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ABSTRACT Rate coefficients, k(T), for the gas‐phase Cl atom reaction with hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, L2), k1; octamethyltrisiloxane ([(CH3)3SiO]2Si(CH3)2, L3), k2; decamethyltetrasiloxane ((CH3)3SiO[Si(CH3)2O]2Si(CH3)3, L4, k3; dodecamethylpentasiloxane ((CH3)3SiO[Si(CH3)2O]3Si(CH3)3, L5, k4; hexamethylcyclotrisiloxane ([‐Si(CH3)2O‐]3, D3), k5; octamethylcyclotetrasiloxane ([‐Si(CH3)2O‐]4, D4), k6; decamethylcyclopentasiloxane ([‐Si(CH3)2O‐]5, D5, k7), and dodecamethylcyclohexasiloxane ([‐Si(CH3)2O‐]6, D6, k8) were measured over a range of temperature (273–363 K) using a pulsed laser photolysis (PLP) – resonance fluorescence (RF) technique. The obtained k(296 K) and Arrhenius expressions with 2σ uncertainties including estimated systematic errors are (in units of 10−10 cm3 molecule−1 s−1): L2: k1(296 K) = (1.58 ± 0.07) k1(273–363 K) = (1.36 ± 0.10) exp((43 ± 249)/T) L3: k2(296 K) = (1.93 ± 0.09) k2(273–353 K) = (1.95 ± 0.18) exp((1 ± 632)/T) L4: k3(296 K) = (2.36 ± 0.12) k3(273–353 K) = (2.09 ± 0.16) exp((43± 522)/T) L5: k4(296 K) = (2.84 ± 0.19) k4(296–353 K) = (2.70 ± 0.18) exp((15 ± 490)/T) D3: k5(296 K) = (0.588 ± 0.028) k5(273–353 K) = (0.47 ± 0.03) exp((78 ± 470)/T) D4: k6(296 K) = (1.13 ± 0.06) k6(273–353 K) = (1.14 ± 0.07) exp((4 ± 432)/T) D5: k7(296 K) = (1.72 ± 0.09) k7(273–353 K) = (1.60 ± 0.10) exp((22 ± 432)/T) D6: k8(296 K) = (2.16 ± 0.14) k8(296–353 K) = (1.73 ± 0.14) exp((71 ± 562)/T) The cyclic permethyl siloxanes (cyclic PMS) were found to be less reactive than the analogous linear permethyl siloxane (linear PMS) with an equal number of CH3‐ groups. Both linear and cyclic compounds show a linear relationship between the measured rate coefficient and the number of CH3‐ groups in the molecule. A structure–activity relationship (SAR) is presented that reproduces the experimental data to within ∼10% at all temperatures. For [Cl] ≈ 104 atom cm−3, an approximate free troposphere abundance, the PMS loss due to Cl atom reaction leads to relatively short estimated lifetimes of 7, 6, 5, 4, 20, 10, 7, and 5 days for L2, L3, L4, L5, D3, D4, D5, and D6, respectively. Therefore, the PMSs included in this study are classified as atmospherically very short‐lived substances and Cl atom reaction represents a significant loss process. [ABSTRACT FROM AUTHOR]

Radiative efficiency (RE) is a key climate metric used in the evaluation of a greenhouse gases global warming potential. However, experimental methods are commonly limited to measurements in the spectral region greater than ∼500 cm−1. In this study, density functional theory with the wB97X-D/def2TZVPPD level of theory was employed to calculate the infrared absorption spectra and the contribution of the 0–500 cm−1 spectral region to the total RE for the nearly 800 greenhouse gases, 20 different classes of primarily halogenated compounds, included in the World Meteorological Organization (WMO) Scientific Assessment of Ozone Depletion Annex [1]. In general, the results indicate that the contribution of the low-frequency regime to the total overall RE is not significant, < 2%, with a few exceptions, e.g. several compounds in the hydrocarbon class. [ABSTRACT FROM AUTHOR]