CMDL Annual Report 24: 5.3.2. Oceanic Uptake of Atmospheric Trace Gases

5.3.2. Oceanic Uptake of Atmospheric Trace Gases

The atmospheric lifetime of a trace gas is derived from the sum of its sinks or loss rates. Loss to the ocean is a significant sink for some gases. Over the past few years a gridded, finite-increment model was developed to determine the uptake rate constant and partial atmospheric lifetime with respect to oceanic degradation for any trace gas that reacts in seawater. The model, originally developed to study the oceanic uptake of atmospheric methyl bromide (CH₃Br) [Butler, 1994; Yvon and Butler, 1996; Yvon-Lewis and Butler, 1997], is used here to calculate the oceanic uptake rate and partial atmospheric lifetime of chlorocarbons, HCFCs, and HFCs (Figure 5.33, Table 5.8). The oceanic uptake rate (mol yr^-1) is defined in the following equation:

math equation (8)

The rate constant (kocn) and lifetime (tocn) for this removal process can then be calculated from the following equation:

math equation (9)

Schematic of the air-sea flux of a trace gas

Fig. 5.33. Schematic of the air-sea flux of a trace gas. Terms are as described in Table 5.8.

TABLE 5.8. Definition of Terms Used in Oceanic Lifetimes Computation (Equations 2 and 9)

Parameter or Variable	Symbol or formula	Units	References for Calculations
Partial pressure of X	P_X	atm
Oceanic concentration of X	X_aq	mol m-3
Gas transfer velocity	K_W	m yr^-1	1, 2, 3, 4
Solubility	H	m³ atm mol^-1	5, 6, 7, 8, 9
Atmospheric burden of X	n_x,a	mol
Surface area of ocean	A	m²
Mixed layer depth	z	m	10
Mass of the troposphere	n_tr	mol
Fraction of X in troposphere	r	Unitless	11, 12, 13
Production rate of X	P₀	mol m-3 yr^-1
Mixed layer chemical degradation rate constant	k_d	yr^-1	8, 14, 15, 16, 17,18, 19
Mixed layer biological degradation rate constant	k_biol	yr^-1	14,20
Thermocline diffusion coefficient	D_z	m² yr^-1	10
Thermocline chemical degradation rate constant	k_z	yr^-1	8, 14, 15, 16, 17,18, 19
Grid cell index	i	--
Interhemispheric ratio multiplier (NH and SH)	R_IHR	Unitless	20, 21

¹Wanninkhof [1992], ²Liss and Merlivat [1986], ³DeBruyn and Saltzman [1997a], ⁴Wilke and Chang [1955], ⁵DeBruyn and Saltzman [1997b], ⁶Moore et al. [1995], ⁷Gosset [1987], ⁸McLinden [1989], ⁹Johnson and Harrison [1986], ¹⁰Li et al. [1984], ¹¹Lal et al. [1994], ¹²Fabian et al. [1996], ¹³Chen et al. [1994], ¹⁴King and Saltzman [1997], ¹⁵Moelwyn-Hughes [1938], ¹⁶Gerkins and Franklin [1989], ¹⁷Jeffers et al. [1989], ¹⁸Wine and Chameides [1989], ¹⁹Elliott et al. [1989], ²⁰Lobert et al. [1995], ²¹Montzka et al. [1996]

Initially this equation was applied to a 2º Ž 2º grid of physical properties in and over the global ocean for the calculation of the partial atmospheric lifetime with respect to oceanic uptake for CH₃Br (Figure 5.34). The inclusion of both the chemical and biological degradation rates for CH₃Br in the ocean resulted in a partial atmospheric lifetime of 1.8 (1.1-3.9) years, which is substantially shorter and more certain than the 3.7 (1.4-14) year estimate calculated by Butler [1994], owing to the inclusion of spatial variability in oceanic physical properties and biological degradation rates. The corresponding atmospheric lifetime, including soil to atmospheric sinks for CH₃Br, is 0.7 (0.5-1.2) years.

Global distribution of the oceanic uptake rate constant

Fig. 5.34. Global distribution of the oceanic uptake rate constant, kocn,_i, where kocn,_i = (1/t_ocn)_i for combined chemical and biological aquatic removal of atmospheric CH₃Br. Including biological processes lowers the partial atmospheric lifetime with respect to oceanic loss from 2.7 years [Yvon and Butler, 1996] to 1.9 years [Yvon-Lewis and Butler, 1997].

Oceanic uptake does not appear to be a significant sink for many of the HCFCs and the HFCs (Table 5.9). Because biological degradation processes have not been investigated for most of these trace gases, the term (k_biol) was null in these calculations. Model results for these gases depend upon chemical degradation rates alone. The presence of any biological degradation would result in substantially reduced lifetimes. Evidence for the presence of degradation mechanisms other than the hydrolysis reaction used in this model has been observed in the saturation anomaly data for CH₃Br, CH₃Cl, and CCl₄.

TABLE 5.9. tocn and t for Selected Halocarbons

	t_ocn (y)			t (y)
Trace Gas	This Study	Previous	Reference	WMO (1994)^*	This Study
CH₃Cl	70(70-79)			1.5	1.46
CH₃CCl₃	94(94-123)	59-128⁽¹⁾	1	4.8 (4)	4.8
CCl₄	2250			42	42
HCFC-22	2320	110⁽²⁾	2	13.3	13.3
HFC-125	10600	>1500⁽²⁾	2	36	36
HFC-134a	9100	>1100⁽²⁾	2	14	14
HFC-152a	5530	>460⁽²⁾	2	1.5	1.5
HCFC-124	1840	360⁽²⁾	2	5.9	5.9
HCFC-142b	2060	270⁽²⁾	2	19.5	19.5
HCFC-123	635	180⁽²⁾	2	1.4	1.4
HCFC-141b	2230	77-360⁽²⁾	2	9.4	9.4
CHCl₃	715			0.55	0.55
C₂Cl₄	2130000			0.4⁽⁵⁾ ^*
OCS	13.2	18⁽³⁾	3	4.3⁽⁶⁾ ^*	3.2

^*C₂Cl₄ OCS lifetimes are from the indicated manuscripts rather than WMO [1994]. The only value in this column that included oceanic uptake is that for CH₃CCl₃.

¹Butler et al. [1991], ²Wine and Chameides [1989], ³Ulshöfer and Andreae [1997], ⁴Prinn et al. [1995], ⁵Wang et al. [1995], ⁶Chin and Davis [1995].

The t_ocn values calculated by Wine and Chameides [1989] are shorter than those determined by this model because the investigators neglected stratification below the mixed layer [e.g., Butler et al., 1991]. At this time there are no available data on the degradation rate constants and/or solubilities for many of the halocarbons found in the atmosphere. This lack of data prevents us from calculating the effect of oceanic degradation processes on the lifetimes and budgets of many trace gases. Accounting for the oceanic uptake of CH₃Cl and OCS resulted in reductions of 3% and 25% in the total atmospheric lifetimes for these species.

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