Table 1.

Tropospheric budgets of key species and definition of linear stability modes

		CH4	H2	CO	OH	$-{\lambda }_i^{-1}$(yr)
Steady state	Concentration (ppb)	1890	530	80	106 cm−3
	Sources (ppb/yr)	226	265a	480a	1333
	τ (yr)	8.3	2	0.17	1 s
Linear stability	CH4 mode	1%	0.31%	0.64%	−0.39%	12.3
	H2 mode	−0.01%	1%	0.03%	−0.06%	2
	CO mode	−0.008%	0.001%	1%	−0.36%	0.2
	OH mode	...b	...b	...b	1%	1.5 s

^aSources for CO and H₂ include production from CH₄ oxidation.

^b... is <10⁻⁷.

Sources are obtained from the system (5)–(8) at steady state with the current tropospheric concentrations. τ is the average lifetime of each gas. The modes are expressed as relative changes normalized so that the dominant species’ ratio is 1%. Reaction rates are defined as follows: k₁ = 3.17 × 10⁻¹⁵ cm³/s; k₂ = 3.8 × 10⁻¹⁵ cm³/s; k₃ = 1.9 × 10⁻¹³ cm³/s; k_s = 0.02 yr⁻¹; k_d = 0.38 yr⁻¹ is such that soil uptake accounts for 75% of atmospheric H₂ removal; k₄[X] = 0.3 s⁻¹ (E_OH = 0.82) is defined so that 45% of OH is consumed by the species X, 36% by CO, 14% by CH₄, and 5% by H₂³⁸. Concentrations are converted to mixing ratios using 1 ppb = 1.57 × 10¹⁰ cm⁻³; sources are converted from ppb/yr to Tg/yr using 4.22 × 10¹⁸ kg as the troposphere mass⁶⁸.