Environmental impacts of atmospheric nitrous oxide

Applied Energy 44 (1993) 197 231

Environmental Impacts of Atmospheric Nitrous Oxide

O. Badr & S. D. Probert Department of Applied Energy, Cranfield Institute of Technology, Bedford MK43 0AL, UK

ABSTRACT Nitrous oxide ( N z O ) is an important trace gas in the atmosphere. Changes in the atmospheric concentration of N20 have evoked considerable concern because of its role in (i) regulating stratospheric' ozone levels, (ii) contributing to the atmospheric greenhouse phenomenon and ( iii) participating in the acid-rain formation process. The global concentration of N z 0 in the atmosphere has been rising since the start of the Industrial Revolution, before which it was ahnost constant at about 285 +_5 ppbv (billion = 1 0 9 ) . In 1990, the concentration reached about 310pphv and is now rising at a rate ~[ 0.5-1.1ppbv (i.e. 0"2~)'3%) per ),ear. In this paper, the environmental impacts ~( the increasing atmospheric concentration of NzO are discussed.

NITROUS OXIDE IN THE EARTH'S ATMOSPHERE In 1772, Joseph Priestley, during a series of experiments via which he also discovered oxygen, prepared a gas which he called 'dephlogisticated nitrous air', and which was later renamed nitrous oxide. ~ Since the beginning of the nineteenth century, N 2 0 has gained widespread popularity as a general anaesthetic. L However, the presence of N 2 0 in the Earth's atmosphere was not established until 1938, and then its discovery occurred almost by accident: Adel, 2'~ while investigating the infrared solar spectrum, detected two absorption bands, which corresponded with those of N z O . This finding was later confirmed. 4 The trace gas N 2 0 is an important component of the atmosphere. It plays significant roles in (i) the chemistry of the stratosphere, (ii) the Earth's 197

Applied Energy 0306-2619/93/$06.00 ~C 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

198

O. Badr, S. D. Probert

radiation-balance and (iii) the global nitrogen-cycle. In the stratosphere, N20 is a source of nitric oxide, which is important in the catalytic destruction of stratospheric ozone (i.e. the ozone layer). Atmospheric N20 absorbs long-wave, infrared radiation effectively, and so it is a potential greenhouse gas, with prospective adverse climatic effects. As with other greenhouse gases, the concentration of N20 in the atmosphere began to increase as industrialisation progressed, s-7 N20 is produced and emitted to the atmosphere naturally due to microbial processes occurring in soils and water (i.e. oceans and fresh-water systems), as well as anthropogenically as a result of agricultural activities (i.e. the use of agricultural fertilisers, biomass burning and deforestation) and the combustion of fossil fuels. 8 Although the mechanisms of the production of N20 are relatively well known, the budget for the N20-exchange between terrestrial ecosystems and the atmosphere remains as yet largely unrevealed. N20 is removed from the atmosphere by absorption by aquatic and soil ecosystems, but principally by destruction, in the stratosphere, by photolysis and reaction with atomic oxygen. Of the currently estimated 22 million tonnes of N20 emitted every year, about 5-5 million tonnes accumulate annually in the atmosphere.7 The 1990 concentration is about 310 ppbv, i.e. about 8% above the pre-industrial level of ,-~280-290 ppbv, and is rising by about 0"2-0"3% annually (i.e. 0"5 l'l ppbv per year). 5-7'9- 16 The annual rates of increase tend to be larger in the Northern Hemisphere (i.e. 0.25-0"7%) than in the Southern Hemisphere (i.e. 0.1-0.2%). 9'x4 The increasing concentration of N20 in the atmosphere, taken in conjunction with its estimated lifetime of about 150 years, v'9'11 implies that the global-source's strength is --~30% greater than the sink's strength, v'9"17 Therefore, even if the emission rate of N20 were to remain constant, the atmospheric abundance of N20 would rise eventually to about 400 ppbv. 9 Assuming an atmospheric lifetime of 170 years for N20, Prinn e t al. ~8 interpreted the current rate of increase in its atmospheric concentration as being due to the global NzO annual emission rate being approximately 40% greater than its stratospheric annual sink capacity, and an approximate 50% growth in N20 emission relative to pre-industrial times. In order to stabilise the N20 concentration in the atmosphere at the present-day level, an immediate reduction of 70-80% of the additional anthropogenic flux, that has ensued since the start of the Industrial Revolution, would be necessary. 16 SINKS FOR ATMOSPHERIC NzO As yet, no tropospheric sink mechanisms of significance have been identified. 12'19-25 Known tropospheric sinks for N20, such as surface

Environment impacts of atmospheric nitrous oxide

199

losses in aquatic and soil ecosystems, are considered to be small, although the exact sizes of these sinks are not yet known. 6' 16 Khalil and Rasmussen 26 and Schutz e t al. 27 suggested that sandy and dusty, dry, heated surfaces (e.g. areas of the Sahara region of North Africa) may provide pathways for the tropospheric removal of atmospheric N20. From their studies on surface samples of cultivated soils in Iowa, USA, Blackmer and Bremner 28 reported that the capacities of these soils for the uptake of N20, under anaerobic conditions, which are favourable for the denitrification (i.e. reduction of nitrate), were much greater than their capacities for the release of the gas. N 2 0 produced by denitrification in subsoils may be reduced to N 2 by soil micro-organisms as it diffuses to the soil surface (see also Ref. 29). However, the process is inhibited by the presence of nitrate. Blackmer and Bremner pointed out that this process deserves much greater attention in assessing the role of soils on the atmospheric concentration of N20, and that the capacity of soils to act as a source or a sink for N 2 0 depends largely on their nitrate contents. Laboratory studies of Freney e t al. 3° suggested that only the very wet soils (i.e. those with a moisture content equal to or higher than the field capacity) have any appreciable ability to act as a sink for atmospheric NzO. Even then, they are more likely to be sources rather than sinks as long as the surface soil has an oxidised layer and mineralisable organic nitrogen is present. Under actual field conditions, the more likely result of intermittent wetting by rain or irrigation would seem to be a net emission of N 2 0 to the atmosphere? ° On the basis of available limited marine data, McElroy e t al. 21 concluded that the net flux of N 2 0 across the ocean-atmosphere boundary could be either up or down. From examination of the vertical profiles of the dissolved N20, they discovered evidence for both sources and sinks in the sea. From measurements in the tropical southeast Pacific and Chesapeake Bay, as well as preliminary observations from a fresh-water pond and a tidal marsh, Elkins e t al. 31 suggested that under conditions of low concentrations of oxygen, water ecosystems acted as sinks for the dissolved N/O. They concluded that, under such conditions, microbial respiration can represent a sink for atmospheric N20. However, they recommended that further studies are required to quantify the part which the microbial uptake may play in the global budget of N20. Cohen and Gordon 32 also suggested that oxygendeficient and truly anoxic waters act as sinks for N20. Although they comprise only a small fraction of the ocean, the sink strengths of these sites need to be evaluated. The major atmospheric sink for N 2 0 is photochemical decomposition in the stratosphere. From either the measured vertical profiles of N 2 0 in the stratosphere or by employing different atmospheric models, the strength of this sink has been calculated by many investigations (see Table 1).

200

O. Badr, S. D. Probert

TABLE 1 Estimated Values for the Strength of the Stratospheric Sink for N20 and its Atmospheric Lifetime

Sink strength (106 tonnes )'ear- 1)

10.0 15.7 23'6 14.8

9.4 17'3 16-5

Atmospheric lifetime (),ears)

Reference

70 10 200 30-100 100-150 10-50 100 175 159 100 100-200

27 33 34 35 36 37 38 39 40 26 41 42 43-45 46 14, 47-49 7, 17,29,50,51 24, 25 6, 16,52 53-56 57, 58 18

120 ~ 164 100-150 150 100-200 170 150 160 166 ___16

15.7 15.7 11-17.3 11-20.4

Because of its long atmospheric lifetime (see Table 1), N 2 0 survives the upward journey to the stratosphere through large-scale motions (principally in the tropics44). There it is destroyed, almost exclusively, by photolysis and reaction with the activated oxygen a t o m s , 29'34'36'38'39'41.4.4,4.5,53,59- 70 viz" UV radiation N20

<337nm UV radiation

N20

~N2+ O

) < 250nm

NO + N

(l)

(2)

O + N20 ~

2NO

(3)

O + N20 ~

N 2 + 02

(4)

The odd oxygen atoms involved in reactions (3) and (4) are produced from the ultraviolet photolysis of the stratospheric ozone, i.e.: UV radiation 03

) 0+02

Environment impacts of atmospheric nitrous oxide

201

Most of the N 2 0 decomposes through reactions (1) and (2).68 It was estimated that reactions (3) and (4) contribute at most 20% to the total N 2 0 loss rate (participations of 23%, only about 5% and a mere 2% were predicted by Johnston et al., 39 Cicerone 45 and Cox et al., 6s respectively), and that approximately 30% of the N 2 0 photo-dissociation occurs at wavelengths < 200 nm. 3s It is also possible that NzO may be removed from the stratosphere through the reaction C1 + N 2 0 ~ C10 + N 2 but this has not yet been established. 64

N 2 0 A N D T H E E N D A N G E R E D OZONE LAYER

The oxidation of N 2 0 is a major source of nitric oxide (NO) in the stratosphere:13"2941"44"59"68: many estimates for the magnitude of this source of NO. As a result of the observed vertical profiles of N 2 0 concentration, Schmeltekopf et al. 38 estimated that from the 23.6 million tonnes of N 2 0 transported to the stratosphere annually, the global rate of production of NO is 1.7 million tonnes per year. Thus, about 10% of the N 2 0 molecules entering the stratosphere are converted to NO molecules. Schmeltekopf et al. suggested that about 75% of the stratospheric NO production from N 2 0 occurs between + 30 ° and - 3 0 ° latitudes, and that most of the produced NO molecules are transported downward to the troposphere (see also Ref. 71). It is believed that the estimate of Schmeltekopf et al. is too high by a factor of about two, because they considered the photochemical process to last for 24 h every day. 41 Johnston e t al. 39 avoided this error in their analysis, and predicted a NO production rate of 1.07 million tonnes per year. Crutzen 41 suggested that if the estimate by Johnston et al. has an uncertainty of about 40%, the production of NO from the stratospheric oxidation of N 2 0 may range between 0-75 and 1"5 million tonnes annually. Stratospheric ammonia (NH3) is believe& T M to be removed mainly by reactions such as: OH + NH 3 ~ NH 2 + H 2 0

(5)

Although the subsequent details of this reaction are uncertain, it is reasonable to expect that NO and NO 2 will be the major products. 41'64 However, reaction (5) might possibly lead to a net sink for stratospheric NOx

O. Badr, S. D. Probert

202

and a source of

N 2

through reactions such

a s : 41'64

NH 2 + NO ---* N 2 + H20 NH2 + NO2 --* N20 + H20 NH + NO ---* N 2 + OH NOx (i.e. NO + NO2) are also produced at the Earth's surface (e.g. anthropogenically due to the combustion of fossil fuels and biomass and from the primary-metals industries, and naturally as a result of bacterial activities in soils and lightning)--see, as an example, Ref. 72. Most of the NO~ compounds are removed from the troposphere through wet and dry deposition, mostly after conversion to soluble species such as HNO 2, HNO 3 and N 2 0 5 .36"44"61 However, there is a possibility of some upward leakage to the stratosphere. 4~'61 Absorption of cosmic rays (i.e. solar protons of energies > l0 MeV) initiates complex chemical reactions, involving charged particles, as a result of which nitrogen oxides are formed in the stratosphere. 4~'59"61'64'7°'v3 In polar regions, galactic cosmic rays produce NO at rates of between 0"026 and 0-039 million tonnes annually, depending on the phase of the solar cycle.4~ Sporadic solar cosmic-ray events may occasionally produce almost 10 times as much NO per event as galactic cosmic rays in a whole year. 4~ More than 10 solar cosmic-ray events can occur in a year when the sun is very active, or there might be none at all in a quiet year. 73 The possibility of some significant contribution to the stratospheric NO x budget from the high levels of the atmosphere (i.e. the measophere, thermosphere and ionosphere) cannot be totally ruled o u t . 41'59'61'64'70'74'75 In the ionosphere, 200-400 million tonnes of NOx (as nitrogen) are produced annually by the ionising action of short-wave ultraviolet radiation. 4~'v5 Most of the NOx produced by this mechanism is photodissociated via the reactions: 4~'59 UV radiation

NO

~N+O < 191 nm

net:

NO + N ----~N2 + O 2NO ---* N 2 + 20 --o N 2 +

02

The NO~ species that escape this sink are transported downward into the stratosphere by eddy processes.41'61'74 Atmospheric nuclear explosions may also be responsible for injecting NO,, at stratospheric levels. 13,59.66,76 After 1963, a moratorium on nuclear tests in the atmosphere was agreed upon by the main world powers. However, it seems likely that significant amounts of NO were dumped into the stratosphere by nuclear tests prior to 1963. 59 If a nuclear attack, large enough to wipe out completely any of the

Environment impacts of atmospheric nitrous oxide

203

world powers (i.e. > 5 0 0 M t o n n e s TNT), were launched, it has been estimated that this would cause an input of NO into the stratosphere at least 10 times larger than the annual natural supply provided by the oxidation of N 2 0 . 59 High-flying supersonic transport (SST) aeroplanes (e.g. the AngloFrench Concorde and the Soviet Tupolev- 144) cruise in the stratosphere and inject their NOx-containing exhaust gases there. It has been estimated that a fleet of 250 Concordes (or Tupolevs), each flying for 11 h per day, would supply an equivalent of about 10% of the natural input of NO to the stratosphere. 59 This problem has caused considerable concern because of the estimated adverse environmental impacts, 36'44"59"61 -65, 76- 86 and was a contributing factor to the decision by the US Congress to cancel further financial support for the development of the Boeing SST in 1971. In the stratosphere, the natural cycle of ozone is governed by the Chapman mechanism: 87 natural reactions maintain the harmonious balance between ozone production and destruction at any time. The destruction of stratospheric ozone (i.e. its reversion to molecular oxygen) can be accelerated as a result of the existence of other catalytic trace-gases by a series of double-step cycles involving the free-radical species in the HO,., NO,., ClOx families, viz: X -~- O 3 ---+ X O --1-O 2

and XO+O--~X+O

2

where the catalyst X can be HO, NO, C1 or Br (see Ref. 88). These species can, with different effectivenesses, control the abundance and distribution of ozone in the stratosphere (see Figs 1 and 2). The principal catalytic chain reactions, involving the NO radical and leading to a net destruction of ozone, a r e ; 3 5 ' 3 7 ' 4 1 ' 4 4 ' 4 5 ' 5 3 ' 5 9 ' 6 0 ' 6 3 - 7 0 ' 7 3 ' 7 6 ' 78,81,84.,90- 95

NO + 0 3 --+ NO 2 + 02 NO 2 + O --~ NO + 02 net:

O + 03 ~ 202

and NO + 0 3 --+ N O 2 + NO2 + 03 ---+ N O 3 +

0 2 0 2

UV NO 3

net:

~ radiation

NO + 02

2 0 3 ---+ 30 2

60

50

HOx

~

30 20

10 0 0

I

I

10

I

20

30

I

GO

I

50

I

60

I

I

70

[

80

90

100

PERCENTAfiE CONTRIBUTION TO OZONE REDUCTION BY HOx,NOx,Ox, CLOx

Fig. I.

Relative chemical efficiency of different chain reactions in reducing stratospheric ozone 36 (the c o n t r i b u t i o n of Ox is due to the reaction O + 0 3 -+ 202). 5c 1,5

H0x

4(

3( 25 2C 15

Fig. 2.

10

20 30 4.0 50 60 70 80 FRACTION OF OZONEDEPLETION,[%]

90

100

FRACTIONOF OZONEDEPLETION,[%1 F r a c t i o n o f ozone loss rates due to the Ox, HOx, NOx a n d CIOx chemical cycles in the years 1980 a n d 2024. 89

Environment impacts of atmospheric nitrous oxide

205

However, the first of these two processes has much more impact than the second. 68 In the lower stratosphere, the C1Ox and the NO:, act together to destroy the stratospheric ozone through the catalytic chain reactions:59.76,84.9o.92,96.97 C1 + 0 3 ~

C 1 0 q'- O 2

C10 + NO --* C1 + N O 2 N O 2 + O "-'+ NO + 0 2 net:

0 q-- 0 3 ~

20 2

In addition to their involvement in the direct ozone-destruction, the NOx species play crucial roles in the stratospheric chemistry of the radical species of the HOx and ClOx f a m i l i e s , 37'41"44'45'53'59'60'64'66-70'84'89-94 e.g.: HO 2 +

NO --* OH + N O 2

(6)

CIO + NO --* CI + N O 2

(7)

OH + NO 2 --* HNO 3

(8)

HO2 + NO 2 --~ HNO4

(9)

C10 + NO 2 --* CIONO 2

(10)

Before the debates over nitric oxides arising as a result of SSTs (which started at the beginning of the 1970s), there was no real interest in the processes occurring in the stratosphere and so affecting the ozone layer. 86 The first indications by Johnston 7s were that a fleet of 500 Boeing SSTs, each flying at an altitude of 20 km for seven hours a day, would deplete the stratospheric ozone by 22-50%. By 1974, many investigators had modelled the ozone changes as a result of the NOx emissions in the exhausts of SSTs, and their results were in reasonably good agreement: a 50% increase in the stratospheric budget of NOx, the consensus indicated, would decrease the ozone levels by between 7 and 12%. 77 It was estimated that the mentioned fleet of Boeing SSTs, flying mainly at mid-latitudes of the Northern Hemisphere, would cause a 12% reduction of the global ozone level (i.e. 16% in the Northern Hemisphere and 8% in the Southern Hemisphere). 59 It was also predicted that a nuclear attack, wiping out one of the major world powers, would reduce the amount of ozone in the Northern Hemisphere by approximately 50% of the present abundance, s9 Crutzen 59 developed the following approximate relationship between the steady-state relative change (AO3/O3) of the total amount of stratospheric ozone present and the corresponding change (AN20/N20) of the atmospheric concentration of N20: AO 3

1AN20

03

5 N20

206

O. Badr, S. D. Probert

An increase of 20% in the atmospheric abundance of N20 was, therefore, expected to yield 4% decrease in the total amount of ozone present. The scaling factor between atmospheric N 2 0 change and ozone reduction was taken as 5 in the Climatic Impact Assessment Programme (CIAP) of the US Department of Transport, s° and as ,-~10 by implication from the OH measurements of Anderson. 9s Crutzen and Ehhalt 36 proposed a scaling factor of 6-5, which applies approximately for small deviations from the prevailing atmospheric N 2 0 budgets. Later, Crutzen 4~ predicted a loss of about 12% in total stratospheric ozone as a result of the doubling of the N 2 0 atmospheric concentration, all other factors that effect the prevalance of ozone being maintained invariant. Hunten sl concluded that NzO is the major source of odd nitrogen compounds (i.e. NO, NO 2 and HNO3) in the stratosphere. Without this source, he estimated that the total amount of stratospheric ozone would be greater by a factor of two. Hunten correlated the model results of McElroy eta/., 79 regarding the response of ozone concentration (O3) to the increment f ( i n ppbv) of the stratospheric odd nitrogen, via the equation: 1 0 0 ~ 3 = 1.405f- 0"0105f 2 Hardy and Havelka 99 pointed out that the use of artificial fertilisers (which is growing at a rate of 6% per year on average w o r l d - w i d e ) could increase from 40 million tonnes of nitrogen in 1974 to 200 million tonnes by the year 2000. McElroy e t al. z ~ estimated that the resulting reduction of the total ozone present could be approximately 20% by the year 2013. This prediction was based on the assumptions that the atmospheric lifetime of N 2 0 is only 20 years and that oceans represent a net sink for NzO (with a strength of ~ 63 million tonnes per year). Crutzen,~ 9 using a similar estimate for the atmospheric lifetime of N 2 0 (i.e. between 10 and 20 years), but assuming the oceans to be a source of NzO (which represents 50-93% of the total global source) rather than a sink, predicted an upper limit of ozone reduction of between 1-5 and 9% by the year 2025. However, Sze and Rice z3 estimated that this might be as large as 9% if60% of the fertiliser applied to arable lands is denitrified promptly. If only 10% of the fertiliser is immediately denitrified, they calculated an ozone reduction of 1"6%. Liu e t al. 35 argued that the increase in the use of nitrogen fertilisers will produce only a relatively small change in the fixed-nitrogen global reservoir. When assuming an N 2 0 atmospheric lifetime of 30-100 years, they estimated only between 0"2 and 2.8% reduction in stratospheric ozone by the year 2025 due to the projected increased use of industrial fertilisers. However, they concluded that the long-term (i.e. > 300 years), O3-reductions could be as high as 10%. Crutzen and Ehhalt 36 estimated that, with an N 2 0

Environment impacts of atmospheric nitrous oxide

207

atmospheric lifetime of 160 years, an increased input of fixed-nitrogen fertilisers from 40 to 200 million tonnes of nitrogen per year could lead to a global ozone-depletion in the range from 4 to 15% by AD 2177. They predicted that the ozone reductions would amount to < 2 % at the beginning, and ~ 10% by the end of the 21st century. McElroy e t aL 37 assumed a moderately stable world population by the middle of the next century and that climatic conditions would remain favourable for agriculture over the next 150 years. These would allow modest improvements in living standards (model A), otherwise a major dietary deprivation could ensue (model B). For these models, they predicted the future changes in the rates of emission of N 2 0 from agriculture and combustion sources (Fig. 3) and the corresponding reductions of stratospheric ozone (Fig. 4). The models' predictions indicate that large reductions in 0 3 would ensue. For model A, the reduction in stratospheric ozone approaches 20% in the latter half of the 21st century. If the currently observed rate of increase of the atmospheric N 2 0 concentration were to continue throughout the next 100 years, its level will rise, during this period, from --~300 to 360 ppbv (i.e. by ~ 200/0).1°° Typical estimates ~°°-~°3 suggest a resulting ozone reduction of ~ 4 % of the total column. However, a review of the predictions from representative models 9 indicated steady-state reductions in the range from 1.1 to 2.6% for an atmosphere with about a 1.3 ppbv chlorine burden. Using iterative runs between the one-dimensional radiative-convective model t°4 and the one-dimensional photochemical-diffusion model, 1°5 Wang and Sze 66 predicted the change in the atmospheric column of ozone tOO0

I

MODEAL

MODEL B

4

i 6 0 c B O C :' ' 5OO

AI 201970 ~ , 1990 , Fig. 3.

i

i

2010

, 2030 2050 , 2070 YEAR (AD) h

i

/ i

2090

1970

~

1~)0

k

i

2010

i

2030

J

J

2~0 ' 2~/0 ' 2~)0 200

YEAR (AD)

Bands of the estimated global anthropogenic emission rate for

N 2 0 . 37

O. Badr, S. D. Probert

208

MODEL A

MODEL B

°

o=

c)

OI

I

1970 1990 2010

I

2030

I

YEAR (AO)

Fig. 4.

I

I

2050 2070 2090

I

1970 1990

1

2010

I

I

2030 2050 2070 2()90 YEAR (AD)

B a n d s o f the projected r e d u c t i o n s in s t r a t o s p h e r i c ozone. 3v

shown in Fig. 5. The results indicated relatively large perturbations in local 0 3 concentrations: up to an increase of ~ 18% at 12km height (i.e. in the troposphere) and a reduction of -,~ 19% at 38 km height (i.e. in the stratosphere). The increase of ozone below ~ 26 km altitude (i.e. in the lower stratosphere and upper troposphere) was attributed to the strong couplings between NOx, HOx and C1Ox--see reactions (6)-(10)--which reduce the efficiencies of both the HOx and C1Ox catalytic chain-reactions, leading to the destruction of 0 3. Farman e t al., 93 by analysing the losses of the total ozone in Antarctica, reported that 85% of the 0 3 destruction, in the altitude range from 20 to 40kin, results from the NOx and the C1Ox catalytic reactions. At 40km height, the contributions of NOx and C10~ are roughly equal. The share of NOx decreases rapidly to 10% of the combined effect of 30 km altitude, and to 3% at 20 km height. By employing a two-dimensional transport-chemistry model, l°6 Isaksen and Stordal s3 estimated the effects of increasing the atmospheric concentration of N/O upon the ozone level (see Fig. 6). At high latitudes, N20 increases lead to ozone depletion at all altitudes. There is a large latitudinal gradient in the total ozone destruction: for a 40% increase in the concentration of N20, the predicted reduction in the total column of 03 increases from --~2% at the equator to ~ 12% at high latitudes in the

Environment impacts of atmospheric nitrous

209

oxide

CHANGE (A03) OF OZONECONCENTRATION IN THE LOCALATMOSPHERE,I%] -20

-10

0

I

I

10

20

I

i/

"-

~.ATQ

"'

3

\\

I.u ¢'-i

///

)

,o

0 -2

i v" I ~ -1 0 1 CHANGE(ATe) IN AIR TEMPERATURE,[K]

2

Fig. 5. Estimated changes in ozone concentration (AO3) and in air temperature (ATd) with altitude due to the doubling of the N 2 0 concentration in the atmosphere from the AD 1980 value of 320 ppbv. 66 Northern Hemisphere. The reductions in the Southern Hemisphere are less pronounced, and the seasonal differences are small. The 40% increase in the atmospheric concentration of N 2 0 was predicted to lead to a decrease in the globally averaged ozone column by 3.6%. Up to a doubling of the N 2 0 atmospheric level, the model indicated that the ozone response to the N 2 0 changes is linear, which is in agreement with the results obtained from the earlier one-dimensional models of Cicerone et a l . l ° 7 and Stordal.l°s By employing a one-dimensional, radiative-convective-photochemical model, Callis e t al. 1°9 (see also Ref. 44) concluded that a doubling of the N 2 0

O. Badr, S. D. Probert

210

i

6( _

~

,

,x~_

r

i

,

i

,

-~- -10 -9 -

~

5

2(

-

Z

tu

~o

,

~

-2 - EQUATOR

-14

-6(

--

HONTH OF THE YEAR

(a)

-6(

13 17 21-25 29:3 37 41 /~5/.9 ALTITUDE,[km]

(h)

Fig. 6. Contour maps showing the percentage of ozone changes due to a 40% increase in the atmospheric concentration of N2 O'53 (a) change in total column of ozone and (b) change in ozone concentration. Tropospheric concentrations of 305 ppbv for N 2 0 and 1.6 ppmv for CH4 and an upper stratospheric chlorine level of 2.4 ppbv were assumed.

surface flux (which leads to a 93% increase in its surface concentration) would lead to a decrease in the stratospheric 0 3 column by about 7%. However, in the troposphere (i.e. in the altitude range from 0 to 10km), the estimated change in the ozone column ranges from a reduction of 4% to an increase by 2%, depending on the background levels of the tropospheric NOx.

N20

CONTRIBUTION TO GLOBAL W A R M I N G

Between approximately 70 and 90% of the terrestrial radiation emitted from the Earth's surface and the clouds escapes to space through the atmospheric window (i.e. in the spectral range 7-13/2m). 9'44'1 lO-112 If there is a sizeable increase in atmospheric concentration of a gas with strong absorption features in this spectral region, this will tend to cause a warming of the Earth's surface by the greenhouse effect. Nitrous oxide has three infrared absorption-bands (see Table 2). Ramanathan et al. 44 reported that improvements in the accuracies of the available data are imminent. The trapping of the long-wave thermal radiation due to the presence of N 2 0 in the troposphere is proportional to the square root of its atmospheric concentration' 1o.,, 5 (see Fig. 7). Nitrous oxide, whose strongest absorption-

Environment impacts of atmospheric nitrous oxide

211

TABLE 2 Locations and Strengths of the N20 Infrared Absorption-Bands

Data from Refs 13, 44, 66 & 111

Data from Refs 13, 43, 113 & 114 Band strength at standard pressure and tempera lure (cm 2 atm-1)

Band centre (~m)

7.78

242 384 (234.5)" 8.5-12 (10) 20'7~,0"3 (27)

8.56 16"98

(Itm)

Band strength at 296 K (cm- 2 atm- 1)

4-414.71 7.41 8.33

1 247 218

Rough spectral range

15"15-19.23

24

a Values in parentheses represent mean strengths adopted by Ramanathan et al. 43

bands are on the short-wavelength edge of the atmospheric window, loses about half of its trapped radiation due to the overlap between its bands and those of water vapour and carbon dioxide. 43'11° Donnor and Ramanathan 114 used a modified version of the two-dimensional radiation model of Ramanathan and Dickinson 116 and predicted the effects of perturbing the N 2 0 atmospheric concentration upon the relative energy input to the Earthtroposphere system (see Fig. 8). Removing N 2 0 from the atmosphere cools the earth-troposphere system at a rate of between 0-9 and 1-6 W m-2, with the strongest effect occurring at low latitudes. The most pronounced latitudinal variations ensue during winter. The annual mean radiative rate

L-

2

E

J

~o

2~o

36o

~o

~o

NzO CONCENTRATION,[ppbv]

Fig. 7.

Greenhouse heating due to the presence of N20 in the atmosphere. 111

O. Badr, S. D. Probert

212 2.0 1.8 1.6

(a) SEASONALEFFECT OF COMPLETELY REHOVlNG N2O FROM THE ATHOSPHERE

(b) ANNUAL EFFECTS OF:OF NzO

1A. 1.2 q"

1.0

i=

0.8

=z_

0..~i

3=

0.6

THE ATMOSPHERIC CONCENTRATIONOF NzO

~'~

>-

0.2

-0. -0.L Ix

z

-0.( -0.~ -1.C -1.1 -1.l, -1.~ -1.1~ -2.0

0~

S (iii) EOHPLETELY REHOVINGN2O FROM THE ATMOSPHERE

60* 0o LATITUDE , (DEGREES)

Fig. 8.

Estimated effects of N 2 0 upon the earth-troposphere radiative heating in the Northern Hemisphere.114

of heating (i.e. between 0.4 and 0.8 W m - 2, and a mean hemispherical value of 0.65Wm-2), caused by a doubling of the N20 atmospheric concentration, is less than that for removing N20 entirely. By employing the two-dimensional radiative-dynamical model of Wang e t al., 117 Wang and Molnar ~18 predicted that doubling the N20 atmospheric concentration (i.e. from 300 to 600 ppbv) would yield a 1-07 W m - 2 warming of the Earth-troposphere system, while the stratosphere would be cooled by 0.2Wm -2. Dickinson and Cicerone 110 reported that the 1985 N20 atmospheric concentration, of 304 ppbv, caused a trapping of 1"3 W m-2 of thermal infrared radiation in the Earth-troposphere system. They estimated that the change in thermal trapping due to the increase in the N20 concentration, from the pre-industrial level of 285 ppbv to that of 1985, was 0.05 W m - 2 (see also Ref. 111). They also predicted that, by the year 2050, the N20 atmospheric concentration is expected to reach a value of

Environment impacts of atmospheric nitrous oxide

213

between 350 and 450 ppbv and this would increase the trapping of thermal radiation by 0.14).3 W m - 2 in comparison with the 1985 level (see also Refs 13, 43 and 44). Mitchell, ~11 assuming a concentration of 380ppbv by the year 2035, calculated a further trapping of 0.15 W m - 2 relative to 1985. Based on the data of Ramanathan et al., 43 Wigley 1~5 proposed the following correlation for estimating the radiative forcing AQ (in W m - 2) as a result of changing the atmospheric concentration of N 2 0 from an initial value of CN~o(to) in a reference year t o to CN2o(t) in year t: AQ = 0"105{[CN20(t)] °'5 -- [CN2o(to)] °'5 } where the concentrations are in ppbv. For a doubling of the N 2 0 concentration, from 280 to 560 ppbv, Wigley calculated a radiative forcing of 0.73 W m-2. He predicted that the relative contributions of N 2 0 to the total radiative forcing of all atmospheric trace-gases was negligible prior to AD 1900, around 4% for the period 1900-1950, and about 6% thereafter up to the year 2030. Working Group I of the Intergovernmental Panel on Climate Change (IPCC), 56 of the World Meteorological Organisation (WMO) and the United Nations' Environment Programme (UNEP), developed expressions for evaluating the radiative forcing as a result of changing the atmospheric concentrations of greenhouse gases. The proposed equation for N20, which takes into account the effect of the overlap between the absorption bands of N 2 0 and methane (CH4), is: AQ = 0"14{[CN2o(t)] °'5 - [CN~o(to)]°'5 } -- 0"475(1n {1 + 2"01 × 10 -5 [ CcH, ( to)CN20( I )] 0.75 + 5"31 x 10-15CcH,(lo)[CcHa(lo)CN20(l)]I52} - In { 1 + 2.01 x 10- 5[Cc~(to)CN~o(to)]°75 + 5"31 × 10-X5CcH,(to)[CcH,(to)CN~o(to)]I52}) where AQ is in W m - 2 and the concentrations, CN,o and CcH, are expressed in ppbv. The equation is valid for concentrations of N 2 0 and CH 4 each of less than 5000 ppbv. The past concentrations of trace gases, from AD 1765 to 1990, were used to evaluate the N 2 0 contribution to radiative forcing shown in Table 3. Working Group III of the IPCC studied the effects of future technical developments and environmental controls on the emission rates of greenhouse gases up the year 2100. x19-121 Working G r o u p 1119 predicted the future atmospheric concentration of the gases which would arise from the emissions according to the four developed scenarios (i.e. A: the 'Business as Usual' or the 'AD 2030 High Emissions' approach; B: the 'AD 2060 Low Emissions' approach; C: the 'Control Policies' approach; and D: the 'Accelerated Policies approach). The predicted future changes in the

O. Badr, S. D. Probert

214

TABLE 3 Changes in the Earth-Troposphere Radiative Forcing Due to the Presence of N 2 0 in the Atmosphere, Relative to its Concentration in AD 176556

Year

1900 1960 1970 1980 1990

Change in radiative forcing(Wm 2) due to changes in the concentration of

N20

All greenhouse gases

0-027 0"045 0"054 0'068 0'100

0'53 1"17 1'48 1"91 2"45

radiative forcing due to N 2 0 , for the four policy-scenarios, are presented in Table 4. Ramanthan 112 proposed the following relationship between the global average change, ATS,in the Earth's equilibrium surface-temperature and the change, AQ, in the Earth-troposphere radiative heating:

ATs- AQ 2 2 being a climatic-feedback parameter, which is inferred from model studies to be in the range from 0"9-2"7 W m-2 K Wang and S z e 66 predicted that a doubling of the atmospheric concentration of N20, from an initial value of 320 ppbv, would cause a direct rise AT S in the Earth's surface-temperature by 0.44K. NzO also perturbs the atmospheric distributions of ozone (see Fig. 5) and nitric acid, (HNO3)--see, for example, reaction (8)--which are effective greenhouse gases. Their model calculations indicated that the associated positive climatic feedbacks from 03 and HNO3 perturbations, as a result of the doubling of the N20 atmospheric concentration, could contribute as much as a 0.23 K increase to the surface temperature (see Fig. 5). In Refs 13 and 109, it was reported that doubling the N20 atmospheric concentration, from an initial value of 300ppbv, would increase the global mean surfacetemperature by between 0.3 and 0.4K, and between 0.34 and 0.36K, respectively. Lacis et aL 122 developed a one-dimensional model in order to calculate the vertical atmospheric temperature profile from the net radiative and convective energy fluxes. The model was employed to predict the equilibrium change in the Earth's surface-temperature caused by the

Environment impacts of atmospheric nitrous oxide

215

TABLE 4 Predicted Future Changes in the Earth-Troposphere Radiative Forcing Due to the Presence of N20 in the Atmosphere, Relative to its Concentration in AD 1765, According to the IPCC Policy-Scenarios56

Year

Change in radiative forcing ( W m- 2) due to changes in the concentrations of" N20

All greenhouse gases

Scenario A (Business as Usual) 2000 2025 2050 2075 2100

0"12 0-21 0"31 0'40 0-47

2"95 4'59 6"49 8"28 9'90

Scenario B (Low Emissions) 2000 2025 2050 2075 2100

0' 11 0"18 0'23 0'28 0"33

2"77 3"80 4"87 5-84 6"68

Scenario C (Control Policies) 2000 2025 2050 2075 2100

0' 11 0' 17 0-22 0"25 0"27

2"74 3'63 4.49 5-00 5"07

Scenario D (Accelerated Policies) 2000 2025 2050 2075 2100

0"11 0'17 0'21 0"24 0"26

2"74 3'52 3"99 4"22 4"30

measured increases in the concentrations of greenhouse gases in the atmosphere between 1970 and 1980. From the calculated surfacetemperature rise of 0.242 K, due to all greenhouse gases considered, the N 2 0 contribution was 0.016K (i.e. 6"6%). The model results, for arbitrary changes in the relevant trace gases, fitted the expression AT s = 0.57~ACcH,) °5 + 2"8(ACN2o)°'6 - 0"057 ACcn, ACN2o + 0-15 ACR. 11 + 0"18 ACR.I 2 + 2.5 In [1 + 0"005ACco2 + 10- s(ACco)/]

216

O. Badr, S. D. Probert

where all the concentrations are expressed in ppmv, except for R-11 and R-12, which are in ppbv; the CO 2 abundance is in ppmv above a reference value of 300 ppmv. The third term, on the right-hand side of the equation, represents the necessary correction for the overlap of the CH 4 and N20 absorption bands. The portion of the equilibrium warming that is expected to be experienced depends on the effective thermal capacity of the oceans. Lacis e t al. estimated that the net greenhouse warming for the 1970s was between 0"1 and 0.2 K. They predicted a likely warming of between 0-2 and 0-3 K during the 1980s. By employing their two-dimensional model, Wang and Molnar 118 estimated that the doubling of the atmospheric concentration of N20, from an initial value of 300 ppbv, would result in an average surface-temperature rise of the Northern Hemisphere of: (i) 0.35 K for the fixed cloud-altitude condition and (ii) 0.78 K for the fixed cloud-temperature parameterisation. A detailed analysis of the changes to be expected from the simultaneous increases in the atmospheric concentrations of several greenhouse gases was presented by Ramanathan e t al. 43 The predictions, from the onedimensional radiative-convective model employed, indicated that a 25% increase in the atmospheric concentration of N20 would cause the Earth's surface-temperature to rise by 0.1 K: a 0.2 K increase was predicted when the overlap between the absorption bands of N20 and those of the other greenhouse gases was ignored. The correponding increase in the surface-air temperature (which is typically larger than the surface-temperature change by about 10-13%) was estimated as 0.12 K. Increasing the N20 atmospheric concentration by 50% was predicted to result in a 0.21 K rise in the surfaceair temperature. Ramanathan e t al. 4a calculated that the increase in atmospheric N20 from 285 to 300 ppbv, which took place between AD 1880 and 1980, resulted in a surface-temperature rise of 0.02 K. The results of the model calculations, employing the projected increases in the concentrations of atmospheric trace-gases, are shown in Fig. 9. By considering the 'best estimate' line, it can be concluded that the surface warming A Ts, due to all the trace gases considered, is 1"54K. The projected increase in N20 concentration, from 300 to 375 (with a possible range 350-450) ppbv, contributes ,-~0"083 K (i.e. ,-~5.4%). The World Resources Institute, USA, studied the effects of various energy-economics policy strategies on the future build-up of greenhouse gases in the atmosphere and the corresponding global-warming commitments (i.e. equilibrium surface temperature increases)54--see Table 5. Predicted concentrations of the greenhouse gases were converted to their corresponding warming effects by extrapolating the changes in surface temperature estimated by Ramanathan e t al. 4a The warming due to increase in the N20 concentration was scaled to the difference in the square

217

Environment impacts of atmospheric nitrous oxide

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.......

3.5

3

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u

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I

~

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TRAZEGASES Fig. 9. Cumulativeequilibrium Earth's surface-temperaturerise as a result of the predicted increases in the concentrationsof the atmospheric trace-gases for a period of 50 years starting from AD 1980.9't2'43"44

roots of the concentration in the future and that as at AD 1980 (i.e. 310 ppbv) via the equation AT s = 0-057{[CN2o(t)] °'s -- [CN2o(to)] °'s} The total change in the Earth's surface-temperature was estimated to be the sum of the contributions of the individual greenhouse gases multiplied by a feedback factor of between 0.75 and 2.25. R a m a n a t h a n et aL 44 analysed the effects of atmospheric trace-gases on past and future climate trends. They did not employ explicitly a climate model, but instead predicted the equilibrium change in the surface-air temperature, (ATa) 0, with zero climate-feedback from a modified version of

218

O. Badr, S. D. Probert

TABLE 5 Commitment to Equilibrium Surface-Temperature Rise, in K, Relative to the 1980 Atmosphere, for the Energy Use Scenarios Adopted by the World Resources Institute 54 Year

N 20

Total

contribution Base-case scenario (Business as usual)

1990 2000 2010 2020 2030 2040 2050 2060 2075

0-1 0"1 0"1 0'1 0"2 0"2

0"2 (0-24)'5) 0"5 (0-4-1"1) 0"8 (0"6-1"7) 1-1 (0"8-2'4) 1'4 (1"1-3"2) 1'8 (1"3-4-0) 2"1 (1"6-4"8) 2-5 (1"9-5"7) 3"2 (2.4-7-1)

Modest policies scenario

1990 2000 2010 2020 2030 2040 2050 2060 2075

0"1 0'1 0-1 0'1 0"2

N 20

Total

contribution

0-2 (0'14)'4) 0'4 (0"3-1'0) 0"7 (0"5-1'5) 0'9 (0-7-2"1) 1"2 (0-9-2"7) 1-5 (1'1-3"3) 1"7 (1"3-3-9) 2-0 (1"5-4"5) 2-4 (1"8-5'5)

High emissions scenario

0"1 0'2 0"2 0"3 0'4

0'2 (0"24)'5) 0"6 (0-5-1'4) 1'1 (0"9-2'6) 1"7 (1"3-3-9) 2"4 (1-8-5"5) 3"2 (2"4-7'1) 4-0 (3"0-8.9) 4-9 (3"7-11"0) 6'4 (4"8-14-5)

Slow build-up scenario

0'2 (0"14).4) 0"3 (0"34)-8) 0"5 (0"4-1"1) 0-6 (0'5-1-4) 0'8 (0-6-1-7) 0'9 (0"7-2"0) 1"0 (0'8-2"3) 1-1 (0'8-2-5) l'2 (0"9-2"7)

In this table, void spaces for the N20 contribution indicate values below 0'1 K. Values in parentheses represent the possible range according to assumed range of values for the feedback factor.

the radiative-convective model of Lacis e t a/. 122 The expected change, ATa, in the surface-air temperature may then be computed by multiplying (ATa)o by a feedback factor ( = 1.2-3-6). The use of (ATa)o avoids the current uncertainties in climate sensitivity. Also, because it refers to an equilibrium response, it is independent of uncertainties in climate response time. The predicted decadal increases in (ATa)o from AD 1850 to 2030 are presented in Fig. 10. It is apparent that the N 2 0 contribution to global warming is increasing. The one-dimensional, radiative-convective model of Lacis et al. 122 was also employed by Hansen e t al. 123 to develop expressions for predicting the global rises in the equilibrium surface-air temperature as functions of the corresponding changes in the concentrations of greenhouse gases in the atmosphere, if no climate feedback occurred. The equation proposed for the

Environment impacts of atmospheric nitrous oxide 0.12

03, CFCs & STRATOSPHERIC ~ 20

03 '

& STRATOSPHERIC [FCs

"4

0.01~

o,.

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.

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.

1850--1960

(per decode)

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STRA~RIE H20

.

219

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•,~ R-11

1960s

1970s

1980s

DECADAL INCREMENTS OF GREENHOUSEFORCING (A D 1850 - 1990)

CFCs& S T C I '~ ST TR RA A TO OSPHER~ H20 ~,

03,

0.201 0,16~

J-

_

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2010s

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2020s

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Fig. 10. Estimated additions per decade to the equilibrium global average surface-air temperature with no climate feedback.l°'4*'1 t2,123 Temperature rises shown by dotted lines are highly speculative.

N20 contribution, which accounts for the overlap between the absorption bands of N 2 0 and CH4, is: (ATa) o = f { Ccn,(to), CN2o(t)} -- f { CcH,(to) , CN~o(to) } where the concentrations are in ppbv, and:

f{Ccn,(t), CN2o(t)}':= 0"394[CcH'(t)]°'66 + 0"16 exp [I'6CcH,(t)] 1 + O'169[CcH,(t)] °'°62 109-8 + 3"5CN2o(t) ~, + 1"5561n 1 + [CN2o(t)] °77 1 0 - 6 ~ b _ l ~ Z j -- 0"14 In { 1 + 0.636[Ccn,(t)CN~o(t)] °'Ts + 0.007 CCH,(t)[CcH,(t)CN~o(t)] 1.52 }

220

O. Badr, S. D. Probert

The formula provides values of(ATa) o with an accuracy of + 1% compared with those predicted via the model for the CH~ and N20 concentrations each < 5000ppbv. Hansen et al. predicted a value of (ATa)o of 0-07 K as a result of a 25% increase in the atmospheric concentration of N20. The predicted increments of (ATe)o, between AD 1850 and 2030, due to the observed and forecasted increases in the atmospheric concentration of N20 and other greenhouse gases, are presented in Fig. 10. Ciborowski 7 reported that a 50% increase in the atmospheric concentration of N20 would raise the mean global surface temperature by between 0.2 and 0.6 K. Without a deliberate policy intervention, the N20 concentration is expected to reach a value of 375 ppbv by the year 2035. This will introduce a surface warming of 0-09-0.26 K with respect to the 1985 concentration (i.e. ,-~6% of the total effect of all the greenhouse gases, which amounts to 1.5-4.8K). If the rise in the future N20 atmospheric concentration is assumed to result exclusively from emissions from the energy sector, its long-term level would reach a value of between 335 and 600 ppbv. This would contribute 0.05-0.42 K to the expected total globalwarming of 2"5-4 K. MacDonald, 124 using a simple energy-budget model and assuming an increase in the N20 concentration from 300 ppbv in 1980 to 350-450 ppbv in 2030, calculated comparable temperature changes in the year 2030 with those predicted by Ramanathan et al. 4a For a total equilibrium surfacetemperature rise of 1"8-2"2 K, the N20 contribution was estimated to be 0.2 K. MacDonald reported that the calculated total equilibrium surface temperature change corresponds to an actual warming of 1.3-1.5K, if current estimates of the thermal lag of the oceans are appropriate. The Office of Policy, Planning and Evaluation of the USA Environmental Protection Agency 125 studied the effects of future patterns of economic and technological development on the build-up of greenhouse gases in the atmosphere. Four scenarios were constructed: (i) a slowly changing world, (ii) a rapidly changing world, (iii) a slowly changing world with stabilising policies and (iv) a rapidly changing world with stabilising policies. The predicted changes in the concentrations of the trace-gases, for the four scenarios adopted, were employed to produce estimates for the global surface-temperature rises up to the year 2100. In all the scenarios analysed, carbon dioxide accounts for more than 65% of the increased commitments to global warming (see Fig. 11). This presents a significantly higher estimate for the role of CO2, compared with roughly 50% in the last few decades and the prediction of Ramanathan et a l f f a for the year 2030 (see Fig. 9). Much of this difference was attributed to the smaller increases in the emissions of CFCs, N20 and CH 4 projected in these scenarios, whereas the CO2 concentration was assumed to increase gradually. ~25

Environment impacts of atmospheric nitrous oxide (o) SLOWLY-CHANfilNI3 WORLD

It) RAPIDLY-CHANGINGWORLD

(b) SLOWLY-CHANGING WORLD WITH STABILISINDPOLICIES

{d) RAPIDLY-CHANGING WORLD WITH STABILISINGPOLICIES

221

1Z w/o

,0%

11%

Fig. 1 I.

CARBON DIOXIDE

~

NITROUS OXIDE

METHANE

~ - - ~ ZFCs

~OZONE

Estimated relative contributions of greenhouse gases to global warming by the year 2100. t25

The relative contribution of N20, to the total global warming caused by the cumulative effect of all greenhouse gases, was predicted to be between 0.2 and 0"3% between AD 1880 and 1980.125 - 128 Between AD 1765 and 1990, its contribution was estimated a s 4 % . 6'56'129 In the 1980s, the N 2 0 contribution was reported to be 6 % , 6'56'58'119"125'127'128'130 whereas the 1990 emission is expected to contribute 4 % over a future time horizon of 100 years. 56'119 The instantaneous radiative forcing due to a unit increase in the concentration of N20 in the atmosphere, from 0.31 to 0"41ppmv, was estimated as 3 . 8 W m - 2 p p m v -1, compared with a value of 0-015 W m - 2 p p m v - 1 for CO 2 as its concentration changed from 350 to 450 ppmv. 58 On this basis, the contribution of N 2 0 to the global warming is 233 times that of the CO 2. Ehrlich 6 and Lashof and Tirpak ~25 reported a value of 230. Derwent 55 calculated the increase in surface-temperature as a result of a unit increase in the atmospheric concentration of N20 as 0"81 K

222

O. Badr, S. D. Probert

p p m v - L When comparing this with the predicted effect of CO 2 (i.e. 0.0049 K p p m v - 1), it can be concluded that N 2 0 is 165"3 times more potent a greenhouse gas than CO 2 (see also Ref. 57). Rodhe TM proposed a value of 200. Because different greenhouse gases have different residence times in the atmosphere, the relative cumulative impact of a gas may be quite different from its relative instantaneous radiative-forcing effect. A consistent cumulative basis for comparing emissions of different trace-gases was introduced: sS'56,ss'131'132 the global warming potential (GWP) of a gas i can be defined as: t'n aiCi(t)

GWP -

dt

j0

f~ aco2Cco2(t)dt where a i is the instantaneous radiative forcing due to a unit increase in the atmospheric concentration of gas i; Ci(t)is the concentration of the trace gas i remaining in the atmosphere at time t after the release; and n is the number of years for which the calculation is performed. The corresponding values for CO2 are in the dominator. For integration time-horizons, n, of 20, 100 and 500 years, Derwent 55 calculated GWPs for N 2 0 of 210, 220 and 150, respectively. After review, by Working Group I of the IPCC, s6'119 these were modified to the respective values of 270, 290 and 190. For a time horizon of 100 years, Rodhe TM estimated the GWP of N 2 0 to be 300. Lashof and Ahuja ~8 carried out the integrations for n ~ ~ and arrived at a GWP for N 2 0 of 180.

N20 AND ACID RAIN The oxidation of N 2 0 is a major source of the oxides of nitrogen in the stratosphere. F r o m these, nitric acid (HNO3) may be formed in the lower stratosphere by the reactions: 41,45,60,66,78,90,133 - 135 OH + NO 2 ~

HNO 3

H O 2 + N O --~ H N O 3 H 2 0 + N 2 0 5 -'~ 2HNO3

which are followed b y ."41'45'60'78'90 UV radiation

HNO 3

<300nm

HO + HNO 3 ~

~ OH + NO2 H20 + NO 3

Environment impacts of atmospheric nitrous oxide

223

TABLE 6 Estimated Concentrations of Nitric Acid in the Stratosphere 6°

Attitude (km)

HNO 3 concentration (ppbv)

15 20 25 30 35 50

2"0 3-7 7.0 8'7 6"3 1'6

The model calculations of Crutzen 6° resulted in the values of the concentrations of H N O 3 shown in Table 6. These are of the same order of magnitude as the values observed by Rhine et al. 136 and the mean concentration of ~ 3 ppbv deduced by Murcray et al. 137 (see also Ref. 138). Wang and S z e 66 estimated that doubling the atmospheric concentration of N20, from an initial value of 320ppbv, would increase the column abundance of H N O 3 by 85%. When condensed in the stratosphere, as the measurements in the Arctic indicated, nitric acid acts as a necessary chemical pre-conditioner for the chlorine catalytic ozone destruction. 138 HNO3, transported to the troposphere, is an effective greenhouse gas. Doubling the N 2 0 atmospheric concentration would result in an indirect Earth's surface-temperature increase of --,0.05 K due to the formation o f H N O 3 .66 In the troposphere, nitric acid is removed from the atmosphere by solution and rain-out. .1'.5'76'81'84 Atmospheric N 2 0 also causes an increase in the tropospheric concentration of 0 3, which is a poison and an effective greenhouse gas. It was estimated that doubling the N 2 0 atmospheric level would increase the surface temperature by ~ 0.18 K due to the resulting increase in the level of tropospheric 03 .66 The latter also has an effect on the atmospheric processes that transform nitrogen oxides into nitric acid (thereby contributing to acid rain) and photochemical s m o g . 139'14° Acid precipitation can reduce the rate of growth of vegetation and, therefore, has a serious adverse impact on forest and agricultural yields. 14°'141 It also leads to substantial damage to historic buildings and monuments constructed of marble or limestone. 14° There is now little question that acid deposition can adversely affect fresh-water ecosystems (i.e. lakes and streams). Acid rain has already caused widespread acidification of surface water of many aquatic ecosystems in the northern USA, Canada, Norway, Sweden and the UK, ~40,142.14.3 with recorded evidence for serious biological disruption occurring. ~40.142.1~4

224

O. Badr, S. D. Probert REFERENCES

1. Pierotti, D. & Rasmussen, R. A., The atmospheric distribution of nitrous oxide. J. Geophys. Res., 82 (1977) 5823-32. 2. Adel, A., Further details in the rock-salt prismatic solar spectrum. Astrophys. J., 88 (1938) 186-8. 3. Adei, A., Note on the atmospheric oxides of nitrogen. Astrophys. J., 90 (1939) 627-8. 4. Adel, A., The grating infrared solar spectrum. Astrophys. J., 93 (1941) 509-10. 5. Badr, O. & Probert, S. D., Nitrous oxide in the Earth's atmosphere. Applied Energy, 41(3) (1992) 177-200. 6. Ehrlich, A., Agricultural contributions to global warming. In Global Warming: The Greenpeace Report, ed. J. Leggett. Oxford University Press, Oxford, UK, 1990, pp. 400-20. 7. Ciborowski, P., Sources, sinks, trends and opportunities. In The Challenge of Global Warming, ed. D. E. Abrahamson. Island Press, Washington, DC, 1989, pp. 213-30. 8. Badr, O. & Probert, S. D., Sources of atmospheric nitrous-oxide. Applied Energy, 42(3) (1992) 129-76. 9. Watson, R. T., Geller, M. A., Stolarski, R. S. & Hampson, R. F., Present state of knowledge of the upper atmosphere: An assessment report--Processes that control ozone and other climatically important trace gases. NASA Reference Publication 1162, NASA, Washington, DC, May 1986. 10. Ramanathan, V., Observed increases in greenhouse gases and predicted climatic changes. In The Challenge of Global Warming, ed. D. E. Abrahamson. Island Press, Washington, DC, 1989, pp. 239-47. 11. Blake, D. R., Methane, CFCs and other greenhouse gases. In The Challenge of Global Warming, ed. D. E. Abrahamson. Island Press, Washington, DC, 1989, pp. 248-55. 12. Bolle, H.-J., Seiler, W. & Bolin, B., Other greenhouse gases and aerosols: Assessing their role for atmospheric radiative transfer. In The Greenhouse Effect, Climatic Change and Ecosystems, ed. B. Bolin, B. R. Doos, J. Jager & R. A. Warrik. SCOPE Report no. 29, John Wiley, Chichester, UK, 1989, pp. 157-203. 13. World Meterological Organisation (WMO), Global Ozone Research and Monitoring Project, Report of the Meeting of Experts on Potential Climatic Effects of Ozone and other Minor Trace Gases, Boulder, CO, 13-17 Sept. 1982. Report no. 14, WMO, Geneva, Switzerland, 1983. 14. Gammon, R. et al., Tropospheric trace gases: Sources, distribution and trends. In WMO, Global Ozone Research and Monitoring Project, 'Atmospheric Ozone 1985: Assessment of our Understanding of the Processes Controlling its Present Distribution and Change'. Report no. 16, vol. 1. WMO, Geneva, 1986, pp. 57-116. 15. Chanin, M.-U et aL, Global trends. In WMO, Global Ozone Research and Monitoring Project, 'Scientific Assessment of Stratospheric Ozone: 1989'. Report no. 20, vol. 1, WMO, Geneva, 1990, pp. 163-281. 16. Watson, R. T., Rodhe, H., Oeschger, H. & Siegenthaler, U., Greenhouse gases and aerosols. In Climate Change: The IPCC Scientific Assessment, ed. J. T.

Environment impacts of atmospheric nitrous oxide

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