOLAS) International Project for the Cryosphere and Atmospheric Chemistry (CATCH) Workshop 2017.
Appointed to the Editorial Board of Atmosphere 2016.
ACS Young Academic Investigators Awardee in Organic Chemistry 2016.
College Research Activity Award for the Goldschmidt Conference 2013.
ACS Division of Environmental Chemistry Certificate of Merit for 10 years of service, 2014.
College Research Activity Award for the Goldschmidt Conference 2011.
Harvard Origins of Life Post-Doctoral Fellowship 2007–2010.
ISSOL award to young scientists 2008.
Young scientist best poster award. Gordon Research Conference: Origin of Life 2008.
Vito Vanoni Caltech Institute Fellowship 2002 – 2003.
Andrew W. Mellon Fellowship. The Metropolitan Museum of Art, New York 2002.
Research Council Fellowship, National University of Tucuman 1999–2001.
Honors medal to the best Chemistry Graduate from the National University system, class of 2000. Awarded by the Argentine Chemical Society.
The Guzman group studies processes occurring in environmental interfaces and is interested in research problems of interdisciplinary scope. We are currently investigating photooxidative reactions of organic molecules of environmental relevance to understand the processing of pollutants "on the surface of" and "in" atmospheric aerosols, clouds, and fogs. Some of our recent accomplishments include to have provided comprehensive photochemical and heterogeneous oxidation reaction mechanisms for the processing of dissolved organic matter molecular probes "in" and "on the surface of" aqueous aerosol mimics. The laboratory applies soft ionization methods such as online electrospray ionization mass spectrometry (OESI-MS), multiple chromatographic separations, one and two dimensional nuclear magnetic resonance, and various spectroscopies to contrast the fast oxidation of biomass burning and combustion emissions at the air-water interface versus the air-solid interface. One of our latest environmental chemistry developments is the creation of integrated unmanned aerial systems for environmental monitoring of trace gases. We are also interested on the potential use of photocatalysis for fuel production and to jumpstart a prebiotic chemical cycle related to the origin of life. Because past work has focused on the study of reactions in ice matrices, the laboratory is also experienced in ice chemistry. Learn more about our work at the group website: http://www.guzmanlab.com/research
Physical and Environmental Organic Chemistry
We currently have no openings for postdoctoral researchers and short-term visitors unless they are self-supported.
Prof. Guzman will be recruiting Ph.D. graduate students to work in applied chemistry research for the Fall 2022 admission. We are searching for inspired and self-motivated graduate students interested in joining the lab. Graduate students looking to join the lab should review the information provided https://chem.as.uky.edu/chemistry-graduate-program/. Should you have questions about the lab, research, or potential openings, please feel free to contact Prof. Marcelo Guzman by email at Professor Marcelo I. Guzman (marcelo dot guzman at uky dot edu).
The pioneering contributions to explain the effect of airborne transmission on COVID-19 were covered by Phys.Org on May 15, 2021.
65) Oxidation of Phenolic Aldehydes by Ozone and Hydroxyl Radicals at the Air-Solid Interface. M.S. Rana and M.I. Guzman. ACS Earth and Space Chemistry (2022), 6, 2900-2909, DOI: 10.1021/acsearthspacechem.2c00206.
64) Oxidation of Catechols at the Air-Water Interface by Nitrate Radicals. M.S. Rana and M.I. Guzman. Environmental Science and Technology (2022), 56, 15437-15448. DOI: 10.1021/acs.est.2c05640.
63) Chemical State of Potassium on the Surface of Iron Oxides: Effects of Potassium Precursor Concentration and Calcination Temperature. M.A. Hoque, M.I. Guzman, J.P. Selegue, and M.K. Gnanamani. Materials (2022), 15,(20), 7378. DOI: 10.3390/ma15207378.
61) Reactivity of aminophenols in forming nitrogen-containing brown carbon from iron-catalyzed reactions. H.A. Al-Abadleh, F. Motaghedi, W. Mohammed, M.S. Rana, K.A. Malek, D. Rastogi, A.A. Asa-Awuku, and M.I. Guzman. Communications Chemistry (2022) 5, 112. DOI: 10.1038/s42004-022-00732-1. PDF
60) Surface Oxidation of Phenolic Aldehydes: Fragmentation, Functionalization, and Coupling Reactions. M.S. Rana and M.I. Guzman. Journal of Physical Chemistry A (2022), 126, 126, 6502–6516; DOI: 10.1021/acs.jpca.2c04963.
59) Characteristics and health effects of particulate matter emitted from a waste sorting plant. A. Barkhordari, M.I. Guzman, G. Ebrahimzadeh, A. Sorooshian, M. Delikhoon, M.J. Rastani, S. Golbaz, M. Fazlzadeh, R. Nabizadeh, A.N. Baghani. Waste Management (2022), 150, 244-256. PDF
58) Research on oxygen solubility in aqueous amine solvents with common additives used for CO2 chemical absorption. T.B. Jorgensen, K. Abad, M. Sarma, M.I. Guzman, J.G. Thompson, K. Liu. International Journal of Greenhouse Gas Control (2022) 116, 103646, https://doi.org/10.1016/j.ijggc.2022.103646. PDF
57) Characteristics and assessing biological risks of airborne bacteria in waste sorting plant. A.N. Baghani, S. Golbaz, G. Ebrahimzadeh, M.I. Guzman, M Delikhoon, M.J. Rastani, A. Barkhordarie, R. Nabizadeh. Ecotoxicology and Environmental Safety (2022) 232, 113272. https://www.sciencedirect.com/science/article/pii/S0147651322001129
56) Aqueous photochemistry of 2-oxocarboxylic acids: Evidence, mechanisms, and atmospheric impact. M.I. Guzman and A.J. Eugene. Molecules (2021), 26 (17), 5278, https://doi.org/10.3390/molecules26175278.
55) Modes of transmission of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) and factors influencing on the airborne transmission: a review. M. Delikhoon, M.I. Guzman, R. Nabizadeh, A.N. Baghani. International Journal of Environmental Research and Public Health (2021), 18 (2), 395; DOI: 10.3390/ijerph18020395. PDF
54) An Overview of the Effect of Bioaerosol Size in COVID-19 Transmission. M.I. Guzman. International Journal of Health Planning and Management (2020). DOI: 10.1002/hpm.3095.PDF
53) Dark iron-catalyzed reactions in acidic and viscous aerosol systems efficiently form secondary brown carbon. H.A. Al-Abadleh, M.S. Rana, W. Mohammed, and M.I. Guzman. Environmental Science and Technology (2021), 55, 209-219, DOI: 10.1021/acs.est.0c05678. PDF
52) Application of a Small Unmanned Aerial System to Measure Ammonia Emissions from a Pilot Amine-CO2 Capture System. T.J. Schuyler, B. Irvin, K. Abad, J.T. Thompson, K. Liu and M.I. Guzman. Sensors (2020), 20 (23), 6974; DOI: 10.3390/s20236974. PDF
51) Atmospheric Measurements with Unmanned Aerial Systems (UAS). M.I. Guzman. Atmosphere (2020), 11 (11), 1208, DOI: 10.3390/atmos11111208. PDF
50) Oxidation of Phenolic Aldehydes by Ozone and Hydroxyl Radicals at the Air-Water Interface. M.S. Rana and M.I. Guzman. Journal of Physical Chemistry A (2020), 124, 8822-8833, DOI: 10.1021/acs.jpca.0c05944.
49) University of Kentucky measurements of wind, temperature, pressure and humidity in support of LAPSE-RATE using multi-site fixed-wing and rotorcraft unmanned aerial systems. S.C.C.Bailey, M.P. Sama, C.A. Canter, L.F. Pampolini, Z.S. Lippay, T.J. Schuyler, J.D. Hamilton, S.B. MacPhee, I.S. Rowe, C.D. Sanders, V.G. Smith, C.N. Vezzi, H.M. Wight, J.B. Hoagg, M.I. Guzman, and S.W. Smith.Earth Syst. Sci. Data (2020), DOI: 10.5194/essd-12-1759-2020. PDF
47) Understanding the Effect of Host Structure of Nitrogen Doped Ultrananocrystalline Diamond Electrode on Electrochemical Carbon Dioxide Reduction. N. Wanninayake, Q. Ai, R. Zhou, M.A. Hoque, S. Herrell, M.I. Guzman, C. Risko, and D.Y. Kim. Carbon (2020), 408-419, DOI: 10.1016/j.carbon.2019.10.022.
46) Production of Singlet Oxygen (1O2) During the Photochemistry of Aqueous Pyruvic Acid: The Effects of pH and Photon Flux under Steady State O2(aq) Concentration. A.J. Eugene and M.I. Guzman. Environmental Science and Technology (2019), 53, 12425-12432, DOI: 10.1021/acs.est.9b03742.
45) Monitoring Tropospheric Gases with Small Unmanned Aerial Systems (sUAS) during the Second CLOUDMAP Flight Campaign. T.J. Schuyler, S.C.C. Bailey, and M.I. Guzman. Atmosphere (2019), 10 (8), 434, DOI: 10.3390/atmos10080434. PDF
44) Crystal Structure of Zymonic Acid and a Redetermination of its Precursor, Pyruvic Acid. D. Heger, A.J. Eugene, S.R. Parkin and M.I. Guzman. Acta Crystallographica Section E: Crystallographic Communications (2019), 75 (6), 858-862, DOI: 10.1107/S205698901900707. PDF
43) Intercomparison of Small Unmanned Aircraft System (sUAS) Measurements for Atmospheric Science during the LAPSE-RATE Campaign. L. Barbieri, S.T. Kral, S.C.C. Bailey, A.E. Frazier, J.D. Jacob, J. Reuder, D. Brus, P.B. Chilson, C. Crick, C. Detweiler, A. Doddi, J. Elston, H. Foroutan, J. Gonzalez-Rocha, B.R. Greene, M.I. Guzman, A.L. Houston, A. Islam, O. Kemppinen, D. Lawrence, E.A. Pillar-Little, S.D. Ross, M. Sama, D.G. Schmale III, T.J. Schuyler, A. Shankar, S.W. Smith, S. Waugh, C. Dixon, S. Borenstein, and G. de Boer. Sensors (2019), 19 (9), 2179, DOI: 10.3390/s19092179. PDF
42) Using a Balloon Launched Unmanned Glider to Validate Real-Time WRF Modeling. T.J. Schuyler, S.M.I. Gohari, G. Pundsack, D. Berchoff, and M.I. Guzman. Sensors (2019), 19 (8), 1914, DOI: 10.3390/s19081914. PDF
41) The Effects of Reactant Concentration and Air Flow Rate in the Consumption of Dissolved O2 During the Photochemistry of Aqueous Pyruvic Acid. A.J. Eugene and M.I. Guzman. Molecules (2019), 24 (6), 1124, DOI: 103390/molecules24061124. PDF
40) Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis. M.A. Hoque and M.I. Guzman. Materials (2018), 11 (10), 1190, DOI: 10.3390/ma11101990. PDF
39) An Overview of Dynamic Heterogeneous Oxidations in the Troposphere. E.A. Pillar-Little and M.I. Guzman. Environments (2018), 5 (9), 104, DOI: 10.3390/environments5090104. PDF
38) Cross Photoreaction of Glyoxylic and Pyruvic Acids in Model Aqueous Aerosol. S.-S. Xia, A.J. Eugene, and M.I. Guzman. Journal of Physical Chemistry A (2018), 122, 6457-6466, DOI: 10.1021/acs.jpca.8b05724. PDF
37) Enhanced Acidity of Acetic and Pyruvic Acids on the Surface of Water. A.J. Eugene, E.A. Pillar, A.J. Colussi, and M.I. Guzman. Langmuir (2018), 34, 9307-9313, DOI: 10.1021/acs.langmuir.8b01606. PDF
36) Reply to "Comment on 'Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid.'" A.J. Eugene and M.I. Guzman. Journal of Physical Chemistry A (2017), 121, 8741-8744, DOI: 10.1021/acs.jpca.7b08273. PDF
35) Unmanned Aerial Systems for Monitoring Trace Tropospheric Gases. T.J. Schuyler and M.I. Guzman, Atmosphere (2017), 8 (10), 206, DOI:10.3390/atmos8100206. PDF
34) Cu2O/TiO2 heterostructures for CO2 reduction through a direct Z-scheme: Protecting Cu2O from photocorrosion. M.E. Aguirre, R. Zhou, A.J. Eugene, M.I. Guzman, and M.A. Grela. Applied Catalysis B: Environmental (2017), 217, 485-493, DOI: 10.1016/j.apcatb.2017.05.058. PDF
33) Oxidation of Substituted Catechols at the Air-Water Interface: Production of Carboxylic Acids, Quinones, and Polyphenols. E.A. Pillar and M.I. Guzman. Environmental Science and Technology (2017), 51, 4951-4959, DOI: 10.1021/acs.est.7b00232. PDF
32) Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid. A.J. Eugene and M.I. Guzman. Journal of Physical Chemistry A (2017), 121, 2924-2935, DOI: 10.1021/acs.jpca.6b11916. PDF
31) Catalyzed Synthesis of Zinc Clays by Prebiotic Central Metabolites. R. Zhou, K. Basu, H. Hartman, C.J. Matocha, S.K. Sears, H. Vali, and M.I. Guzman. Scientific Reports (2017), 7, 533. DOI: 10.1038/s41598-017-00558-1. PDF
30) Nitrate Radicals and Biogenic Volatile Organic Compounds: Oxidation, Mechanisms and Organic Aerosol. N.L. Ng, S.S. Brown, A.T. Archibald, E. Atlas, R.C. Cohen, J.N. Crowley, D.A. Day, N.M. Donahue, J.L. Fry, H. Fuchs, R.J. Griffin, M.I. Guzman, H. Herrmann, A. Hodzic, Y. Iinuma, J.L. Jimenez, A. Kiendler-Scharr, B.H. Lee, D.J. Luecken, J. Mao, R. McLaren, A. Mutzel, H.D. Osthoff, B. Ouyang, B. Picquet-Varrault, U. Platt, H.O.T. Pye, Y. Rudich, R.H. Schwantes, M. Shiraiwa, J. Stutz, J.A. Thornton, A. Tilgner, B.J. Williams, R.A. Zaveri. Atmospheric Chemistry and Physics (2017), DOI: 10.5194/acp-17-2103-2017. PDF
28) Photocatalytic Reduction of Fumarate to Succinate on ZnS Mineral Surfaces. R. Zhou and M.I. Guzman. Journal of Physical Chemistry C (2016), 120, 7349-7357, DOI: 10.1021/acs.jpcc.5b12380, 2016. PDF
26) Secondary Organic Aerosol (SOA) Formation from β-pinene + NO3 System: Effects of Humidity and Peroxy Radical Fate. C.M. Boyd, J. Sanchez, L. Xu, A.J. Eugene, T. Nah, W.-Y. Tuet, M.I. Guzman, and N.L. Ng. Atmospheric Chemistry and Physics (2015), 15, 7497–7522, DOI: 10.5194/acp-15-7497-2015. PDF
25) Catechol oxidation by ozone and hydroxyl radicals at the air-water interface. E.A. Pillar, R.C. Camm, and M.I. Guzman. Environmental Science & Technology (2014), 48, 14352-14360, DOI: 10.1021/es504094x. PDF
23) A review of air-ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow. T. Bartels-Rausch, H.-W. Jacobi, T.F. Kahan, J.L. Thomas, E.S. Thomson, J.P.D. Abbatt, M. Ammann, J.R. Blackford, H. Bluhm, C. Boxe, F. Domine, M.M. Frey, I. Gladich, M.I. Guzman, D. Heger, Th. Huthwelker, P. Klan, W.F. Kuhs, M.H. Kuo, S. Maus, S.G. Moussa, V.F. McNeill, J.T. Newberg, J.B.C. Pettersson, M. Roeselova, J.R. Sodeau. Atmospheric Chemistry and Physics (2014), 14, 1587-1633, DOI: 10.5194/acp-14-1587-2014. PDF or PDF
22) Negative production of acetoin in the photochemistry of aqueous pyruvic acid. A.J. Eugene, S.-S. Xia, and M.I. Guzman. Proceedings of the National Academy of Science of the United States of America (2013), 110, E4274-E4275, DOI: 10.1073/pnas.1313991110. PDF
21) Conversion of iodide to hypoiodous acid and iodine in aqueous microdroplets exposed to ozone. E.A. Pillar, M.I. Guzman, and J.M. Rodriguez. Environmental Science & Technology (2013), 47, 10971-10979, DOI: 10.1021/es401700h. PDF
20) Organics in Environmental Ices: Sources, Chemistry, and Impacts. V.F. McNeill, A.M. Grannas, J.P.D. Abbatt, M. Ammann, P. Ariya, T. Bartels-Rausch, F. Domine, D.J. Donaldson, M.I. Guzman, D. Heger, T.F. Kahan, P. Klan, S. Masclin, C. Toubin, D. Voisin. Atmospheric Chemistry and Physics (2012), 12, 9653-9678, DOI: 10.5194/acp-12-9653-2012. PDF
19) Chemisorption on Semiconductors: the Role of Quantum Corrections on the Space Charge Regions in Multiple Dimensions. F. Ciucci, C. de Falco, M.I. Guzman, S. Lee, and T. Honda. Applied Physics Letters (2012), 100, 183106, DOI: 10.1063/1.4709483. PDF
18) Concentration Effects and Ion Properties Controlling the Fractionation of Halides during Aerosol Formation. M.I. Guzman, R.R. Athalye, and J.M. Rodriguez. Journal of Physical Chemistry A (2012), 116, 5428-5435, DOI: 10.1021/jp3011316. PDF
17) Abiotic Photosynthesis: From Prebiotic Chemistry to Metabolism. M.I. Guzman in Origins of Life: The Primal Selforganization. R. Egel et al. (eds.), Springer Verlag, Berlin-Heidelberg (2011), pp 85-105, DOI: 10.1007/978-3-642-21625-1_4, ISBN 978-3-642-21624-4.
16) Second-generation products contribute substantially to the particle-phase organic material produced by β-caryophyllene ozonolysis. Y.J. Li, Q. Chen, M.I. Guzman, C.K. Chan, and S.T. Martin. Atmospheric Chemistry and Physics (2011), 11, 121-132, DOI: 10.5194/acp-11-121-2011. PDF
15) From Prebiotic Chemistry to Metabolic Cycles. M.I. Guzman in Astrobiology: From the Big Bang to Civilizations. G.A. Lemarchand and G.Tancredi (ed.), (2010), pp. 223-247. ISBN 978-92-9089-163-5. Montevideo, UNESCO. PDF
14) Photo-Production of Lactate from Glyoxylate: How Minerals Can Facilitate Energy Storage in a Prebiotic World. M.I. Guzman and S.T. Martin. Chemical Communications (2010), 46, 2265-2267, DOI:10.1039/b924179e. PDF
12) Optical absorptivity versus molecular composition of model organic aerosol matter. A.G. Rincon, M.I. Guzman, M.R. Hoffmann, and A.J. Colussi. Journal of Physical Chemistry A (2009), 113, 10512-10520, DOI: 10.1021/jp904644n. PDF
11) Prebiotic Metabolism: Production by Mineral Photoelectrochemistry of α-Ketocarboxylic Acids in the Reductive Tricarboxylic Acid Cycle. M.I. Guzman and S.T. Martin. Astrobiology (2009), 9, 833-842, DOI:10.1089/ast.2009.0356. PDF
10) Synthesis of Pyrimidines and Triazines in Ice: Implications for the Prebiotic Chemistry of Nucleobases. C. Menor-Salván, M. Ruiz-Bermejo, M.I. Guzman, S. Osuna-Esteban, S. Veintemillas-Verdaguer. Chemistry-A European Journal (2009), 15, 4411-4418, DOI: 10.1002/chem.200802656. PDF
9) Oxaloacetate-to-Malate Conversion by Mineral Photoelectrochemistry: Implications for the Viability of the Reductive Tricarboxylic Acid Cycle in Prebiotic Chemistry. M.I. Guzman and S.T. Martin. International Journal of Astrobiology (2008), 7, 271-278, DOI: 10.1017/S1473550408004291. PDF
8) An overview of snow photochemistry: evidence, mechanisms and impacts. A.M. Grannas, A.E. Jones, J. Dibb, M. Ammann, C. Anastasio, H. Beine, M. Bergin, J. Bottenheim, C.S. Boxe, G. Carver, J.H. Crawford, F. Domine, M.M. Frey, M.I. Guzman, D. Heard, D. Helmig, M.R. Hoffmann, R.E. Honrath, L.G. Huey, M. Hutterli, H.W. Jacobi, P. Klan, B. Lefer, J. McConnell, J. Plane, R. Sander, J. Savarino, P.B. Shepson, W.R. Simpson, J. Sodeau, R. von Glasgow, R. Weller, E.W. Wolff, T. Zhu. Atmospheric Chemistry and Physics (2007), 7, 4329-4373, DOI: 10.5194/acp-7-4329-2007. PDF
7) Photolysis of Pyruvic Acid in Ice: Possible Relevance to CO and CO2 Ice Core Record Anomalies. Guzman M.I., M.R. Hoffmann, and A.J. Colussi. Journal of Geophysical Research (2007), 112, D10123, DOI: 10.1029/2006JD007886. PDF
6) Cooperative Hydration of Pyruvic Acid in Ice. M.I. Guzman, L. Hildebrandt, A.J. Colussi, and M.R. Hoffmann. Journal of the American Chemical Society (2006), 128, 10621-10624, DOI: 10.1021/ja062039v. PDF
4) Photoinduced Oligomerization of Aqueous Pyruvic Acid. M.I. Guzman, A.J. Colussi, and M.R. Hoffmann. Journal of Physical Chemistry A (2006), 110, 3619-3626, DOI: 10.1021/jp056097z. PDF
3) Photogeneration of Distant Radical Pairs in Aqueous Pyruvic Acid Glasses. M.I. Guzman, A.J. Colussi, and M.R. Hoffmann. Journal of Physical Chemistry A (2006), 110; 931-935, DOI: 10.1021/jp053449t. PDF
2) Characterization of the effect of white lead on some properties of proteinaceous binding media. SA Centeno, M.I. Guzman, A. Yamazaki-Kleps and C.O. Della Védova. Journal of the American Institute for Conservation (2004), 43, 139-150, DOI: 10.2307/4129649. PDF
1) Synthesis, stereochemistry and absolute configuration of deodarols and deodarones. M.B. Villecco, L.R. Hernandez, M.I. Guzman, C.A.N. Catalán, M.A. Bucio and P. Joseph-Nathan. Tetrahedron: Asymmetry (2001), 12 (21), 2947-2953, DOI: 10.1016/S0957-4166(01)00521-3. PDF