Diurnal Variability and Emission Pattern of Decamethylcyclopentasiloxane (D5) from the Application of Personal Care Products in Two North American Cities

Decamethylcyclopentasiloxane (D-5) is a cyclic volatile methyl siloxane (cVMS) that is widely used in consumer products and commonly observed in urban air. This study quantifies the ambient mixing ratios of D-5 from ground sites in two North American cities (Boulder, CO, USA, and Toronto, ON, CA). From these data, we estimate the diurnal emission profile of D-5 in Boulder, CO. Ambient mixing ratios were consistent with those measured at other urban locations; however, the diurnal pattern exhibited similarities with those of traffic-related compounds such as benzene. Mobile measurements and vehicle experiments demonstrate that emissions of D-5 from personal care products are coincident in time and place with emissions of benzene from motor vehicles. During peak commuter times, the Ds/benzene ratio (w/w) is in excess of 0.3, suggesting that the mass emission rate of D-5 from personal care product usage is comparable to that of benzene due to traffic. The diurnal emission pattern of D-5 is estimated using the measured Ds/benzene ratio and inventory estimates of benzene emission rates in Boulder. The hourly D-5 emission rate is observed to peak between 6:00 and 7:00 AM and subsequently follow an exponential decay with a time constant of 9.2 h. This profile could be used by models to constrain temporal emission patterns of personal care products.