Identification of chemistry-dependent artifacts on gravimetric PM fine readings at the T1 site during the MILAGRO field campaign. M. Moya 1,2 , S. Madronich 3 , A. Retama 4 , R. Weber 5 , K. Baumann 6 , A. Nenes 5,7 , M. Castillejos 8 , C. Ponce de León 2,9 . [1] Instituto Nacional de Enfermedades Respiratorias Ismael Cosio Villegas. Calz. Tlalpan 450, Section XVI, CP 14080. Tlalpan. Mexico City, Mexico. [2] Posgrado en Ciencias Químicas, UNAM. Mexico City, México. [3] National Center for Atmospheric Research. 1850 Table Mesa Drive. Boulder, Colorado, 80305-5602, USA. [4] Dirección de Monitoreo Atmosférico. Gobierno del Distrito Federal, Mexico City, Mexico. [5] School of Earth and Atmospheric Science. Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0100, USA [6] Atmospheric Research and Analysis, Inc. North Carolina, USA. [7] Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0100, USA [8] Universidad Autónoma Metropolitana, Mexico City, Mexico. [9]Now at the Facultad de Ciencias, UNAM. Mexico City, México.