Secondary organic aerosols from OH oxidation of cyclic volatile methyl siloxanes as an important Si source in the atmosphere

Abstract

Cyclic volatile methyl siloxanes (cVMSs), which can volatilize into the atmosphere, are active ingredients in widely used consumer products, thus attracting much attention due to their potential environmental risks. While in the atmosphere the cVMSs undergo oxidation, yielding both gaseous and particulate products. The aerosol yields and compositions from the OH oxidation of four cVMSs (D3-D6) were determined under low- and high-NOx conditions in an oxidation flow reactor. The aerosol yields increased progressively from D3 to D6, consistent with the volatilities and molecule weights of these cVMSs. NOx can restrict the formation of secondary organic aerosols (SOAs), leading to lower SOA yields under high-NOx conditions than under low-NOx conditions, with a yield decrease between 0.05-0.30 depending on the cVMSs. Ammonium sulfate seeds exhibited minor impacts on SOA yields under low-NOx conditions but significantly increased the SOA yields in the oxidation of D3-D5 at short photochemical ages under high-NOx conditions. The mass spectra of the SOAs showed a dependence of its chemical compositions on OH exposure. At high exposures equivalent to photochemical ages of >4ĝ€¯d in the atmosphere, D4-D6 SOAs mainly consisted of CxHy and CxHyOzSin under low-NOx conditions, whereas they primarily contained NmOz, CxHy, CxHyO1, CxHyO>1 and CxHyOzSin under high-NOx conditions. The potential contributions of cVMSs to SOA formation in the atmosphere were evaluated using the reported cVMSs annual production and the yield obtained in the present study. A global cVMS-derived (D4-D6) SOA source strength is estimated to be 0.01ĝ€¯Tgĝ€¯yr-1, distributed over major urban centers.