Atmospheric emerging pollutants of cyclic volatile methyl siloxanes across the Yangtze River Delta region revealed by mobile measurements.

Cyclic volatile methyl siloxanes (cVMS) are a large class of high-production-volume chemicals that have garnered increasing scrutiny due to their potential adverse effects on air quality and human health. Among cVMS, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) dominate global production, with China playing a pivotal role in their manufacture and consumption. Despite their widespread use, the atmospheric occurrence and sources of these compounds in China, even in the Yangtze River Delta (YRD)-the nation's most densely populated and economically developed region-remain largely unknown. Here, we present the first mobile measurements of atmospheric D4 and D5 across the YRD, employing a highly sensitive Vocus Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS). The average concentration of D5 (21.4 ± 20.5 pptv) was comparable to levels found in developed countries, while D4 concentrations (50.1 ± 41.8 pptv) in the YRD were approximately an order of magnitude higher than D4 levels typically reported for urban areas in those same developed countries. The D4/D5 ratio (mean 1.3) was significantly higher than the global average (<1.0), indicating that industrial emissions are a major driver of D4 concentrations in the region. Elevated D4 levels (up to 1119.4 pptv) were observed near industrial facilities, reinforcing their role as prominent localized sources. The spatial distribution of D5, however, was influenced by both population density and the level of economic development (per capita income). These findings offer new insights into the spatial distribution and socioeconomic drivers of atmospheric cVMS in rapidly developing regions, providing a valuable contrast to previous studies primarily focused on developed economies.