The 12th International Conference on “Instrumental Methods of Analysis” IMA-2021 was successfully held, organized by the Aristotle University of Thessaloniki (Prof. M. Mitrakas, Analytical Chemistry Laboratory, Dep. Chemical Engineering) and the National Technical University of Athens (Prof. Maria Ochsenkühn-Petropoulou and Assist. Prof. Fotis Tsopelas, Laboratory of Inorganic and Analytical Chemistry, School of Chemical Engineering) during 20-23 September 2021. The conference, which took place in virtual mode for the first time, hosted the recent progress in the field of Instrumental Chemical Analysis and the modern trends in the development and application of analytical methods in cutting-edge sectors such as the environment, pharmaceutics, life sciences, food and materials. The program of the 4-day event attended by 260 participants from 23 countries,included 14 invited speakers, 73 oral presentations and 98 poster contributions. Special sessions focused on aerosol metrology (supported by EU Project AEROMET II), advanced X-ray techniques (supported by the European X-ray Spectrometry Association) and virus spread analytics were also organized within the frame of IMA-2021. Details on the program of the conference and the book of abstracts which is going to become available soon through the journal MDPI Applied Sciences can be found in the website (www.ima2021.gr). In the special session on aerosol metrology, 8 oral presentation from AEROMET II partners were included, with two plenary lectures given by the coordinator of the project, Dr. Burkhard Beckhoff and Dr. Konstantinos Eleftheriadis (NCSR Demokritos, GR). In the following the titles, names and affiliations of the presentations, as well as the link to the abstracts are shown, according to the presentation order: 1.Traceable analysis of aerosols and advanced materials at the nano- and microscales by X-ray spectrometry B.Beckhoff, K. Bzheumikhova, P. Hönicke, Y. Kayser, C. Stadelhoff, R. Unterumsberger, A. Wählisch, N. Wauschkuhn, J. Weser, and C. Zech Physikalisch-Technische Bundesanstalt (PTB), Abbestraße 2-12, 10587 Berlin, Germany 2. Element Mass Concentrations in Ambient Aerosols, a Comparison of Results from Filter Sampling / ICP-MS and Cascade Impactor Sampling / Mobile Total Reflection X-ray Fluorescence Spectroscopy S.Seeger1, J. Osan2, O. Czömpöly2, A. Gross3, H. Stosnach3, L. Stabile4, M. Ochsenkuehn-Petropoulou5, L. A. Tsakanika5, T. Lymperopoulou5, S. Goddard6, M. Fiebig7, F. Gaie-Levrel8, Y. Kayser9, B. Beckhoff9 1Bundesanstalt für Materialforschung und -Prüfung BAM, D-12203 Berlin, Germany 2Centre for Energy Research (EK), H-1121 Budapest, Hungary 3Bruker Nano GmbH, D-12489 Berlin, Germany 4Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio UNICAS, 03043 Cassino, Italy 5Laboratory of Inorganic and Analytical Chemistry, School of Chemical Engineering, National Technical University Athens NTUA, Iroon Polytechniou 9, 15773 Athens, Greece 6National Physical Laboratory, NPL, Teddington TW11 0LW, UK 7Norwegian Institute for Air Research NILU, N-2027 Kjeller, Norway 8Laboratoire National de Métrologie et d’Essais, 75724 LNE Paris, France 9Physikalisch-Technische Bundesanstalt, PTB, D-38116 Braunschweig, Germany 3.X-ray fluorescence and absorption methods for identifying sources of urban aerosol pollution episodes J.Osán1, O. Czömpöly1, V. Groma1, E. Börcsök1 and S. Pollastri2 1Environmental Physics Department, Centre for Energy Research, Konkoly-Thege M. út 29-33., H-1121 Budapest, Hungary 2Elettra Sincrotrone Trieste, Strada Statale 14 – km 163,5, I-34149 Basovizza, Trieste, Italy 4.NPL Studies into dental surgery airborne particulates for COVID-19 mitigation J.T. Tompkins, D.M. Butterfield, A.S. Brown Air Quality and Aerosol Metrology Group, National Physical Laboratory, Teddington, TW11 0LW, UK 5.The role of aerosol science on understanding and preventing SARS-CoV-2 transmission in the community K.Eleftheriadis NCSR “Demokritos”, 15310 Ag. Paraskevi, Athens, Greece 6.Total reflection X-ray fluorescence reference materials for cascade impactor air quality monitoring systems L.Vigna1, M. Gottschalk2, E. Cara3, F. Ferrarese-Lupi3, A. Verna1, S. L. Marasso1,4, S. Seeger2, L. Boarino3, N. De Leo3, F. C. Pirri1,5 and M. Cocuzza1,4 1Politecnico di Torino, Torino, 10129, Italy 2Bundesanstalt für Materialforschung und -prüfung (BAM), Berlin, 12489, Germany 3Istituto Nazionale di Ricerca Metrologica (INRiM), Torino, 10135, Italy 4CNR-IMEM, Parma, 43124, Italy 5Istituto Italiano di Tecnologia, Center for Sustainable Future Technologies, Torino, 10144, Italy 7.Experimental measurement protocol of aerosol deposition relating to a bend in a large ventilation network with rectangular ducts Costa1, 2, J. Malet1 and E. Géhin2 1Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PSN-RES, SCA, LEMAC, Gif-sur-Yvette, 91192, France 2Univ Paris Est Creteil, CERTES, F-94000 Creteil, France 8. Aethalometer multiple scattering correction in an Arctic environment S.Vratolis, K. Eleftheriadis ERL, Institute of Nuclear & Radiological Sciences & Technology, Energy & Safety, National Centre for Scientific Research “Demokritos”, 15310 Ag. Paraskevi, Attiki, Greece.