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

. Cyclic volatile methyl siloxanes (cVMSs), which can volatilize into the atmosphere, are active in-gredients 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-NO x 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. NO x can restrict the formation of secondary organic aerosols (SOAs), leading to lower SOA yields under high-NO x conditions than under low-NO x 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-NO x conditions but signiﬁcantly increased the SOA yields in the oxidation of D3–D5 at short photochemical ages under high-NO x 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 > 4 d in the atmosphere, D4–D6 SOAs mainly consisted of C x H y and C x H y O z Si n under low-NO x conditions, whereas they primarily contained N m O z , C x H y , C x H y O 1 , C x H y O > 1 and C x H y O z Si n under high-NO x 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 0.01 Tg yr − 1 , distributed over major urban centers.


Text S1. Si/O and Si/C ratios of CxHyOzSin in D5 and D6 SOAs
The calculation processes of the Si/O and Si/C ratios at each PA are shown as follows: (1) The normalized peak intensity of CxHyOzSin (Cx1Hy1Oz1Sin1, Cx2Hy2Oz2Sin2 …CxiHyiOziSini) is obtained from the HR-ToF-AMS, which is named as A1, A2…Ai, respectively.
(2) The fraction of each CxHyOzSin ion (F1, F2, F3…Fi) is calculated by Equation S1, Fi = Ai / SUM (A1, A2, A3…Ai) (S1) (3) The Si/O and Si/C ratio of CxHyOzSin ions at each equivalent day are calculated by Equation S2-S6, where M is the molar mass of one specific element (Si, O, C and H) or CxHyOzSin ions; m is the total mass of Si, O or C in all CxHyOzSin ions.For the calculation results, there may be some uncertainties due to the assignments of peaks in the HR-ToF-AMS and the fragmentation processes of the AMS ionization (Aiken et al., 2007).

Figure S1 .
Figure S1.Details of experimental setup

Figure S3 .
Figure S3.Number and mass size distributions of D5 SOAs measured by SMPS under (a) low-NOx

Figure S4 .
Figure S4.The relationship between SOA yields and mass concentration for cVMSs in low-NOx

Figure S5 .
Figure S5.Comparison of low-NOx (black) and high-NOx (red) SOA yields for cVMSs (a-d) for unseeded

Figure
Figure S7.HR-ToF-AMS mass spectra of D4 (a) and D5 (b) in high m/z range at OH exposure of 9.0 ×

Figure S8 .
Figure S8.The weighted average values of Si/C (n/x) and Si/O (n/z) ratios for CxHyOzSin groups in SOAs

Figure S11 .
Figure S11.The weighted average values of Si/C (n/x) and Si/O (n/z) ratios for CxHyOzSin groups in SOAs

Table S1 .
Formula and structures of cyclic volatile methyl siloxanes

Table S2 .
The concentration of cVMSs at the reactor inlet and outlet.
Note: 'i' and 'o' means the reactor inlet and outlet, respectively; the OH exposure is 1.85×10 12 and 1.10×10 12 molecules cm -3 s in low and high-NOx experiments, respectively.

Table S3 .
Background data of SMPS at different OH exposures in low and high-NOx experiments without

Table S4 .
The mass concentrations of D5 SOAs on the basis of effective aerosol density and average SMPS data in the last 10 minutes at different OH exposures under unseeded conditions in low-NOx

Table S5 .
Comparison of experimental results and conditions for D5 SOAs in this study with those in other literatures

Table S6 .
Details about sites and concentrations of cVMS SOAs.The concentrations of cVMS SOAs were calculated on the basis of the cVMS concentrations reported from multiple literatures and SOA yields measured in this work.Here, the chosen concentrations of cVMSs were maximum that have been reported in each site.
Note: '--' means that the data was not obtained for this site; 'PO' means polar sites; 'BA' means background sites; 'UR' means urban sites.