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Substantial amounts of secondary organic aerosol (SOA) can be formed from isoprene epoxydiols (IEPOX), which are oxidation products of isoprene mainly under low- NO conditions. Total IEPOX-SOA, which may include SOA formed from other parallel isoprene oxidation pathways, was quantified by applying positive matrix factorization (PMF) to aerosol mass spectrometer (AMS) measurements. The IEPOX-SOA fractions of organic aerosol (OA) in multiple field studies across several continents are summarized here and show consistent patterns with the concentration of gas-phase IEPOX simulated by the GEOS-Chem chemical transport model. During the Southern Oxidant and Aerosol Study (SOAS), 78% of PMF-resolved IEPOX-SOA is accounted by the measured IEPOX-SOA molecular tracers (2-methyltetrols, C5-Triols, and IEPOX-derived organosulfate and its dimers), making it the highest level of molecular identification of an ambient SOA component to our knowledge. An enhanced signal at C5H6O+ (m=z 82) is found in PMF-resolved IEPOX-SOA spectra. To investigate the suitability of this ion as a tracer for IEPOX-SOA, we examine fC5H6O (fC5H6O D C5H6O+=OA) across multiple field, chamber, and source data sets. A background of ~1.7±0.1% (%Dparts per thousand) is observed in studies strongly influenced by urban, biomass-burning, and other anthropogenic primary organic aerosol (POA). Higher background values of 3.1±0.6% are found in studies strongly influenced by monoterpene emissions. The average laboratory monoterpene SOA value (5.5±2.0 %) is 4 times lower than the average for IEPOX-SOA (22±7 %), which leaves some room to separate both contributions to OA. Locations strongly influenced by isoprene emissions under low-NO levels had higher fC5H6O (~6.5±2.2% on average) than other sites, consistent with the expected IEPOX-SOA formation in those studies. fC5H6O in IEPOX-SOA is always elevated (12-40 %) but varies substantially between locations, which is shown to reflect large variations in its detailed molecular composition. The low fC5H6O (<3 %) reported in non-IEPOX-derived isoprene-SOA from chamber studies indicates that this tracer ion is specifically enhanced from IEPOX-SOA, and is not a tracer for all SOA from isoprene. We introduce a graphical diagnostic to study the presence and aging of IEPOX-SOA as a triangle plot of fCO2 vs. fC5H6O. Finally, we develop a simplified method to estimate ambient IEPOX-SOA mass concentrations, which is shown to perform well compared to the full PMF method. The uncertainty of the tracer method is up to a factor of ~2, if the fC5H6O of the local IEPOX-SOA is not available. When only unit mass-resolution data are available, as with the aerosol chemical speciation monitor (ACSM), all methods may perform less well because of increased interferences from other ions at m=z 82. This study clarifies the strengths and limitations of the different AMS methods for detection of IEPOXSOA and will enable improved characterization of this OA component. [ABSTRACT FROM AUTHOR]