Acyclic halogenated monoterpenes from marine macroalgae: Estimated atmospheric lifetimes, potential degradation products, and their atmospheric impacts

Figures & data

Figure 1. Halogenated acyclic monoterpenes isolated from four Plocamium species: 1–4 from P. cornutum (South Africa; Afolayan et al., 2009); 5–8 from P. corallorhiza (South Africa, Mann et al., 2007; Afolayan et al., 2009,) 9, 10 from P. brasiliense (Brazil; Vasconcelos et al., 2010); 11 from P. cartilagineum (Portugal, Abreu and Galindro, 1996); 12–14 from P. corallorhiza (South Africa, Mann et al., 2007) and 15–19 from P. cartilagineum (California; Mynderse and Faulkner, 1975).

Table 1. Group rate constants used for addition of O₃ to alkenes and conjugated dienes and trienes.

Structure	10^17 kO3	Comment
Alkenes
CH2 = CH(-R)	1	Taken from Calvert et al. (2000).
CH2 = C(-R)2	1.1	Taken from Calvert et al. (2000).
cis-CH(-R)=CH(-R)	12	Taken from Calvert et al. (2000). Average value of 15.5 used for CH(-R)=CH(-R) structure in the present work.
trans-CH(-R)=CH(-R)	19
CH(-R)=C(-R)2	40	Taken from Calvert et al. (2000).
C(-R)2 = C(-R)2	110	Taken from Calvert et al. (2000).
Conjugated dienes
CH2 = C(-R)CH = CH2	1.3	Based on isoprene (Atkinson and Arey, 2003).
CH2 = C(-R)C(-R)=CH2	2.6	Based on 2,3-dimethyl-1,3-butadiene (Atkinson and Arey, 2003).
CH2 = CHCH = CH(-R)	4	Based on 1,3-pentadiene (Atkinson and Arey, 2003).
CH2 = C(-R)CH = CH(-R)	8	Based on 2-methyl-1,3-pentadiene (Atkinson and Arey, 2003).
CH2 = CHC(-R)=CH(-R)	8	Addition of -R increases k by factor of 2 (relative to 1,3-pentadiene).^a
CH2 = C(-R)C(-R)=CH(-R)	16	Addition of -R increases k by factor of 2 (relative to 2-methyl-1,3-pentadiene).^a
CH(-R)=CHCH = CH(-R)	34	Based on 2,4-hexadiene (Atkinson and Arey, 2003).
CH(-R)=CHC(-R)=CH(-R)	68	Addition of -R increases k by factor of 2 (relative to 2,4-hexadiene).^a
CH(-R)=C(-R)C(-R)=CH(-R) CH(-R)=C(-R)CH = C(-R)2	136	Addition of -R increases k by factor of 2 (relative to CH(-R)=CHC(-R)=CH(-R)).^a
Conjugated trienes
CH2 = CHCH = CHCH = CH2	2.6	1,3,5-hexatriene (Atkinson and Arey, 2003).
CH(-R)=CHCH = CHCH = CH(-R)	5.4	Based on 1,3,5-cycloheptatriene (Atkinson and Arey, 2003).
Other conjugated trienes		Addition of successive -R groups in conjugated trienes increases k by a factor of 1.44.^b

Note: Units of k is cm³ molecule⁻¹ s⁻¹.

^a The factor of 2 is derived from the trend observed on going from 1,3-butadiene (0.63), to isoprene (1.3) and 2,3-dimethyl-1,3-butadiene (2.6); and from 1,3-pentadiene (4.3) to 2-methyl-1,3-pentadiene (8.0).

^b The factor of 1.44 ( =  (5.4/2.6)^1/2) is derived from the data for 1,3,5-hexatriene (2.6) and 1,3,5-cycloheptatriene (5.4). For the reference structure CH(-R) = C(-R)C(-R) = CHCH = C(-R)₂, containing five -R substituents (being relevant to Compound N), a rate coefficient of 2.6 × 1.44⁵ = 16.1 (in units of 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹) was therefore used.

Table 2. Substituent factors used for addition of O₃ to alkenes and conjugated dienes and trienes.

Substituent group	Factor^a	Comment
–X (X = Cl, Br)	0.01	Based on a comparison of 2-chloropropene with 2-methylpropene.^b
–CH2X, –CHX2, –CH(X)–, –C(X)< (X = Cl, Br)	0.24	Based on a comparison of 3-chlorobut-1-ene with 3-methylbut-1-ene.^c
–CHO	0.11	Based on a comparison of methacrolein with 2-methypropene.^d
–C(=O)–	0.54	Based on a comparison of methyl vinyl ketone with 3-methylbut-1-ene.^e
–CH2OH, –CH(OH)–, –C(OH)<	1	Based on a comparison of 2-methyl-3-buten-2-ol with 3-methylbut-1-ene.^f

^a The substituent factor relates to substitution of an alkyl group by the given group.

^b Reference alkene data taken from Atkinson and Arey (2003). Data for chloropropene taken from Leather et al. (2011). It should also be noted that data for cis-1,2-dichlororoethene, trans-1,2-dichlororoethene, trichloroethene and tetrachloroethene (Leather et al., 2011) are consistent with respective factors of 0.006, 0.02, 0.01 and 0.006 for each Cl group added (average ≈ 0.01).

^c Reference alkene data and data for methacrolein taken from Atkinson and Arey (2003).

^d Reference alkene data and data for methylvinyl ketone taken from Atkinson and Arey (2003).

^e Reference alkene data taken from Atkinson and Arey (2003).

^f Approximate data for 2-methyl-3-buten-2-ol (Fantechi et al., 1998) are consistent with little influence from the hydroxyl group.

Table 3. Summary of compounds assessed and their estimated rate coefficients and lifetimes based on an OH radical concentration of 1 × 10⁶ molecule cm⁻³ and an ozone concentration of 30 ppbv (∼ 7.5 × 10¹¹ molecule cm⁻³ at the Earth’s surface).

Compound	Name	SMILES	Molecular formula	10^11kOH (cm^3 molecule^−1 s^−1)	10^20kO3 (cm^3 molecule^−1 s^−1)	kO3/kOH	Gas-phase Lifetime Hours	Vapour Pressure Pa
1	5,6-Dichloro-2-chloromethylene-6-methyl-octa-3,7-dienal	C = CC(C)(Cl)C(Cl)/C = C/C(C = O)=C\Cl	C10H11Cl3O	4.75	258	5.43 × 10^−8	5.6	0.111
2	1,5,6-Trichloro-2-dichloromethyl-6-methyl-octa-1,3,7-triene	C = CC(C)(Cl)C(Cl)/C = C/C(C(Cl)Cl)=C\Cl	C10H11Cl5	5.09	279	5.48 × 10^−8	5.3	0.083
3	5,6-Dichloro-2-dichloromethyl-6-methyl-octa-1,3,7-triene	C = C(C(Cl)Cl)/C = C/C(Cl)C(C)(Cl)C = C	C10H12Cl4	12.9	701	5.45 × 10^−8	2.1	0.346
4	1-Bromo-5,6-dichloro-2,6-dimethyl-octa-1,3,7-triene	C = CC(C)(Cl)C(Cl)/C = C/C(C)=C\Br	C10H13BrCl2	5.80	403	6.95 × 10^−8	4.6	0.937
5	3,5-Dibromo-8,8-dichloro-2,6-dimethyl-octa-1,6-diene	C = C(C)C(Br)CC(Br)/C(C)=C/C(Cl)Cl	C10H14Br2Cl2	9.09	2570	2.83 × 10^−7	2.5	7.13
6	4,6-Dibromo-3,7-dimethyl-octa-2,7-dienal	C = C(C)C(Br)CC(Br)/C(C)=C/C = O	C10H14Br2O	7.97	1320	1.66 × 10^−7	3.1	15.6
7	1,4,8-Tribromo-3,7-dichloro-3,7-dimethyl-octa-1,5-diene	BrC(C(Cl)(C)\C = C/Br)/C = C/C(Cl)(C)CBr	C10H13Br3Cl2	4.69	1050	2.23 × 10^−7	5.1	5.39
8	6,7,8-Trichloro-3,7-dimethyl-octa-2,4-dienal	C\C(/C = C/C(Cl)C(C)(Cl)CCl)=C/C = O	C10H13Cl3O	6.66	1800	2.70 × 10^−7	3.5	0.089
9	1,8-Dibromo-3,4,7-trichloro-7-chloromethyl-3-methyl-octa-1,5-diene	ClC(C(Cl)(C)\C = C/Br)/C = C/C(CCl)(Cl)CBr	C10H12Br2Cl4	4.70	1050	2.23 × 10^−7	5.1	0.140
10	1,4,8-Tribromo-5,6-dichloro-2,6-dimethyl-octa-1,3,7-triene	C/C(/C = C(Br)/C(Cl)C(C)(Cl)\C = C/Br)=C/Br	C10H11Br3Cl2	2.97	40.5	1.36 × 10^−8	9.3	0.168
11	1,6-Dibromo-2,7-dichloro-3,7-dimethyl-oct-3-ene	CC(C)(Cl)C(Br)C/C = C(C)/C(Cl)CBr	C10H16Br2Cl2	6.80	9600	1.41 × 10^−6	2.0	2.72
12	4,8-Dichloro-3,7-dimethyl-octa-2,4,6-trienal	C\C(CCl)=C/C = C(Cl)/C(C)=C/C = O	C10H12Cl2O	3.09	4.25^b	1.38 × 10^−9a	9.0	0.022
13	4-Bromo-8-chloro-3,7-dimethyl-octa-2,6-dienal	C\C(CCl)=C/CC(Br)/C(C)=C/C = O	C10H14BrClO	10.6	10700	1.00 × 10^−6	1.5	2.37
14	8-Bromo-6,7-dichloro-3,7-dimethyl-octa-2,4-dienal	BrCC(Cl)(C)C(Cl)/C = C/C(C)=C/C = O	C10H13BrCl2O	6.65	1800	2.70 × 10^−7	3.5	0.371
15	1,8,8-Tribromo-3,4,7-trichloro-3,7-dimethyl-octa-1,5-diene	ClC(C(Cl)(C)\C = C/Br)/C = C/C(C)(Cl)C(Br)Br	C10H12Br3Cl3	4.69	1050	2.24 × 10^−7	5.1	0.146
16	1,4,6-Tribromo-7-chloro-3,7-dimethyl-oct-1-en-3-ol	CC(C)(Cl)C(Br)CC(Br)C(C)(O)\C = C/Br	C10H16Br3ClO	2.68	155	5.79 × 10^−8	9.9	1.99
17	1,8-Dibromo-5,6-dichloro-2,6-dimethyl-octa-1,3,7-triene	C/C(/C = C/C(Cl)C(C)(Cl)\C = C/Br)=C/Br	C10H12Br2Cl2	4.99	200	4.02 × 10^−8	5.4	1.83
18	8-Bromo-1,5,6-trichloro-2-chloromethyl-6-methyl-octa-1,3,7-triene	CC(\C = C/Br)(Cl)C(Cl)/C = C/C(CCl)=C\Cl	C10H11BrCl4	4.11	76.4	1.86 × 10^−8	6.7	0.425
19	1,1,8-Tribromo-5,6-dichloro-2,6-dimethyl-octa-1,3,7-triene	C/C(/C = C/C(Cl)C(C)(Cl)\C = C/Br)=C(Br)/Br	C10H11Br3Cl2	2.99	40.5	1.35 × 10^−8	9.2	0.168

The MCM methodology considers OH-initiated chemistry for all relevant VOCs. O₃-initiated chemistry is also treated when k_O3/k_OH > 10⁻⁸, and k_O3 > 1.0 × 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ (Jenkin et al., 1997). Neither of these criteria is fulfilled for Compound N, based on the estimation methods applied here.

Table 4. The table shows the contributions of OH- and O₃-initiated oxidation, assuming [O₃]/[.OH] = 7.5 × 10⁵.

Compound	Name	fOH	fO3	Note
1	5,6-Dichloro-2-chloromethylene-6-methyl-octa-3,7-dienal	94.8%	5.2%	a,b
2	1,5,6-Trichloro-2-dichloromethyl-6-methyl-octa-1,3,7-triene	94.8%	5.2%	A
3	5,6-Dichloro-2-dichloromethyl-6-methyl-octa-1,3,7-triene	94.8%	5.2%	A
4	1-Bromo-5,6-dichloro-2,6-dimethyl-octa-1,3,7-triene	93.5%	6.5%	A
5	3,5-Dibromo-8,8-dichloro-2,6-dimethyl-octa-1,6-diene	78.0%	22.0%	A
6	4,6-Dibromo-3,7-dimethyl-octa-2,7-dienal	85.8%	14.2%	a,b
7	1,4,8-Tribromo-3,7-dichloro-3,7-dimethyl-octa-1,5-diene	81.7%	18.3%	a
8	6,7,8-Trichloro-3,7-dimethyl-octa-2,4-dienal	78.8%	21.2%	a,b
9	1,8-Dibromo-3,4,7-trichloro-7-chloromethyl-3-methyl-octa-1,5-diene	81.8%	18.2%	a
10	1,4,8-Tribromo-5,6-dichloro-2,6-dimethyl-octa-1,3,7-triene	98.7%	1.3%	a
11	1,6-Dibromo-2,7-dichloro-3,7-dimethyl-oct-3-ene	41.5%	58.5%	a
12	4,8-Dichloro-3,7-dimethyl-octa-2,4,6-trienal	99.9%	0.1%	a,b
13	4-Bromo-8-chloro-3,7-dimethyl-octa-2,6-dienal	50.0%	50.0%	a,b
14	8-Bromo-6,7-dichloro-3,7-dimethyl-octa-2,4-dienal	78.7%	21.3%	a,b
15	1,8,8-Tribromo-3,4,7-trichloro-3,7-dimethyl-octa-1,5-diene	81.7%	18.3%	a
16	1,4,6-Tribromo-7-chloro-3,7-dimethyl-oct-1-en-3-ol	94.5%	5.5%	a
17	1,8-Dibromo-5,6-dichloro-2,6-dimethyl-octa-1,3,7-triene	96.1%	3.9%	a
18	8-Bromo-1,5,6-trichloro-2-chloromethyl-6-methyl-octa-1,3,7-triene	98.2%	1.8%	a
19	1,1,8-Tribromo-5,6-dichloro-2,6-dimethyl-octa-1,3,7-triene	98.7%	1.3%	a

^a Contributions assume removal is only by reaction with OH radical and O₃, with [O₃]/[.OH] = 7.5 × 10⁵. In all cases additional removal may occur by photolysis, via C-X bond cleavage, although this is likely to be very slow in comparison.

^b Removal may also occur by photolysis at the carbonyl group, although this is likely to be slow in comparison.

Table 5. Isolated yields of halogenated monoterpenes per dry mass of alga.

Compound	Sample collection site	Initial dry mass of alga (g) extracted	Mass of isolated monoterpene (g)	% isolated yield based on dry mass of alga	References
1	Kalk bay, Western Cape of South Africa	26.02	0.065	0.25%	Afolayan et al. (2009)
2	Noordhoek beach, Port Elizabeth South Africa	16.93	1.14	6.73%	Afolayan et al. (2009)
2	Kalk bay, Western Cape of South Africa	26.02	0.186	0.71%	Afolayan et al. (2009)
3	Kalk bay, Western Cape of South Africa	26.02	0.004	0.02%	Afolayan et al. (2009)
4	Kalk bay, Western Cape of South Africa	26.02	0.013	0.05%	Afolayan et al. (2009)
5	South east coast of South Africa	31.5	0.01	0.03%	Mann et al. (2007)
6	South east coast of South Africa	31.5	0.061	0.19%	Mann et al. (2007)
7	South east coast of South Africa	31.5	0.009	0.03%	Mann et al. (2007)
7	Kalk bay, Western Cape of South Africa	14.3	0.0011	0.008%	Knott et al. (2005)
11	Sesimba, west coast of Portugal	12.00	0.028	0.23%	Abreu and Galindro (1996)
12	South east coast of South Africa	31.5	0.002	0.007%	Mann et al. (2007)
13	South east coast of South Africa	31.5	0.001	0.003%	Mann et al. (2007)
14	South east coast of South Africa	31.5	0.002	0.007%	Mann et al. (2007)
17	Sea hare Aplysia California	18.8	0.0104	0.055%	Mynderse and Faulkner (1975)
18	Sea hare Aplysia California	18.8	0.0254	0.135%	Mynderse and Faulkner (1975)
19	Sea hare Aplysia California	18.8	0.0261	0.139%	Mynderse and Faulkner (1975)

Note: No data available for compounds (8), (9), (10), (15) and (16).

Scheme 1. Predicted major peroxy radicals, and terminating and propagating channel products from the reaction of atmospheric OH radical with the Δ³ and Δ⁷ olefins in 3. Both cohorts of channel products are proposed from reaction of RO₂, with NO, NO₂, HO₂ and RO₂ radicals.

Scheme 2. Predicted major Criegee and carbonyl products, acid product and peroxy radical formed from reaction of atmospheric O₃ at the Δ⁵ olefin in 11.

Scheme 3. Proposed major peroxy radical, and terminating and propagating channel products from the reaction of atmospheric OH radical with the Δ⁵ olefin in 11. Both cohorts of channel products are proposed from reaction of RO₂, with NO, NO₂, HO₂ and RO₂ radicals.

Scheme 4. Predicted major Criegee and carbonyl products, acid product and peroxy radicals formed from reaction of atmospheric O₃ at the Δ² and Δ⁶ olefins in 13.

Scheme 5. Predicted major peroxy radicals (RO₂), and terminating and propagating channel products from the reaction of atmospheric OH radical with the Δ², Δ⁶ and Δ⁸ olefins in 13. Both cohorts of channel products are proposed from reaction of RO₂, with NO, O₂, O₂ and R’O₂ radicals.

Table 6. The vapour pressure of the 1st generation OH-initiated oxidation products produced from monoterpenes 3, 11 and 13 and the percentage of gas and particle phase contribution in the formation of the SOA.

1st generation oxidation product	Product formed (in molecules) per 1 g dry algae	Vapour pressure (atm) of the product	Partitioning of the products
			% gas	%particle
	8.61 × 10^18	5.37 × 10^−9	96.6	3.4
	3.67 × 10^18	4.27 × 10^−10	30.6	69.4
	7.35 × 10^19	2.57 × 10^−11	12.0	88.0
	1.59 × 10^18	2.40 × 10^−9	92.7	7.3
	6.78 × 10^17	1.25 × 10^−10	40.0	60.0
	1.36 × 10^19	7.24 × 10^−12	3.7	96.3
	2.71 × 10^19	6.72 × 10^−8	99.7	0.3
	1.99 × 10^20	3.41 × 10^−10	64.5	35.5
	1.15 × 10^19	5.35 × 10^−9	96.6	3.4
	3.65 × 10^18	1.38 × 10^−10	42.3	57.7
	1.82 × 10^17	4.07 × 10^−9	95.6	4.4
	1.24 × 10^18	1.95 × 10^−10	51.0	49.0
	6.19 × 10^16	5.72 × 10^−9	96.8	3.2
	9.31 × 10^17	1.10 × 10^−7	99.8	0.2
	4.65 × 10^16	3.92 × 10^−7	99.9	0.1

Note: The product amount was calculated based on a time lag of 3 h after releasing the halogenated monoterpenes from 1 g dry mass of algae. The calculation of the partition between gas and particle phase is based on the total mass of organic materials in the condensed phase of 1.0 μg/m³. The products formed from RO₂ H-shift have not been included in the calculation.

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DATA AVAILABILITY STATEMENT

The methodological data will be available from the corresponding author on request.