Physical properties of secondary photochemical aerosol from OH oxidation of a cyclic siloxane

Abstract

Cyclic volatile methyl siloxanes (cVMS) are high-production chemicals present in many personal care products. They are volatile, hydrophobic, and relatively long-lived due to slow oxidation kinetics. Evidence from chamber and ambient studies indicates that oxidation products may be found in the condensed aerosol phase. In this work, we use an oxidation flow reactor to produce ~ 100 μgm^-3 of organosilicon aerosol from OH oxidation of decamethyl-cyclopentasiloxane (D₅) with aerosol mass fractions (i.e., yields) of 0.2-0.5. The aerosols were assessed for concentration, size distribution, morphology, sensitivity to seed aerosol, hygroscopicity, volatility and chemical composition through a combination of aerosol size distribution measurement, tandem differential mobility analysis, and electron microscopy. Similar aerosols were produced when vapor from solid antiperspirant was used as the reaction precursor. Aerosol yield was sensitive to chamber OH and to seed aerosol, suggesting sensitivity of lower-volatility species and recovered yields to oxidation conditions and chamber operation. The D₅ oxidation aerosol products were relatively non-hygroscopic, with an average hygroscopicity kappa of ~ 0.01, and nearly non-volatile up to 190 °C temperature. Parameters for exploratory treatment as a semi-volatile organic aerosol in atmospheric models are provided.

Figure 1.

Flow diagram for generation of aerosols in the OFR. Aerosols were analyzed by SMPS, TPS100, V-TDMA, and DMT-CCN instruments. Delivery of the precursor gas was either by diffusion of liquid D₅ controlled by a water bath or flowing air past a personal care product placed in an Erlenmeyer flask. Short dashed lines in the diagram indicate Teflon tubing, long dashed lines represent copper tubing, and solid lines represent conductive silicon tubing.

Figure 2.

SMPS time series of a typical yield experiment. An equilibration period was run for ~ 20 h prior to measuring the D₅ gas concentration upstream and downstream of the OFR (yield experiment period ~ 1.7 h). Aerosol measurements used for yield analysis were from the yield experiment period. After the yield experiment period, the precursor gas was switched to SO₂ for OH quantification. An SO₂ monitor was used to measure SO₂ downstream of the chamber with and without the OFR lights on.

Figure 3.

Measured D₅ oxidation aerosol yield as a function of (a) ROG (reacted D₅), (b) equivalent age assuming an OH concentration of 1.5 × 10⁶ molec cm⁻³, and (c) aerosol mass. Data points are color coded according to OH exposure.

Figure 4.

STEM-EDS analysis of D₅ oxidation aerosols and antiperspirant oxidation aerosols obtained from analysis of TPS100 samples.

Figure 5.

D₅ oxidation aerosol CCN activation curve. Size-specific κ_a and κ_t are tabulated for particles 70–200 nm. The calculated kappa parameter range and average (in parentheses) are tabulated in the upper left. Each point represents an average 30 s CCN/CPC measurement.

Figure 6.

Size-resolved kappa parameters for D₅ oxidation aerosols. Error bars represent the 95% confidence interval.

Figure 7.

Two-product model fit (black long dash) and one-product model fit with a saturation concentration (c*) of 1 μg m⁻³ (red short dash) overlaid with chamber data from this work and D₅ oxidation experiments of Wu and Johnston (2017). Parameters for the two-product model fit are yields of 0.14 and 0.82, respectively, for c* of 0.95 and 484 μg m⁻³. Parameters for the one-product model are a c* of 1 μg m⁻³ and a yield of 0.25.