EGUsphere EGUsphere EGUsphere EGUsphere Copernicus Publications Göttingen, Germany 10.5194/egusphere-2026-3224 Gravimetrically Measured Water Content of Filter Sampled Fine Particulate Matter at Low Relative Humidity: Insight from the Surface Particulate Matter Network (SPARTAN) Oxford Christopher R. https://orcid.org/0000-0003-4799-9141 1 Zhu Haihui https://orcid.org/0009-0005-4259-0277 1 2 Mehrotra Maya 1 Liu Xuan 1 3 Ren Yuxuan https://orcid.org/0000-0001-6608-9501 1 Arnott Maya 1 Adimula Isaac Abionum 4 Ajibolataiye Taiye Benjamin 4 Akoshile Clement 4 Amador-Munoz Omar https://orcid.org/0000-0002-4020-7183 5 Asfaw Araya 6 Chang Rachel Ying-Wen https://orcid.org/0000-0003-2337-098X 7 Dey Sagnik https://orcid.org/0000-0002-0604-0869 8 9 Dillner Ann M. 10 Diner David J. 11 Flynn Connor J. 12 Francis Diana https://orcid.org/0000-0002-7587-0006 13 Gahungu Paterne 14 Garland Rebecca M. https://orcid.org/0000-0002-1855-8622 15 16 Grutter Michel https://orcid.org/0000-0001-9800-5878 5 Hasheminassab Sina 11 Imam Fahad 8 Kim Jhoon https://orcid.org/0000-0002-1508-9218 17 Langerman Kristy 18 Lee Pei-Chen 19 Lestari Puji 20 Lin Po-Hsiung 21 Loria-Salazar S. Marcela 12 Mamo Tesfaye 22 Mayol-Bracero Olga L. 23 Naidoo Mogesh 15 Nelli Narendra https://orcid.org/0000-0003-0066-6040 13 Park Sang Seo 24 Salam Abdus https://orcid.org/0000-0002-5609-6828 25 Sarangi Bighnaraj https://orcid.org/0000-0002-7876-5383 23 Saud Trailokya 26 Schofield Robyn https://orcid.org/0000-0002-4230-717X 27 Schechner Yoav 28 Tripathi Sachchida N. https://orcid.org/0000-0002-6402-4680 29 West Emily K. https://orcid.org/0009-0005-3241-6320 12 Windwer Eli 30 Wu Ming-Tsang 31 32 Zhang Qiang 33 Brauer Michael 34 Rudich Yinon https://orcid.org/0000-0003-3149-0201 30 Turner Jay R. 1 Martin Randall V. https://orcid.org/0000-0003-2632-8402 1 Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80521, United States Scripps Institution of Oceanography, University of California San Diego, San Diego, California 92093, United States Department of Physics, University of Ilorin, Ilorin, 240003, Nigeria Instituto de Ciencias de la Atmósfera y Cambio Climático, Universidad Nacional Autónoma de México, Mexico City, 04510, Mexico Institute of Geophysics and Space Science, Addis Ababa University, Addis Ababa,1176, Ethiopia Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India 110016 Adjunct Faculty, Department of Health, Management and Policy, Korea University, Seoul, South Korea Air Quality Research Center, University of California Davis, Davis, CA, USA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States School of Meteorology, University of Oklahoma, Norman, Oklahoma, United States Environmental and Geophysical Sciences Lab, Earth Science Department, Khalifa University, Abu Dhabi, 127788, United Arab Emirates Institute of Applied Statistics, University of Burundi, Bujumbura, BP1550, Burundi Council for Scientific and Industrial Research, Pretoria, 0001, South Africa Department of Geography, Geo-Informatics and Meteorology, University of Pretoria, Pretoria, 0002, South Africa Department of Atmospheric Sciences, Yonsei University, Seoul, 03722, Republic of Korea Department of Geography, Environmental Management and Energy Studies, University of Johannesburg, Johannesburg, 2006, South Africa Department of Public Health, National Cheng Kung University, Tainan, 701, Taiwan Faculty of Civil and Environmental Engineering, Bandung Institute of Technology, Bandung, 40132, Indonesia Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan Department of Physics, Addis Ababa University, Addis Ababa, Ethiopia Department of Environmental Science, University of Puerto Rico, Puerto Rico, 00931, United States Department of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan, 44919, Republic of Korea Department of Chemistry, University of Dhaka, Dhaka, 1000, Bangladesh National Aerosol Facility, Indian Institute of Technology Kanpur, Kanpur, 208016, India School of Geography, Earth and Atmospheric Sciences, University of Melbourne, Melbourne, 3010, Australia Department of Electrical Engineering, Technion Israel Institute of Technology, Haifa, 3200003, Israel Kotak School of Sustainability, Indian Institute of Technology Kanpur, Kanpur, 208016, India Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, 76100, Israel Department of Family Medicine, Kaohsiung Medical University Hospital, Kaohsiung Medical University, Kaohsiung, 807, Taiwan PhD Program in Environmental and Occupational Medicine, Kaohsiung Medical University, Kaohsiung, 807, Taiwan School of Environment, Tsinghua University, Beijing, 100084, China School of Population and Public Health, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada 22 06 2026 2026 1 30 Copyright: © 2026 Christopher R. Oxford et al. 2026 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3224/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3224/egusphere-2026-3224.pdf

Accurate measurements of the composition of fine particulate matter (PM<sub>2.5</sub>) are important for understanding its sources and health impacts. Water, a portion of PM<sub>2.5</sub> mass, is difficult to measure. We describe developments to the Surface PARTiculate mAtter Network (SPARTAN) to better characterize the chemical composition of PM<sub>2.5</sub> on polytetrafluoroethylene filters. A robotic weighing facility is used to estimate water content of PM<sub>2.5</sub> sampled at three sites (Fajardo, Puerto Rico; Bujumbura, Burundi; Abu Dhabi, United Arab Emirates), and the chemical composition is used to attribute water content to three hygroscopic categories: high growth (sodium chloride), medium growth (ammonium-nitrate-sulfate-potassium), and low growth (organics). The growth rates are tested using two additional sites (Beijing, China; Halifax, Canada). The water content at 35 % relative humidity (RH) is estimated to be 16.5 % (11.6 %&ndash;26.5 %, 95 % confidence) for the high growth category, 3.9 % (3.0 %&ndash;4.2 %, 95 % confidence) for medium growth, and 1.0 % (0.6&ndash;1.3 %, 95 % confidence) for low growth. We calculate the average water content at 35 % RH for 2442 filters from 24 globally distributed sites from December 2019 to September 2024 to be 2.06 % (0.85 %&ndash;5.47 %, range), where 58 % of the aerosol water is associated with the medium growth category (22 % low growth, 19 % high growth).