Atmospheric Lifetime Calculator

Calculate atmospheric lifetime of pollutants from OH radical reaction rate constants.

OH rate constant k_OH (cm³/molecule/s) (cm³/mol/s)

Methane: 6.3e-15, Toluene: 5.6e-12

Average OH concentration [OH] (molecules/cm³) (mol/cm³)

Troposphere: ~1×10⁶ day average

Additional removal: photolysis rate J (s⁻¹) (s⁻¹)

If compound photolyzes

Dry deposition velocity vd (cm/s) (cm/s)

Typical: 0.1-1 cm/s for reactive gases

Please check your inputs and try again.

Why Hydroxyl Radical Controls the Atmosphere

The hydroxyl radical is present at only about one part in 10¹⁴, yet it determines the fate of most atmospheric pollutants. It is extraordinarily reactive, and it is continuously regenerated by sunlight — which is why it is often called the atmosphere's detergent.

τ = 1 / (k_OH[OH] + k_photolysis + k_deposition)

Removal pathways add as rates, so lifetimes combine reciprocally. The fastest pathway dominates the overall lifetime, in the same way the shortest half-life dominates a parallel decay.

Species	τ with OH	Consequence
Toluene	~2 days	Local — affects the source region only
Methane	~9 years	Well mixed globally
CFC-12	~100 years	Reaches the stratosphere intact
SF₆	~3,200 years	Effectively permanent

Lifetime determines geographic reach. Anything living days affects only its source region; anything living years mixes globally, which is why methane concentration is nearly uniform worldwide while urban toluene is not.

Lifetime and Global Warming Potential

GWP combines radiative efficiency with lifetime over a chosen horizon, which is why the horizon matters so much. Methane is roughly 80 times more potent than CO₂ over 20 years but only about 28 times over 100 years — because most of it has decayed by then.

The policy implication is direct: cutting short-lived species such as methane produces rapid climate benefit, while CO₂ reductions matter over centuries. Both are needed, on different timescales.

Worked Examples

Example 1: Methane CH4: k=6.3e-15, [OH]=1e6

k_OH=6.3e-9 s⁻¹

Result: τ=1/6.3e-9=159 million s=5 years

Long lifetime enables global buildup

Example 2: Toluene: k=5.6e-12, [OH]=1e6

k_OH=5.6e-6 s⁻¹

Result: τ=179,000s=2.1 days

Short lifetime: urban air quality, not global concern

Example 3: Multiple removal pathways

k_OH[OH] = 1e−6, photolysis = 3e−6 s⁻¹

Result: τ = 2.9 days, not 11.6

Rates add, so the combined lifetime is shorter than either alone. Photolysis dominates here at three times the OH rate.

Example 4: Why horizon choice matters

Methane GWP at 20 versus 100 years

Result: About 80 versus 28

Methane's roughly nine-year lifetime means most has decayed within a century, so its integrated effect falls sharply with a longer horizon.

Common Mistakes

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Assuming OH reaction is the only removal route

Photolysis, wet deposition and dry deposition also matter. For soluble or photolabile species they can dominate.

⚠️

Using a single global OH concentration

OH varies with latitude, season and time of day, since it is produced photochemically. Global averages hide large local differences.

⚠️

Comparing GWP values without stating the horizon

Methane is about 80× CO₂ over 20 years but 28× over 100. Quoting a GWP without the horizon is meaningless.

⚠️

Assuming long lifetime always means worse

A long-lived gas with low radiative efficiency can matter less than a short-lived potent one over policy-relevant timescales.

Frequently Asked Questions

Why does OH matter so much?

OH radical is the 'detergent of the atmosphere' — primary removal mechanism for most organic compounds. [OH] varies: lower in winter/night, higher in summer daytime. VOCs with k_OH>10⁻¹¹: reactive, short-lived. k<10⁻¹³: long-lived, potentially global.

Global warming potential connection?

Greenhouse gas lifetime determines global warming potential (GWP). Methane τ~9yr: GWP100=27-30. Nitrous oxide τ~120yr: GWP100=273. SF6 τ~3200yr: GWP100=23,500. Long lifetime × strong absorption = high GWP.

Why is the hydroxyl radical so important?

It reacts rapidly with most atmospheric pollutants and is continuously regenerated by sunlight, making it the main removal route for methane, VOCs and many other species.

How do multiple removal pathways combine?

Their rate constants add, so lifetimes combine reciprocally. The fastest removal process dominates the overall atmospheric lifetime.

Why does atmospheric lifetime determine geographic impact?

Species lasting days affect only their source region. Those lasting years mix globally, which is why methane is uniformly distributed while urban pollutants are not.

Why does methane's GWP depend on the time horizon?

Because its lifetime is about nine years. Over 20 years most is still present, giving a GWP near 80; over 100 years most has decayed, reducing it to about 28.

Formula Explorer connections

Interpretation: This formula connects molecular properties, material structure or environmental transport to a macroscopic behavior or exposure estimate. Assumption: Use parameters measured for the same material, solvent, temperature and environment. Empirical correlations may not transfer outside their calibration range.

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