Scientists have an array of analytical and computational tools to help monitor airborne pollutants such as volatile organic compounds (VOCs). But researchers at Peking University, in China, and the University of Colorado, Boulder, have discovered that photochemistry occurring while a pollutant molecule travels from its source to a monitoring instrument can interfere with the accuracy of a popular data analysis method (J. Geophys. Res.: Atmos., DOI: 10.1029/2012jd018236). Min Shao and colleagues used gas chromatography and mass spectrometry to monitor VOC emissions in Beijing, relying on a data-processing technique known as positive matrix factorization (PMF). Although some methods in the same class as PMF try to take into account atmospheric photochemical reactions, such interferences in the more commonly used PMF method have not been well recognized. For example, with the Beijing data, PMF failed to completely separate human and nonhuman VOC sources. The method also mistakenly identified some VOCs as coming from separate sources, when they were actually from the same source but in different oxidation states. The researchers caution that when using PMF-based results, including those already reported in the literature, photochemical influences should be taken into account.