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Environmental fate & pathways

Phototransformation in air

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Description of key information

For the purposes of exposure assessment relevant to risk characterisation, a precautionary approach is taken for phototransformation in air:
Phototransformation in air: Rate constant of 1.01 E-12 cm3 molecule-1 second-1 at 24°C (half-life 15.8 days) for reaction with OH radicals (at tropospheric concentration of OH radicals 5E+05 molecule/cm3 over 24-hour period (ECHA, 2016).

However, for the purposes of assessment of properties such as long-range transport potential, the measured concentrations are given priority, suggesting a half-life in the range 1.7 to 2.7 days.

Key value for chemical safety assessment

Half-life in air:
15.8 d
Degradation rate constant with OH radicals:
0 cm³ molecule-1 s-1

Additional information

An experimental relative rates study (Atkinson, 1991) found that the NO3 radical and O3 reactions are of no importance as tropospheric removal processes for this compound. The dominant gas-phase chemical loss process is by reaction with the OH radical, with measured rate constant of 1.01 E-12 cm3 molecule-1 second-1 at 24°C (calculated half-life 15.8 days, using tropospheric concentration of OH radicals 5 +E05 molecule/cm3 over 24-hour period (ECHA, 2016)). It should be noted that the true hydroxyl radical concentration is considered by some authorities to be higher than the default used in EU. Therefore, degradation rates may be higher than the values reported here. However, the Annex to the CSR indicates the half-lives in air necessary to model long-range transport potential.

A further experimental relative rates study (Sommerlade et al., 1993) found a reaction with the OH radical rate constant of 1.26 E-12 cm3 molecule-1 second-1 at 24°C (calculated half-life 12.7 days, using tropospheric concentration of OH radicals 5 E05 molecule/cm3 over 24 -hour period (ECHA, 2016)).

A reaction with the OH radical rate constant of 1.20 E-12 cm3 molecule-1 second-1 (calculated half-life 13.4 days, using tropospheric concentration of OH radicals 5 E05 molecule/cm3 over 24 -hour period (ECHA, 2016)) was obtained using an accepted calculation method (AOPWIN ver. 1.92). The result is considered to be reliable.

Two recent publications (Safron et al., 2015; Xiao et al., 2015) which provide the reaction rate constants of cVMS with the hydroxyl radicals do not agree with the corresponding values from the earlier studies (Atkinson 1991; Sommerlade et al., 1993), and the corresponding half-life of D4 in air could be reduced by 60% using these new but faster reaction rates constants.

Sommerlade et al.(1993) identified the major products from the reaction of D4 with hydroxyl radicals as heptamethylhydroxycyclotetrasiloxane, along with smaller amounts of heptamethyl(hydroperoxymethyl)cyclotetrasiloxane and 1,2- bis(heptamethylcyclotetrasiloxanyl)ethane, and trace amounts of heptamethyl(hydroxymethyl)cyclotetrasiloxane and bis(heptamethylcyclotetrasiloxanyl)ether.

These degradation products are expected to be more soluble in water than D4, and to have a lower vapour pressure, and so are likely to be removed from the atmosphere by wet and dry deposition (Chandra, 1997). Chandramouli and Kamens (2001) confirmed this deposition process for a related substance (decamethylcyclopentasiloxane, D5). In this study nonamethylhydroxycyclopentasiloxane was identified as the main degradation product from D5 using an outdoor smog chamber that contained fine road dust. More than 99% of the hydroxy derivative formed partitioned onto the dust particles.

Whelan et al. (2004) assessed the atmospheric fate of volatile methyl siloxanes (VMS) and their degradation products. The assessment used a simple equilibrium-partitioning model to investigate the relative rates of removal of two representative VMSs (the linear siloxane decamethyltetrasiloxane, L4, and the cyclic siloxane, D4) and their hydroxyl-substituted degradation products by reaction and atmospheric deposition. The modelling is based on the work of Atkinson (1991) and Sommerlade et al. (1993), which demonstrates that siloxanes break down in the atmosphere to form hydroxyl-substituted degradation products by reaction with OH radicals. As substitution proceeds the silanols become increasingly water-soluble and less volatile, and so tend to be washed out of the atmosphere by wet deposition. The silanols are also assumed to undergo hydrolysis reactions when dissolved in liquid water droplets. Removal of the silanols from the atmosphere by dry deposition is also accounted for. The model indicated that L4 and D4 and the monohydroxy degradation products occur mainly in the vapour phase, whereas the further degradation products occur mainly in the dissolved and particulate phases. Overall, it is concluded that >99% of L4 and D4 are removed from the atmosphere as silanols in wet deposition and <1% are removed in dry deposition.

All the aforementioned studies are focused on homogenous reactions. Although they definitely represent the major characteristics of D4 degradation in the atmospheric environment, the real atmosphere is much more complex. For example, the atmosphere contains a combination of multiple oxidants such as O3, OH and other free radicals, as well as UV radiation and aerosols. In addition, cVMS release also follows distinguishable spatial and temporary patterns: they are released mostly to the urban and suburban atmosphere where the O3, OH radical and aerosol concentrations may be much higher than the rural or remote regions. When cVMS are transported from the source region through the air, they move along with those oxidants, which may increase their exposure to the intensified atmospheric degradation processes. The overall half-lives of cVMS therefore may largely depend on the resident time of cVMS in urban and suburban atmosphere.


In order to better understand the environmental fate of cVMS under the more realistic atmospheric conditions, several projects were initiated both at the University Iowa and Dow Corning Corporation. The specific objectives of those studies are twofold: To determine the removal of gas-phase cVMS by combination of multiple oxidants in the presence of UV radiation and aerosols; to determine the effects of the unique release pattern of cVMS and any additional removal mechanism on the overall half-lives of cVMS in the atmosphere.


In University of Iowa studies, the uptake of D4 and D5 vapours (up to saturated vapour pressure) by carbon black and several types of reactive mineral dust aerosols in the absence and presence of O3 and hydroxyl radicals was simulated in an atmospheric chamber at room temperature and monitored by FT-IR spectroscopy (Navea et al., 2009a,b,c) . It was found that the heterogeneous uptake (removal from gas phase) of D4 and D5 by mineral aerosols such as kaolinite, hematite and quartz was rapid and significant (Navea et al., 2009a). Under dry conditions (< 1% RH) in the absence of O3 and hydroxyl radicals, removal of both D4 and D5 by hematite and kaolinite were characterised by two processes. The initial fast removal process completed within one minute accounts for 30 to 50% total removal for D4, but 50~70% for D5. The subsequent slow removal process had a pseudo-first order kinetics with rate constant k at room temperature varying in the range of 0.5~0.9 x 10-3 s-1 (corresponding to half-lives from 24~13 minutes) for D4, and in a range of 0.2~4 x 10-3 s-1 (corresponding to half-lives from 53 minutes to 12 minutes) for D5 for hematite and kaolinite, respectively. The reactivity of various aerosols was in the order, kaolinite > hematite > quartz, for both D4 and D5 after the reaction rates were normalised to surface area of the aerosols.


In the presence of O3 but no solar radiation, no detectable change was observed in the concentration of the gas phase D4 or D5 after 50 minutes exposure (Navea et al., 2009b), consistent with the low rate constant obtained for O3/D4 reactions in the previous study (Atkinson, 1991). Under dry (< 1% RH) conditions, the introduction of aerosols such as hematite and kaolinite triggered immediate removal of both cVMS and O3 (Navea et al., 2009b). The kinetics of the gas phase removal apparently implied multiple heterogeneous processes, significantly different than that obtained with aerosols in the absence of O3. The major differences were in two aspects: First, addition of O3 slowed the uptake of the both D4 and D5 by the aerosols, while existence of D4 and D5 in the gas phase also slowed the decomposition of gas phase O3 relative to the aerosol-containing control with no cVMS, suggesting the competition of cVMS with O3 for surface sites of the aerosols (Navea et al., 2009b). In addition, the constant concentration profile observed at reaction times greater than 200 minutes in the cVMS/aerosol systems without O3 was replaced with a linear decline in D4 and D5 concentrations with increase of the reaction time when O3 was present. The disappearance of the surface saturation characteristics (constant concentration profile as time increases) increased the total removal over a longer time period (Navea et al., 2009b).


Under simulated solar radiation and in the presence of O3, hematite and kaolinite aerosols remove up to 50 to 70 % gas phase D4 and 60 to 90% gas phase D5 within 400 minutes under dry condition (< 1% RH) (Navea et al., 2009c). An increase in humidity under those conditions actually accelerates the removal presumably due to formation of hydroxyl radicals through photolysis of O3 in the presence of water.


The University of Iowa’s studies were conducted under relatively high concentrations of cVMS (> 10 mg L-1)due to limitation of the non-destructive analytical technique employed for cVMS analysis. In follow-up studies conducted at Dow Corning Corporation (Kim et al., 2009; Kim and Xu, 2009a,b), the mechanism for D4 sorption in the high concentration range is verified as polymerisation of the sorbed D4 catalysed by clay surface. Modelling assessment results suggested that aerosol effect on the overall D4 degradation in natural environment should be relatively small by this polymerization mechanism (Navea et al., 2010). However, D4 concentration in atmosphere is in nanogram to micrograms per cubic meter, or 3 to 6 orders of magnitude lower than those tested in Navea’s studies.


At low D4 concentration range, reactive adsorption via depolymerization was observed on aerosol surface (Kim and Xu, 2009b). Under those conditions (initialCD4< 0.3 mg/L, RH 10~80%), 60% to 97% of sorbed D4 was not desorbable (Kim and Xu, 2009a and b). Surface speciation analysis of the sorbed D4 revealed that almost all sorbed D4 in this low concentration range was transformed to silanols within 2 hrs at 28% RH, due to surface-facilitated hydrolysis (or depolymerisation) (Kim and Xu, 2009b), similar to that found in dry soil (Xu 1999; Xu and Chandra, 1999).

A recent study (Kim, J. and Xu, S., 2016) investigated the sorption and desorption behaviours of airborne VMSs (including D4) on nine major primary and secondary atmospheric aerosols (RH 30%). It was found that sorption and desorption of VMS took place via a two-phase process, which included an initial rapid step, followed by slower subsequent step. The initial rapid step was favoured especially at low concentrations. Some aerosols such as carbon black and sea salts interacted reversibly with D4 whereas other aerosols such as kaolinite and sulfates showed highly irreversible sorption for the VMS, especially at low concentrations. Values of apparent aerosol-air partition coefficients ranged 0.09-50.4 L/m(2) for D4, with carbon black having the largest values.

These results suggest that the heterogeneous interaction of D4 with mineral aerosols, therefore, can be an important mechanism in reducing the concentrations and transport of this volatile siloxane compound in the environment. The exact effects from this depolymerisation by aerosol on the half-life of airborne D4 could not be estimated at this juncture. Nevertheless, the actual half-life of D4 in air should be shorter than that calculated based solely on the homogenous reaction rate. 

More recent work using actual field monitoring data has tested this hypothesis (Xu et al., 2017), which has been drawn from extensively in the Annex to the CSR. Evidence is presented there which shows that on the basis of measurements of the concentration of D4 in the northern hemisphere, the degradation rate of D4 in air could be around six times faster than the rate based on the rate constant (Atkinson 1991) reported above.

The objective of this work was to determine the spatial pattern of airborne cVMS distribution and explore how such spatial distribution may be used to extract persistence and long-range transport potential (LRTP) of cyclic volatile methylsiloxanes (cVMS). About 700 individual measurements on atmospheric concentrations of cVMS, i.e., D4, D5 and D6, from all published data at various places including urban, suburban/background, rural and remote Arctic locations. For all data sets, no screening was performed except that the correlation between concentrations of different cVMS compounds measured at the same location were used to check if any given set of data fall in the 99% confidence intervals of the entire data set. Data falling outside the 99% intervals were considered as outliers and were excluded from spatial pattern analysis (See Annex to the CSR for details).

D4 and D6 were found to be correlated with the D5 concentrations in a majority of the data sets measured at the same times and same locations. Average D4, D5 and D6 concentrations in outdoor air, excluding point sources, decreased exponentially by a factor of 100 in a south-to north transect from the source (urban) to the high latitude remote regions (the Arctic).The Airborne cVMS removal roughly follows the first order kinetic the same as that shown for other environmental contaminants by Shen et al. (2005). More specifically, the logarithms of D4, D5 and D6 concentrations, and D5/D6 ratios are found to be linearly related to the sampling latitude in the range of from 40 °N (source) to 80 °N latitude (remote), similar to those found for other contaminants by Shen et al. (2005). Mathematically, the concentrations of any given cVMS ([cVMS], in ng m-3) may be related to the latitude (LA in °N) in a south-to-north transect in the northern hemisphere:

[cVMS] = [cVMS]°exp(-kapp (111.5/n)LA)              (1)

where [cVMS]° is the hypothetical concentration at zero °N; kapp is the pseudo-first-order rate constant; t the time travel from the source to the given latitude; 111.5 (km/°N) is the average displacement on earth surface per °N of latitude; n is the average wind velocity in a south-to-north direction. Therefore, a plot of the log [cVMS] show a negative linear correlation with latitude (“SL1”) that was used to calculate the empirical characteristic travel distance (eCTD, in km) (Table 4.1.3):

eCTD = 111.5 ´ log (1/e)/SL1 = - 48.4/SL1                (2)

where e is the Euler’s number (~ 2.718). The globally averaged half-life (t, in day) of the cVMS compound ((Table 4.1.3)) was then calculated:

t = -0.099/SL1              (3)

based on an assumption of 4.5 m s-1 for n (the details for the deduction of Eq 1-3 can be found in the Annex)

In addition, D5/D6 concetration ratios are also found to be linearly related to the travel displacement projection at a south-to-north direction: the ratio of cVMS compounds with different average half-lives, (tA vs tB, log ([cVMS A]/[cVMS B])) may be related to the latitude (LA) where they are measured by the following equation:

log ([D5]/[D6]) = constant + ((1/11.6)(1/tD6 – 1/tD5))×LA       (4)


(1/tD6 – 1/tD5) = 11.6 ´ SL2        (5)

where SL2 is the slope of the plot of log ([D5]/[D6]) against LA

The half-lives of D5 and D6 calculated using this approach are listed in Table 4.1.3). However, the same correlation was not observed for D4-to-D6 ratio due to poor data quality surrounding D4 measurements, mostly in the Arctic region. However, the D4/D6 concentration ratios calculated from the interspecies correlation mentioned earlier resulted in half-life of 2.3 days for D4, similar to the value calculated using other approach (Table 4.1.3). The yearly-averaged empirical characteristic travel distances (eCTD) of cVMS extracted from these spatial patterns were much less than the model estimations (mCTD) for all three compounds. Similarly, the empirical air half-lives of cVMS were substantially less than those derived from laboratory studies (Table 4.1.3). These findings support that additional removal processes not accounted for in modelling and laboratory assessments are acting upon cVMS and require further elucidation.


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