Lifetime of nitrous oxide in the atmosphere

The rate of N2O photolysis varies spatially and temporally throughout the stratosphere, peaking near the equator at altitudes between 30 and 35km and at noon when the solar UV radiation is most intense (Butenhoff and Khalil, 2007). N2O abundances are about 0.8ppbv greater in the Northern Hemisphere than in the Southern Hemisphere, consistent with about 60 per cent of emissions occurring in the Northern Hemisphere (IPCC, 2001). Almost no vertical gradient is observed in the troposphere, but N2O abundances decrease in the stratosphere as a result of the UV-driven decomposition, for example falling to about 120ppbv at 30km altitude at mid-latitudes (IPCC, 2001). The average lifetime of N2O is in excess of 100 years; the work of Volk et al (1997) and Prinn and Zander (1999) gives a value of 120 years, but there is a feedback whereby the N2O mixing ratio has an impact on its own lifetime (Prather, 1998), and the application of 2-D stratospheric chemical models indicates that the global mean atmospheric lifetime of N2O decreases about 0.5 per cent for every 10 per cent increase in the amount of N2O present (IPCC, 2001). The globally averaged surface abundance of N2O was 314ppbv in 1998, corresponding to a global burden of 1510Tg N. Division of this value by the lifetime, 120 years, gives an annual stratospheric sink rate of 12.6Tg N (Table 4.1). The most recent lifetime assessment listed by IPCC (2007) is the one by Montzka et al (2003) that recommended a lifetime of 114 years. Taking the atmospheric mixing ratio in the middle of the present decade as 319ppbv, the corresponding atmospheric burden is 1534Tg N, and the stratospheric sink becomes 1534/114 = 13.5TgNyr-1.

Table 4.1 Estimates of the global nitrous oxide budget (in Tg N yr1) from different sources

Reference

Mosier et al (1998)/

Olivier et al (1998)

IPCC (1996) IPCC (2001)

Kroeze et al (1999)

Base year

1994

range

1990

range

1980s 1990s

Sources

Ocean

3.0

1-5

3.6

2.8-5.7

3

Atmosphere

(NH3 oxidation)

0.6

0.3-1.2

0.6

0.3-1.2

Soils:

Tropical wet forest

3.0

2.2-3.7

3

Tropical dry savannas

1.0

0.5-2.0

1

Temperate forest

1.0

0.1-2.0

1

Temperate grassland

1.0

0.5-2.0

1

All natural soils

6.6

3.3-9.9

Natural sub-total

9.6

4.6-15.9

10.8

6.4-16.8

9

Agricultural soils

4.2

0.6-14.8

1.9

0.7-4.3

3.5

Biomass burning

0.5

0.2-1.0

0.5

0.2-0.8

0.5

Industrial sources

1.3

0.7-1.8

0.7

0.2-1.1

1.3

Cattle and feedlots

2.1

0.6-3.1

1.0

0.2-2.0

0.4

Anthropogenic sub-total

8.1

2.1-20.7

4.1

1.3-7.7

5.7 6.9

Total sources

17.7

6.7-36.6

14.9

7.7-24.5

14.7

Imbalance (trend)

3.9

3.1-4.7

3.9 3.8

Total sinks

(stratospheric)

12.3

9-16

2.3

12.6

Implied total source

16.2

16.2

16.4

Source: IPCC (2001)

Source: IPCC (2001)

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