A 3D chemistry transport model study of changes in atmospheric ozone due to aircraft NOx emissions

Pergamon

Atmospheric Environment Vol. 31, No. 12, pp. 1819 1836, 1997 PIE

S1352--2310(96)00332--9

(c) 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 1352-2310/97 $17.00 + 0.00

A 3D C H E M I S T R Y T R A N S P O R T M O D E L STUDY O F C H A N G E S IN A T M O S P H E R I C O Z O N E D U E TO AIRCRAFT NO:, EMISSIONS W. M. F. W A U B E N , P. F. J. VAN VELTHOVEN and H. KELDER Royal Netherlands Meteorological Institute, P.O. Box 201, 3730 AE De Bilt, The Netherlands (First received 7 February 1996 and in final form 12 July 1996. Published March 1997)

Abstract---The effect of present day aircraft emissions of nitrogen oxides (NO~ = NO + NOz) on atmospheric NOr and ozone concentrations is investigated with the global three-dimensional chemistry transport model CTMK. This model uses 12-hourly meteorological data from the ECMWF analysis and includes parameterizations for subgrid scale processes such as convection. CTMK includes an ozone chemistry module containing the methane and carbon monoxide oxidation chain for the troposphere and lower stratosphere. It is found by using the ANCAT aircraft NOx emission invertory that aviation contributes to 20-100 pptv of the NO~ at cruise altitudes in northern mid-latitudes, which corresponds to 30-80 and 20-50% of the background mixing ratios for January and July, respectively.This perturbation in NOx occurs mainly in the North Atlantic Flight Corridor and is transported eastwards. The resulting increase in upper tropospheric ozone is 2-3 ppbv (2%) in January and 5-10 ppbv (3%) in July. The ozone perturbation is almost zonally symmetricand attains maximum values at northern mid-latitudes in January and in the polar region in July. The calculated effect of aircraft emissions is found to be small (i.e. less than 5 pptv for NO~ and less than 1 ppbv for 03) in the southern hemisphere. The perturbation of NOx by the aircraft emissions at cruise altitudes in northern mid-latitudes is large compared to the standard deviation. Therefore, it is expected that the effect of aviation on NOx is distinguishable from the contribution from other NO~ sources. As the modelled natural variability of ozone is already about 30%, it will not be easy to detect the ozone perturbation due to aircraft NOx emissions. © 1997 Elsevier Science Ltd. All rights reserved. Key words index: Ozone, nitrogen oxides, background, variability, aircraft perturbation.

1. INTRODUCTION Aircraft introduces combustion reaction products and unburned fuel components at high altitudes in the atmosphere. The present day aviation mainly consists of subsonic aircraft with cruise altitudes between 8 and 12 km. The emissions from aircraft are relatively small on a global scale, e.g. less than 3% of the anthropogenic emissions of nitrogen oxides (NOx = NO + NO2) origin from aircraft. However, since the emissions occur at high altitudes, where except for lightning no other sources are present and where their residence time is relatively long, they could have an important impact on the atmospheric composition (Johnson et al., 1992). In addition, the sensitivity of radiative forcing on climate to changes in e.g. ozone is largest directly below the tropopause (of. Lacis et al., 1990), which is located near cruise altitudes at midlatitudes. The aircraft emissions of trace gases such as carbon dioxide, sulphur dioxide, water vapour and cloud condensation nuclei contribute to relevant changes in radiative forcing (cf. e.g. Schumann, 1994; Fortuin et al., 1995). Nitrogen oxides emitted by aircraft contribute significantly to the background NO~

concentration in the upper troposphere at northern mid-latitudes (Ehhalt et al., 1992; Kasibhatla, 1993). Its direct impact on climate is negligible (Fortuin et al., 1995), but it plays an important role in the ozone chemistry of the atmosphere (cf. e.g. Crutzen, 1979). The change in ozone due to these aircraft NO~ emissions is of particular interest. First, ozone is a greenhouse gas which affects the energy budget of the atmosphere. Secondly, ozone largely determines the amount of harmful UV radiation that reaches the surface. Furthermore, ozone plays an important role in atmospheric chemistry, and finally, ozone is toxic and direct contact can be harmful to life. Therefore, a study into the effect of aircraft emissions on atmospheric ozone is justified. The environmental impact of nitrogen oxides emitted by aviation has, since it was suggested by Johnston (1971), been addressed by several studies using two-dimensional models (cf. e.g. Beck et al., 1992; Derwent, 1982; Hauglustaine et al., 1994; Johnston et al., 1989; Ramaroson and Louisnard, 1994; Tie et al., 1994). These model studies have shown that in the troposphere and near the tropopause aircraft NO~ emissions lead to ozone production, whereas

1819

1820

W . M . F . WAUBEN et al.

these emissions can lead to ozone depletion in the middle stratosphere (Hidalgo and Crutzen, 1977). However, the aircraft NOx emissions exhibit strong spatial variations. NOx has a typical lifetime in the upper troposphere and lower stratosphere of about 10 days, and therefore the resulting geographical distribution of the aircraft NOx emissions is not zonally symmetric (cf. e.g. van Velthoven et al., 1997). Since the chemical processes of ozone production and destruction are nonlinear, three-dimensional global models are needed in order to calculate the effect of the aircraft emissions correctly. In this paper, the global three-dimensional chemistry transport model of the K N M I (CTMK) is used to study the effect of aircraft emissions. In particular, the effect of aircraft emissions on tropospheric NOx and ozone is investigated here. C T M K uses analysed meteorological data to drive the transport in combination with a computationally efficient chemistry module to perform model simulations spanning several years. Therefore, the seasonal and spatial dependence of the NOx and ozone perturbation caused by the aircraft NOx emissions can be studied in detail. The natural variability of the concentrations of NOx and ozone are also considered. These model results make it possible to judge whether the effect of aircraft NOx emissions on atmospheric NOx and ozone are distinguishable from their respective day-to-day variations.

2. MODEL DESCRIPTION

The three-dimensional chemistry transport model of the K N M I (CTMK) has been adapted from the global tracer transport model TM2 (cf. Heimann, 1995). C T M K calculates the horizontal and vertical transport of tracer mass on the basis of 12-hourly output from the European Centre for medium-range weather forecasts (ECMWF) model (cf. Velders et al., 1994). The analysed meteorological fields of wind, surface pressure, geopotential height, temperature and humidity with a horizontal resolution of 2.5 × 2.5 ° are used for this purpose. These meteorological data contribute to a realistic description of the actual meteorological situation since observations are included in the E C M W F analyses. The meteorological data are preprocessed for use in CTMK. This involves the evaluation of parameterizations of subgrid scale processes and the integration/interpolation of data to the model grid of CTMK. For the present study C T M K was run with a horizontal resolution of 5° in longitude and about 4 ° in latitude. Vertically, the model has 15 a-levels. The centres and boundaries of this vertical grid are given in Table 1. The lower boundary of this coordinate system coincides with the surface whereas its upper boundary is located at 5 hPa. 2.1. T r a n s p o r t The advection of tracers in C T M K is calculated with the slopes scheme of Russell and Lerner (1981).

The amount of tracers in each grid box is specified by its mass and the gradients of the tracer mass in the zonal, meridional and vertical direction. The mass fluxes through each boundary of the grid box are computed from the E C M W F analyses in the preprocessing stage and stored for use in C T M K at 12hourly time intervals. The model time step used for the computations on the 4 × 5° horizontal grid of C T M K is 1 h. One such model time step is a combination of 4 advection time steps in the East-West, 2 in North-South, and 1 in the vertical direction. Afterwards vertical adjustment is applied in order to conserve mass. The time step is such that the Courant-Friedrichs-Lewy criterion is satisfied, i.e. the mass transported in a time step does not exceed the mass of the corresponding grid cell. Near the polar regions the zonal advection step is further subdivided in order to meet this criterion. In addition to vertical advection, subgrid scale vertical transport occurs. This subscale transport is obtained from parameterizations for deep and shallow convection and vertical diffusion. The vertical transport associated with these processes is also determined from E C M W F analyses data in the preprocessing stage. The subscale convection fluxes are evaluated according to the scheme of Tiedtke (1989). This scheme uses the water vapour convergence in combination with the evaporation from the surface in order to determine the presence of a cloud. In case a cloud is present a convection matrix is constructed which gives the exchange of tracer mass between the grid cells in a vertical column due to the convection process which includes entrainment and detrainment. Here the altitude at which surface air condenses when lifted adiabatically is considered to be the cloud base height and the altitude at which the cloud parcel is no longer buoyant defines its top. The air mass flux remaining at the top of the cloud is detrained in the two layers above the cloud top to simulate overshooting. The vertical diffusion is calculated based on the stability of air using the formulation of Louis (1979). This scheme, which utilizes a stability function involving the Richardson number, yields the largest diffusion coefficients in the atmospheric boundary layer. The transport in C T M K has been validated with the results obtained with other models as well as with observations. The results of passive tracer transport runs of radon and aircraft NOx emissions show that C T M K compares well with the results of other established three-dimensional (3D) models (Jacob et al., 1997; van Velthoven et al., 1997). Comparison of the 3D distribution of aircraft NO~ emissions, obtained by passive tracer transport runs where the chemical loss of NO~ is parameterized as an exponential decay with a globally constant half-lifetime of 10 days using C T M K with both the second-order moments advection scheme of Prather (1986) and the slopes advection scheme of Russell and Lerner (1981), indicates that the slopes scheme does not lead to significant differences in the monthly mean fields. The convection

3D Chemistry transport model study of aircraft emissions

1821

Table 1. Centres and boundaries of the vertical grid of CTMK in a-units. The corresponding pressure levels (in hPa) and altitudes (in km) pertaining to the U.S. Standard Atmosphere (NASA, 1976) are also given a-coordinate Level

Boundary

Centre

0.000000 15

Pressure Boundary

0.015322 14

0.025536

30.7

0.045965

51.3

0.068948

11

74.5

0.102145

10

108.0

0.148110

9

154.3

0.199183

8

205.8

0.250255

7

257.3

0.314096

6

321.7

0.403473

5

411.8

0.505618

4

514.8

0.643514

3

653.8

0.798774

2

810.4

0.908069

1

920.6

0.974464

scheme in C T M K is a reduced version of Tiedtke's scheme since it does not consider downdrafts and many buoyancy checks are omitted. Recently, it has been found that this may lead to overestimating the upward transport due to convection (Mahowald et al., 1995) which is important when studying the effect of aircraft emissions because they occur mainly in the upper troposphere. Therefore, future versions of C T M K will use the full Tiedtke scheme. Vertical diffusion has been implicitly tested in the model intercomparisons mentioned above. Note however that the Louis scheme mainly redistributes the trace gases in the planetary boundary layer and contributes little to the exchange between the boundary layer and the free troposphere (Mahowald et al., 1995). 2.2. Chemistry C T M K includes a background C H 4 - C O - N O x - H O x photochemistry module from the M O G U N T I A model (of. Crutzen and Zimmermann, 1991). This scheme evaluates the time evolution of 13 trace gases according to the so-called quasi-steady-state assumption. This means that the chemical production and loss terms, denoted by P and L, respectively, are assumed to be constant during the chemical time step

1.85 1.18

961.8

1.000000

3.55 2.56

879.4

0.948927

5.37 4.65

741.4

0.867211

6.99 6.14

566.3

0.730337

8.71 7.93

463.3

0.556691

10.20 9.57

360.3

0.454545

11.63 10.88

283.1

0.352400

13.46 12.48

231.6

0.275792

15.74 14.63

180.1

0.224719

18.11 17.09

128.6

0.173647

20.49 19.32

87.4

0.122574

23.80 21.93

61.6

0.081716

29.62 26.48

41.0

0.056180 12

36.00 12.7

20.4

0.035751 13

Centre Boundary Centre

5.0 0.007661

Altitude

0.80 0.44

987.3 1013.2

0.22 0.00

At. The time evolution of the concentration of a trace gas C is then given by (1t

d C / d t = P - LC.

The changes in the concentration of a trace gas due to advection, convection and emissions are not included in the above relation. These processes are considered separately in other modules of C T M K . The general solution of the differential equation given above is C,+at = (Ct - P / L ) e x p ( - L A t )

+ P/L

(2)

which is a relatively computer time demanding expression to evaluate. For trace gases with a lifetime much shorter than 1 h ( L - 1 < At) this expression can be approximated by the so-called steady-state solution (i.e. assuming dC/dt = 0 in equation (1)) Ct+A, = P/L.

(3)

O n the other hand, an expansion of the exponential factor in Taylor series yields C,+~t = Ct + (P - L C t ) A t

(4)

which can be used for trace gases with a relatively long lifetime. This last solution is called explicit because the second concentration on the right-hand side of the differential equation describing the loss is

1822

W . M . F . WAUBEN et al.

evaluated at time t. Sometimes, this technique leads to stability p r o b l e m s resulting in the occurrence of negative concentrations. The implicit technique, which uses the c o n c e n t r a t i o n at t + At, yields for trace gases with a relatively long lifetime C,+A, = ( P A t + G)/(1 + L A t )

(5)

a n d is always stable. O n e of the a b o v e - m e n t i o n e d solutions for the differential e q u a t i o n describing the time evolution of a trace gas is used for each trace gas, d e p e n d i n g o n its lifetime. Generally, the gases with shorter lifetimes are evaluated first. The gases with relatively long lifetimes, i.e. 0 3 , NO× (=NO+NO2), H 2 0 2 , CH4, CO, H N O 3 a n d C H 3 O O H , are t r a n s p o r t e d in C T M K , where after their change due to chemistry is computed. The conc e n t r a t i o n s of the trace gases with relatively short lifetimes, i.e. H C H O , H O 2 , C H 3 0 2 , O(1D), O H a n d the partitioning of NO× into N O a n d N O 2 are calculated in the chemistry m o d u l e by the a s s u m p t i o n of steady state. The a d v a n t a g e of this chemical m o d u l e is t h a t it allows a time step of 1 h a n d hence does not require m u c h c o m p u t a t i o n a l effort. Results o b t a i n e d with the quasi-steady-state a p p r o x i m a t i o n have been

c o m p a r e d with results calculated using a n Eulerian b a c k w a r d iterative m e t h o d a n d G e a r ' s m e t h o d (e.g. Roelofs a n d Lelieveld, 1995). This showed t h a t the lesser accuracy of the quasi-steady-state approximation is largely c o m p e n s a t e d by the reduction of comp u t a t i o n a l burden. The chemistry m o d u l e has also been c o m p a r e d with o t h e r chemical schemes. T a k i n g explicit N O 3 / N 2 0 5 chemistry a n d H N O 4 into acc o u n t did not affect the calculated NO× a n d H N O 3 distributions significantly (Dentener et al., 1995). However, inclusion of peroxyacetyl nitrate (PAN), which acts as a reservoir of NO×, will increase NO× c o n c e n t r a t i o n s in remote locations. Singh et al. (1992) showed t h a t 50 7 0 % of the NO× in the n o r t h e r n hemisphere high latitude t r o p o s p h e r e m a y be derived from PAN, but model calculations also showed that inclusion of C 2 - C 3 - P A N chemistry does not strongly influence the ozone c o n c e n t r a t i o n s ( K a n a k i d o u et al., 1991). D u r i n g daytime the chemical m o d u l e contains seven p h o t o d i s s o c i a t i o n reactions a n d 18 t h e r m a l reactions (cf. Table 2). In the chemical m o d u l e daytimeaveraged photolysis rates for O3, NO2, H202, H N O 3 , C H 3 O O H a n d H C H O are used, which have

Table 2. The chemical reactions considered in CTMK during daytime and expressions for their reaction rates. Here n, T, p and H 2 0 denote the density, temperature, pressure and water vapour concentration, respectively, of the air in a grid box Code

Reaction

JI J2 J3 J4 J5 J6 J7 Rl R2 R3 R4 R5 R6 R7 R8 R9 RI0 RI1 R12 R13 R14 R15 R16 RI7 RI8 R19

+ hv ~ O(1D) + O2 H202 + hv -+ 2OH NO2 + Oz + hv --+ NO + 03 HNO 3 + hv --* NO 2 + OH HCHO + hv --+ He + CO HCHO + 202 + hv --+ 2HOz + CO CH3OOH + 02 + by--+ HCHO + HO2 + OH O(1D) + 0 2 + M--+O3 + M O(ID) + H20 --' 2OH HO2 + OH --+ H20 + O2 H202 + OH -+HO2 + H20 03 + OH --+ HO2 + 02 HO2 + HO2 ~ H202 + O2 03 + HO2 --+OH + 202 HNO3 + OH + 0 2 --+ NOz + 03 + H20 NO2 + OH + M --. HNO3 + M NO + HO e --+ NO2 + OH NO + 03 -+ NOz + 02 CH4 + OH + Oz + M ~ C H 3 0 2 + HzO + M CO q- OH q- 02 --+CO2 + HO2 HCHO + OH + O2--+ HOz + HzO + CO C H 3 0 / + HO2 ~ CH3OOH + 0 2 CH3OOH + OH -, CH302 + H20 CH3OOH + OH --* HCHO + OH + H20 CH302 + NO + O2 ~ HCHO + HO2 + NO2 NO2 + O 3 ~ N O 3 + O2

Rates

0 3

kR1 = [1.4058 × 10 11exp(l10/T) + 6.704 × 10 12 exp(70/T)] n kR9 =kR91/(l + kR91/kR9h) 0.6"*(1/(1 + (log(kR91/kR9h))2)) where kR91 = 2.6 × 10 - 3o (T/300) - 3.2 n and kR9h = 2.4 × 10-11 (T/300)- 1.3 kR6a = 2.3 × 10-13 exp(600/T ) kR6b= 1.7 × 10 -33 exp(1000/T) n kR6c= 1.0 + 1.4 × 10 -21 exp(2200/T) H20 kR8a = 7.2 × 10-15 exp(785/T ) kR8b=4.1 × 10 -16 exp(1440/T) kR8c = 1.9 × 10 -33 e x p ( 7 2 5 / T ) n

kR1 2.2 x 10 ~0 4.8 × 10-1%xp(250/T) 2.9× 10-12exp(- 160/T) 1.6x 10-12exp(-940/T) (kR6a + kR6b) kR6c 1.1 × 10-~4exp(-500/T) kR8a + kR8c/(1 + kRSc/kR8b) kR9 3.7 × 10-1Zexp(250/T) 2.0 × 10-12exp(-1400/T) 2.9×10 ~Zexp(- 1820/T) 1.5 × 10-13(1 q- 0.6p/1013.25) 1.0 × 10 11 3.8 x 10-13exp(800/T) 0.3 x 3.8 x 10-12exp(200/T) 0.7 × 3.8 × 10-12exp(200/T) 4.2 x 10 12exp(180/T) 1.2x 10 13exp(- 2450/T)

3D Chemistry transport model study of aircraft emissions been computed off-line with the model described in Brfihl and Crutzen (1989). The effect of the ozone column density, aerosol and clouds are taken into account in the photolysis calculations by using climatological data. The rate coefficients of the thermal reactions taken from De More et al. (1992) and Atkinson et al. (1992) are also listed in Table 2. These reaction rates are calculated by using the pressure, temperature and relative humidity fields from the E C M W F analyses. During night time the chemistry module evaluates an off-line parameterized heterogeneous reaction. This parameterization, based on Dentener and Crutzen (1993), converts NO2 and 03 into H N O 3. Additional sinks of trace gases are dry deposition of 03, NO2, HzOz, HNO3, CH3OOH and NO, and wet deposition of H202, HNO3 and CH3OOH. Constant deposition velocities are used for each of the tracers involved using different deposition velocities over land and sea surfaces when appropriate (cf. Hein, 1994). The precipitation data used for the parameterization of the wet deposition comes from the E C M W F forecast for 1987, whereas its vertical distribution is given by climatological data from Newell et al. (1974). Furthermore, destruction of C H 4 by chlorine atoms in the stratosphere is taken into account by using destruction rates calculated with a two-dimensional stratosphere model (Briihl and Crutzen, 1993). 2.3. Main chemical reaction mechanisms In this section the main chemical reaction mechanisms will be briefly described. This description includes the oxidation of CH4 via H C H O and CO into CO2 by OH, the influence of NOx on the various paths that can be followed in this oxidation chain, and the resulting effect for 03. Hence, the effect of aircraft NOx emissions on atmospheric chemistry will be briefly discussed. The chemical scheme continuously converts CH4 and NOx via several intermediary products, which contains various feedback reactions, into C 0 2 and HNO3, respectively. Emissions of CH4 and NOx therefore end up as CO2, which is not chemically active, and HNO3, which is efficiently removed from the atmosphere by dry and wet deposition. These chemical mechanisms influence the concentration of 03 and that of other trace gases. Note that the 03 production and destruction by chemical reactions in the troposphere exceeds the stratospheric input of 03 into the troposphere (cf. e.g. Crutzen, 1995; Roelofs and Lelieveld, 1995). A more detailed description, involving all the reactions listed in Table 2 and more, can be found in e.g. Dentener (1993), Crutzen (1995) and AERONOX (1995). OH plays an important role in the oxidation of most trace gases. The primary source of OH is photodissociation of 03 (J1) followed by reaction with water vapour (R2), i.e. 03 + H 2 0 + hv ~ 2 0 H

+ 02.

(6)

1823

The oxidation of CH4 by OH gives CH302 (R12). In NO-rich air this CH302 oxidizes to H C H O (R18). In NO-poor regions it leads to CH3OOH (R15), which oxidizes further to H C H O (R17, J7). HO2 and CH302 are products of the oxidation of CH4, which form NO2 when being combined with NO (R10, R18). Photolysis of NO2 leads to 03 production in the troposphere and returns the N O again (J3). The net reaction in the NOx-catalysed oxidation of CH4 is, e.g. (R12 + R18 + R10 + 2J3): CH4 + 402 + hv ~ HCHO + H20 + 203.

(7)

Note that n o 2 and NO can also lead to the destruction ofO3 (R7, R11). The H C H O which is produced in the oxidation of methane is eventually transformed into CO (J6, J7, R14). The net oxidation of H C H O to CO is, e.g. (J6 + 2R10 + 2J3) HCHO + 402 --~ CO + 2OH + 203.

(8)

Oxidation of CO into C O 2 o c c u r s via (R13). The net change due to oxidation of CO is either (R13 + R10 + J3) CO + 202 ---+C02 +

0 3

(9)

or (R13 + R7) CO + 03 ~ C O 2 + 02.

(lO)

Hence, the oxidation of CO and CH4 may lead to ozone production as well as destruction. In the presence of sufficient NO~ reaction (R10) will dominate reaction (R7), so that the oxidation leads to ozone production. In general, net chemical production of 03 occurs in the NOx-rich and destruction in the NOxpoor regions of the troposphere. The ratio between N O and NO2 is mainly determined by the reaction of NO with 03 that produces NOz ( R l l ) and photolysis of NO2 that converts it into NO again (J3). These reactions are so fast that during daytime a steady state is reached where NO2/NO = k1103/J3

(11)

with kl 1 the rate coefficient of(R11) and j3 the photolysis rate of(J3). NO2 can be converted to HNO3 (R9), which is efficiently removed by dry and wet deposition. The above mechanisms are driven by photolysis and occur during daytime. At night the main sources of O(1D), OH and N O are absent, so that these concentrations rapidly diminish. The only reaction that is considered in C T M K is then an off-line parameterized heterogeneous reaction. This parameterization, which is based on the work of Dentener and Crutzen (1993), converts precalculated fractions of the NO3 formed by the reaction of NO2 with 03 (R19) directly into HNO3 according to 2NO2 + O3 + H 2 0 - * 2 H N O 3 + O 2

(12)

1824

W.M.F. WAUBEN et al.

whereas this reaction actually proceeds via the formation of NO3 and subsequently N205, which in the presence of aerosol is heterogeneously converted into nitric acid. 2.4. Emission data The three-dimensional distribution of the trace gases mentioned above can be calculated with C T M K . For that purpose the sources of the trace gases must be taken into account. In particular, the emissions of nitrogen oxides (NO~ = NO + NO2), which play an important role in the ozone chemistry, need to be specified. C T M K uses estimated present day NO~ emissions originating from soil microbial production industry and surface traffic

3.9 Tg (N) yr 22 Tg (N) yr 1

biomass burning

5.3 Tg (N) yr

lighting

5.0 Tg (N) y r -

aircraft

0.85 Tg (N) yr-

stratospheric production

0.64 Tg (N) yr ~.

These emissions have been constructed or gathered in the framework of the AERONOX project and are described in more detail in AERONOX (1995) and Lee et al. (1997). Here the so-called ANCAT aircraft NOx emission inventory is used (ECAC/ ANCAT and EC working group, 1995; Gardner et al., 1997). Note that NOx is mainly emitted as NO, but since the chemistry module assumes that NO and NO2 are in steady state in daytime, it does not matter in which form the NO~ emissions are introduced in the CTMK. The stratospheric NO~ production contains NOx as well as other nitrogen species. Because the time required to transport NOx downward to cruise altitudes is much larger than the chemical lifetime of NOx, the model will determine the partitioning between NO~, and HNO3. In addition to the NOx emissions, C T M K uses prescribed surface concentrations for C H 4 and CO according to Fung et al. (1991) and Dianov- Klokov and Yurganov (1981), respectively. Furthermore, the ozone concentrations in the stratosphere are prescribed at 50 hPa according to climatological observations (Fortuin and Langematz, 1995). This is needed since the chemistry module in C T M K contains only the relevant processes for the troposphere and lower stratosphere and as a result underestimates the amount of ozone formed under the influence of ultraviolet radiation in the stratosphere. By imposing this upper boundary condition the model will explicitly calculate the amount of ozone exchanged between the stratosphere and troposphere, which is an important source of ozone in the troposphere (e.g. Danielsen, 1968). Simulations with C T M K showed that a horizontal resolution of 4 x 5 is probably sufficient for calculating this stratospheretroposphere exchange (van Velthoven and Kelder, 1996).

3. "BACKGROUND" CONCENTRATIONS OF NOx AND 03

C T M K has first been run by using all the NOx emissions given in Section 2.4, i.e. including the ANCAT aircraft emissions, for a period of 2 yr. The first year was used to initialize the model. The chemical initialisation of the transported tracers has been derived from a C T M K run with an integration time of several years using a horizontal resolution of 8 × 10<~, which in turn has been initialised with distributions obtained from a 2D model. The results of the second year, obtained with the meteorological data from 1990, are analysed here. Monthly means and standard deviations were computed from the daily mean values. The monthly mean results for NOx and 03 are discussed below. Zonal mean cross sections (latitude vs altitude) and geographical distributions at 200 hPa (longitude vs latitude) of NOx and 03 are considered for January and July. The zonal mean plots give an impression of the global distribution of the trace gases. These plots can also directly be compared with results obtained with 2D models. The geographical distribution at 200 hPa is of particular interest because it is in the range of cruise altitudes of aircraft, where a major fraction of their emissions occurs. Both January and July are considered because the seasonal differences in transport and photochemistry are reflected in large differences between the results for these months. The zonal and monthly mean volume mixing ratios of NOx and 03 for January and July are presented in Fig. 1. The distribution of NO~ reflects the geographical distribution of the NOx sources. The anthropogenic NO~ emissions from industry and traffic dominate in the northern hemisphere. The NOx concentrations in the lower troposphere are lower in summer than in winter due to the shorter lifetime of NO.w, which is a result of the higher solar insolation. The emissions from biomass burning and soil occur predominantly in the extra tropics. Above the major surface source regions, the NO~ volume mixing ratio first decreases with altitude, reaches a minimum, and increases again with altitude in the upper troposphere and lower stratosphere due to the NOx transported downward from the stratosphere. In areas remote from industrialized regions, such as the southern polar region, the NOx volume mixing ratio generally increases with altitude. In the upper troposphere in the sub tropics near 200 hPa, the NOx concentrations for July are larger (smaller) in the northern (southern) hemisphere than the corresponding concentrations for January, since the lightning sources are largest in the summer hemisphere. There is no clear evidence of the NOx lightning sources in the lower troposphere near the tropics in the NO~ distribution. This is probably the result of the short lifetime of NO~ due to the high photochemical activity and the effective wet deposition in the tropics. The NOx fields obtained with C T M K seem realistic, since they compare reasonably well with the NO~ fields obtained by other 2D and 3D

3D Chemistry transport model study of aircraft emissions NOx January

1825

NOx July -0--

----0

o.............

_ .......

0. . . .

--

.... 90

lOO~ -60 i

-130

, 0

130

100~--- . . . . . . 160

9(]

600 .

--

.

----I000 ,90 -160

.

.

.

.

.

.

.

.

.

.

......................... -,30 ~0

03 January ......

,oo_

y_

i i iii ii

.

600-----~

3O

lOOt, . . . . . . . . . . . . . r60

03 July

0

0.

.

.

.

.....

0 ..............................

0 ................

7 o ~ .

.

.

....

80O .

-t

.

.

.

.

.

.

~? 0

60o . . . . . . .

.

.

.

,0

.

8

,

. . . . .

1

----lOOO-90 160

30

,0

,3D

i000 ........... 60

~

Fig. 1. Zonal and monthly mean volume mixing ratios of NOx (top) in pptv and 03 (bottom) in ppbv for January (left) and July (right) obtained with CTMK by using all NOx sources including aviation. The dashed horizontal lines indicate pressure levelsat 100 hPa intervals. Isolines are drawn at 1, 2, 5, 10, 20, 50, 70, 100, 200, 500 and 1000pptv for NOx and 10, 20, 30, 40, 50, 70, 100, 200, 500 and 1000 ppbv for 03.

models (e.g. Hough, 1991; Kasibhatla et al., 1993; AERONOX, 1995) as well as with observations (cf. e.g. Ehhalt and Drummond, 1988; Weinheimer et al., 1994; Fehsenfeld and Liu, 1994). The main features observed in the NOx distribution during the STRATOZ 1II and T R O P O Z II campaigns are represented by the model, i.e. (IPCC, 1995) (i) the NOx mixing ratios are generally lower in the southern hemisphere during both summer and winter; (ii) generally, the NOx mixing ratios increase with altitude and have high values in the planetary boundary layer over the continents; (iii) the shape of the NOx profile at northern mid-latitudes changes dramatically between summer and winter. A more detailed comparison of CTMK results with NOx observations showed that CTMK using a coarse horizontal resolution of 4 x 5° and a simplified chemistry scheme produced NOx distributions that are not much different from other model results or observations (Emmons et al., 1997). The main model deficiencies in CTMK con-

cerning the NOx distribution are the low NOx mixing ratios in the Arctic troposphere caused by the absence of local sources and the neglect of PAN in the chemistry scheme (Singh et al., 1992), and the low NOx mixing ratios in the lower stratosphere where values of 300-500 pptv are observed during STRATOZ III (Ehhalt and Drummond, 1988). Note that the observed NOx mixing ratios near the tropopause show large differences, e.g. Weinheimer et al. (1994) report approximately 30 pptv over the pole to 90 pptv at 40°N, with local excursions up to 140 pptv. The low monthly mean NO~ mixing ratios obtained with CTMK also occur in regions with high temporal variability. Higher NOx in combination with lower ozone mixing ratios were obtained in the upper troposphere and lower stratosphere in previous runs with CTMK when an ozone flux was prescribed at 175 hPa instead of the prescribed ozone concentrations at 50 hPa (AERONOX, 1995). The reason for this discrepancy between observed and modelled NO~ and

1826

W.M.F. WAUBEN et al.

HNO3 values is currently under investigation and could be ascribed to heterogeneous chemistry (Chatfield, 1994) or oxidation of N 2 0 (Bregman and Lelieveld, 1997). In order to obtain realistic NOx and HNOa concentrations in the lower stratosphere in the future, we plan to prescribe observed NO,, NO r, 03 ratios. The three-dimensional ozone distribution calculated with C T M K shows good agreement with other model calculations (e.g. Crutzen and Zimmermann, 1991; Roelofs and Lelieveld, 1995; AERONOX, 1995) and with observations (cf. Komhyr et al., 1989; Oltmans et al., 1989; Crutzen, 1995). Some general features of the ozone distribution, such as the higher values in the northern than in the southern hemisphere, the excursion of low values near the equator towards high altitudes, the generally increasing values towards the poles at high altitudes, and the characteristic dip near 30°North, are also obtained in other model computations (cf. e.g. Crutzen and Zimmermann, 1991) and are confirmed by observations (cf. e.g. Crutzen, 1995). The zonal mean ozone distribution for January and July generally show the same features, but the ozone values in the lower troposphere of the southern hemisphere is lower in January compared to July due to the lower NOx values. The ozone volume mixing ratio increases with altitude from the surface to the lower stratosphere. Note that the ozone gradient between the troposphere and stratosphere is not very strong in the monthly mean mixing ratios because averaging makes the results smoother. The characteristic sine curve of the 50 ppbv isoline results from the ozone flux from the stratosphere into the troposphere at mid-latitudes and the upward transport of low ozone values in the tropics due to convection. The stratosphere troposphere flux is generally higher in the northern than in the southern hemisphere and higher in winter than in summer (cf. e.g. Holton, 1990; van Velthoven and Kelder, 1996). A detailed comparison of modelled ozone profiles with observed profiles shows good agreement (cf. Wauben et al., 1997). The latitudinal and seasonal dependence of the ozone profiles are well reproduced by CTMK. However, the ozone mixing ratios in the upper troposphere and lower stratosphere are generally overestimated by C T M K at mid-latitudes probably due to an overestimation of the stratospheretroposphere exchange resulting from the coarse vertical resolution in the tropopause region in combination with the absence of heterogeneous reactions in C T M K which reduce ozone (Granier and Brasseur, 1992). Furthermore, the seasonal dependence of the ozone concentrations at the surface is generally also not reproduced well by the model, possibly due to the neglect of non-methane hydrocarbons and to the simplified treatment of deposition. The geographical distributions of NO~ and 03 for January and July at 200 hPa are shown in Fig. 2. In January, at 200 hPa over the continents south of the equator the NO~ emissions from lightning domi-

nate over the other emissions. There are local minima over the equatorial regions of the Pacific and the Indian Ocean due to the absence of NOx sources. The NOx emissions from aircraft are visible over western Europe. In July, the aircraft and lightning sources are closer together as the lightning source occurs then mainly in the northern hemisphere. Additionally, the NO~ values near 200 hPa in the northern hemisphere are larger in July compared to January, since upward transport of NOx emitted at the surface by convective events is larger in summer (cf. e.g. K6hler et al., 1997). The NOx volume mixing ratio increases from the poles to the equator. The maximum values are reached south (north) of the equator in January (July). In the southern polar region the NOx concentrations are higher for January than for July. The isolines at 200 hPa for 03 are more zonally symmetric compared to NOx due to the much larger lifetime of 03. The low ozone values over the equatorial regions of the Pacific and the Indian Ocean coincide with the local minima for NOx. At 200hPa the ozone volume mixing ratio increases towards the poles, and the values poleward of 30 ° N for July are smaller than for January. This behaviour is in agreement with the corresponding variations in the altitude of the tropopause, which is at lower altitudes near the poles and which is lower in winter than in summer. The standard deviation of NOx and ozone generally show the same features as the monthly mean distributions given in Figs 1 and 2. In order to improve the presentation of the standard deviation, it is given relative to the corresponding monthly mean values. The zonal mean relative values of the standard deviation of NOx shown in Fig. 3 are typically about 20 30% of the monthly mean values for the upper troposphere and attain values of up to about 50% in the polar region of the winter hemisphere where no NO~ sources are present. Generally, the standard deviation is large at low altitudes because the lifetime of NOx is small and the effect of fluctuations in upward transport of surface emissions is large there. The standard deviation of NO~ is very large in the lower and middle troposphere of the northern polar region in January where maximum values of more than 100% are reached and is more than 70% in the (sub)polar region in the southern hemisphere. In relative numbers, the standard deviation generally increases from the tropics towards the poles and is very high in the winter polar regions (up to 70%). These situations with high variability in the NOx concentrations occur in regions with no local NO~ sources during winter, since the fluctuations in transport from the major source regions of NO~ are important in these situations, especially since the lifetime of NOx is of the same order of magnitude as that of meteorological variability induced by extratropical disturbances. The standard deviation of NOx at 200 hPa (cf. Fig. 4) at northern mid-latitudes is typically about 20%. It is up to 70% in the winter polar regions. Secondary

3D Chemistry transport model study of aircraft emissions

r~

z

z

m

z r~

%

1827

A O

0~ o

r e~ tz~

'I )i

°. ° ~f

_>,

I I ¢

m

x

C"

to

0

0

Z

7> m Q

I_

I'

z

4

z

z o

z

o

¢n

o c-i

z

o

o

z

z

o

o

~ -

o

o

t-

O

--a c-

H

09

0

Z

E~

C,

E ,c: 5e-

z

o

z ,o

1828

W.M.F. WAUBEN et al.

NOx July

NOx January ----0

....

~ .......................

.....

0 ...........

O. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 ............

. . . . . . . .

......

•- - I

000- . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

90

-160

-130

I 0

130

I000- ............ 160

.... 9C

90

600 . . . . . . . . . . .

- -

.

.

.

-

.

.

.

.

.

.

"

.

.

.

....

-160

-?0

.

.

.

.

i 0

130

160

0 .........................

"

:Z:j-

~

.

'S'o

03 July 0. . . . . . . . .

m

4q

1000 .................................. I00~ ...........

03 January 0,

200 ......

--

0 ............

,'~'----"~'-=

w,. ,~,

"'"

"

.

.... 1000 .................................. 90 -t60 -130 I0 130 i 0 0 0

160

9C

-----1000 ................................ I 0 0 0 .............. 90 160 130 i0 130 160

Fig. 3. Standard deviation in % of zonal and monthly mean volume mixing ratios of NOx (top) and 03 (bottom) for January (left) and July (right) obtained with CTMK by using all NOx sources, Isolines are drawn at 10, 20, 30, 40, 50, 70 and 100%.

maxima of about 50% occur over the oceans in the (sub) tropical region. The zonal mean standard deviation for ozone (cf. Fig. 3) is typically 20% near the surface and increases to about 40 and 30% at cruise altitudes in January and July, respectively. The higher standard deviation near the tropopause is a result of the stratosphere-troposphere exchange which is larger in January than in July. The zonal mean standard deviation of ozone in the southern polar region reaches values of 50%. The standard deviation of ozone is much less than for NOx due to the longer lifetime of ozone and the absence of strongly localised source regions. At 200 hPa the standard deviation of ozone (cf. Fig. 4) is typically 20 30% with values of 40-50% occurring in the stormtrack regions at middle and high latitudes of the winter hemisphere. Note that the standard deviation of the monthly mean values obtained from a model simulations over 4 yr is larger than the year-to-year variation of the monthly mean results (AERONOX, 1995).

4. P E R T U R B A T I O N

OF

NOx AND

O~ C O N C E N T R A T I O N S

BY AVIATION

Computations similar to the ones reported in Section 3 have been performed without taking the aircraft emissions into account. The difference between the results obtained with and without aircraft emissions gives the impact of the aircraft NOx emissions upon the atmospheric composition. The absolute contribution of aircraft emissions to the zonal mean and geographical distribution of tropospheric NOx and 03 is presented in this section. The geographical distribution at 200 hPa is considered because it is in the range of cruise altitudes used by subsonic aircraft. Again monthly mean fields of NOx and 03 for January and July are presented. The differences between the zonal and monthly mean NO~ obtained with and without aircraft emissions are shown in Fig. 5 for January and July. The absolute contribution of aircraft to the zonal mean

3D Chemistry transport model study of aircraft emissions

m

z

1829

z

x

0

0

Z

©

m

Q

c-

Q

Z

o

1830

W.M.F. WAUBEN et al.

NOx January

NOx July

I--~O

[

,0. . . . . . .

- 0---,-.

.

.

.

.

.

. . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

....

. . . . . i000- . . . . . . . . . . . . . . . . . . . . 90 160 -130

..... 160

I000---

I0

13O

.....

90

03 January - - - - 0

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

6OO-

....................

-6O0"/--~'~--

IO00 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

-,60

?0

,0

?0

10o0 .

. . . . . .

,6°

03 July .

.

.

0.

.

.

.

.

.

.

.

.

......

0 . . . . . . . . . . . . . . . . . . . . . . . . . .

0 . . . . . . . . . .

-----200 . . . . . . . . . . . . . .

..... -90

i00¢- .

Io00 . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . .

-160

-130

I 0

130

160

- ..... 91

Fig. 5. Absolute contribution of aircraft emissions to the zonal and monthly mean NO~ (top) in pptv and 03 (bottom) in ppbv for January (left) and July (right) as simulated with CTMK. Isolines are drawn at 1, 2, 5, 10, 20, 30, 40 and 50 pptv for NOx and 1, 2, 3..... 9 and 10 ppbv for 03.

NO~ is typically 20 pptv near 200 hPa at northern mid-latitudes, with maxima of about 50 and 40 pptv in January and July, respectively. The perturbation is higher in winter due to the slower photochemical conversion of NO~. With decreasing altitude, the chemical lifetime of NOx decreases, and therefore the contribution of aircraft becomes smaller. Although a considerable fraction of the aircraft NO~ emissions occurs in the lower troposphere at northern mid-latitudes, the resulting change in NO~ is less than 1 pptv. The absolute contribution of aircraft emissions to the NO~ mixing ratio at 200 hPa is presented in Fig. 6. The contribution of aircraft at northern mid-latitudes is higher in January due to the smaller photochemical activity compared to July. Maxima of about 100 pptv occur at cruise altitudes over Europe for both January and July. The absolute contribution of aircraft to the NO~ at 200 hPa for January and July is about 20 pptv in the northern polar region and less than 5 pptv in

the southern hemisphere. This shows that as a result of the relatively short lifetime of NOx only a small fraction of the aircraft NOx emissions are transported towards the northern polar region, and an even smaller fraction crosses the equator. The distribution of the NOx perturbation caused by aircraft NOx emissions (cf. Figs 5 and 6) is similar to that found in passive transport studies using a hierarchy of 2D and 3D models (van Velthoven et al., 1997). The values presented here are lower since the chemical half-lifetime of NO~ is less than the 10 days that were used in the passive transport studies. In particular, the small effect of aircraft in the lower troposphere is not reproduced by the passive transport studies. However, although the passive transport studies do not give the correct vertical distribution of NOx, they can reproduce the general pattern of the NOx perturbation by aircraft at cruise altitudes and can be used to explain certain features in terms of transport.

3D Chemistry transport model study of aircraft emissions

1831

© Z t'¢3

0

Z

2~ ¢-

"c

to

ro

o "3

z

z

,~

z

~ o

z

o

o

£

ro -

r

°

2

0

.x3

t'¢t~

"-3

X

0

Z

CO

0

Z

Z o

1832

W, M. F. WAUBEN et al.

A simplified linear chemical scheme consisting of reactions J3, J4, R8, R9, R l l and R19 (cf. Table 2) and using prescribed OH and 03 fields has been used in the AERONOX (1995) project in order to obtain the contribution of each of the individual NO~ sources to the NOx and HNO3 background concentrations (cf. K6hler et al., 1997; Kraus et al., 1997). This reduced chemical scheme gave results for NOx that were comparable to those obtained with the chemical module that has been used in this work. Therefore, such a reduced chemical scheme is useful for sensitivity studies. It has, however, the disadvantage that 03 fields have to be prescribed. Also, the effect of aircraft emissions on 03 cannot be investigated with such a reduced linear chemical scheme. The chemistry module used in C T M K calculates the 3D ozone distribution itself. The differences between the monthly mean ozone fields obtained with and without aircraft emissions are given in Figs 5 and 6 for the zonal mean and 200 hPa results, respectively. In the zonal mean, the contribution to the northern hemisphere 03 concentration is about 1-4 ppbv in January and 1-10 ppbv in July. Clearly, the higher photochemical activity in summer enhances the ozone perturbation. The contribution of aircraft to 03 is maximum near 200 hPa and decreases with decreasing altitude to less than 1 ppbv near the surface in the northern hemisphere. The contribution is also less than 1 ppbv in the southern hemisphere for both January and July. In January, the aircraft contribution to ozone at 200 hPa is less than 1 ppbv in the southern hemisphere and more than 3 ppbv at northern mid-latitudes with local maxima of 4 ppbv. In July, the aircraft emissions increase the ozone at 200 hPa from about 1 ppbv in the southern hemisphere up to about 10 ppbv in the northern polar region. The maximum contribution to 03 at 200 hPa (Fig. 6) occurs at northern mid-latitudes in January, whereas it is located in the northern polar region in July. This is a result of the poleward transport of the ozone resulting from the aircraft NO~ emissions due to the lifetime of ozone in the order of a month as well as the larger 03 production in the polar region due to the longer fraction of a day over which photochemistry is active and the lower NO~ background concentrations. The absolute contribution of the aircraft emissions to 03 at 200 hPa is almost zonally symmetric due to the relatively long lifetime of 03. C T M K has previously been used to investigate the effect of aircraft NOx emissions on the chemical composition of the atmosphere using other NO~ emission inventories, using a coarser horizontal resolution of 8 × 10 ° and using a prescribed ozone flux at 175 hPa instead of the prescribed ozone concentrations at 50 hPa (cf. Wauben et al., 1994; 1995; AERONOX, 1995). The prescribed ozone concentrations used in this paper resulted in higher ozone mixing ratios in the lower stratosphere and upper troposphere which are in better agreement with observations (cf. Wauben et al., 1997). In contrast, the NOx mixing ratios in the lower

stratosphere and upper troposphere are reduced considerably and are now too low compared to observations, especially in the lower stratosphere. A coarser horizontal resolution of 8 × 10° did hardly alter the monthly mean mixing ratios of NOx and 03. The finer horizontal resolution of 4 × 5° increased the ozone concentrations in the upper troposphere due to the enhanced stratosphere-troposphere exchange (van Velthoven and Kelder, 1996). In addition, the standard deviation of the monthly mean ozone results are typically about 10% higher when using the finer horizontal resolution. The differences in background NOx and 03 fields mainly affected the relative contributions of the NOx and 03 perturbation caused by the aircraft emissions, but had little effect on the absolute values of the calculated perturbation. The absolute values for the NOx perturbation calculated with the prescribed 03 concentrations at 50 hPa are slightly smaller than AERONOX (1995) results obtained with a fixed ozone flux at 175 hPa, whereas the relative NO~ perturbation caused by aircraft emissions is now higher due to the lower NOx background concentrations, especially in January. The resulting ozone perturbation is almost identical to that of the previous study in absolute numbers, but the relative numbers are reduced by about a factor of 2 near the tropopause because of the higher ozone background values. In spite of the differences in the magnitude and global distribution of the emissions the perturbations caused by the aircraft NO~ emissions were qualitatively similar to the ones reported here. The differences in NO~ lightning sources, which are of particular interest because they are next to aviation the only sources of NOx in the free troposphere and hence may influence the effect of aviation, did not affect the perturbation due to aviation, probably because these emissions occur mainly in the sub tropics during summer, far away from the aviation sources. Table 3 gives an overview of both the absolute and relative contribution of the changes induced by aircraft NOx emissions to the concentrations of NO~, 03, HNO3 and OH for January and July. The contributions are given for (i) the total global tracer amount from the surface up to 125 hPa, (ii) the maximum contribution to the zonal mean tracer amount, and (iii) the maximum contribution to the local tracer amount. As might be expected for short-lived tracers, the maximum local contributions are larger than the maximum zonal contributions, which in turn are larger than the relative contribution to the total tracer amount. The maximum local contributions are generally found near the North Atlantic Flight Corridor (NAFC). The trace gases with relatively short lifetimes, such as OH, show large differences between the maximum local and zonal contributions, whereas this difference is small for long-lived trace gases like 03. The effect of NOx emitted by aircraft on the global amount of this trace gas is rather small since the net effect is relatively small in the lower part of the atmosphere, which contains a large fraction of the total

3D Chemistry transport model study of aircraft emissions

1833

Table 3. Contributions of aircraft NOx emissions to monthly mean NOx, 03, HNO3 and OH mixing ratios for January and July. Reported are absolute (in pptv) and relative (in %) contributions to the total tracer amount up to 125 hPa, the maximum contribution to the zonal mean tracer amount, and the maximum local contribution January Total Absolute (pptv) NOx 03 HNO3 OH Relative (%) NO:, 03 HNO3 OH

2 746 12 0.004 4 1 8 2

Zonal 49 3472 196 0.038 81 3 31 31

global amount of trace gases. The relative contributions to the trace gases depend on the background concentrations which vary strongly with season. Also the local contributions, in particular to 03, have to be judged with care, as the background variability of ozone is in the same range as the aircraft signal. Also note that local effects are limited by the resolution of the global model, which is 4 x 5 ~ in this work. The absolute contribution of aircraft to NO~ listed in Table 3 is larger in January than in July. Its relative contribution is even larger in January due to the smaller background concentrations for NO~. The absolute contribution of aviation to 03 is larger in July while the relative contribution in January and July are more similar. Table 3 also shows that the increase of NO~ due to aircraft emissions is accompanied by an increase of nitric acid and hydroxyl via reactions (R9) and (R10), respectively. In accordance with the increase of OH the concentration of HO2 decreases. The absolute contribution of aircraft NOx emissions to the zonal mean NOx obtained with CTMK (cf. Fig. 5) show a similar pattern than two-dimensional model results (e.g. AERONOX, 1995; Strand and Hov, 1996). However, CTMK gives a somewhat lower perturbation, in particular at cruise altitudes and at lower altitudes. Three-dimensional models also give smaller changes to ozone than the two-dimensional models. A comparison of the passive transport of aircraft NOx emissions in two- and three-dimensional models did not show important differences (cf. van Velthoven et al., 1997). Hence, the difference must be due to the chemistry, in particular the non-linearity of the chemical processes. For example, Beck et al. (1992) using a two-dimensional model estimated maximum changes in ozone of about 20 ppbv in the NAFC. The aircraft NO~ emissions are more locally confined in three-dimensional models than in twodimensional models, where an instantaneous mixing in the East-West or North South direction occurs by definition. The resulting higher NOx concentrations in three-dimensional models may, because of saturation, yield less ozone production (cf. Ehhalt and

July Local

Total

Zonal

Local

121 3908 220 0.080

2 1003 14 0.005

39 10,068 274 0.054

106 10,234 294 0.136

5 1 9 3

40 4 33 14

62 5 37 30

88 4 36 60

Rohrer, 1994). CTMK calculates a NOx perturbation from aircraft emissions of typically about 50 and 30 pptv at cruise altitudes in northern mid-latitudes for January and July, respectively, with local maxima of about 100 pptv. These perturbations compare well with values of 25-50 pptv with local maxima of about 90 pptv obtained with a 3D Lagrangian chemistry transport model using the same emission sets but with a more detailed chemistry module (Stevenson et al., 1997). In relative numbers the NOx perturbation calculated with CTMK is typically 80% in January and 40% in July. The relative contribution of aviation to NO~ in January is high compared to other estimates (40% Ehhalt et al., 1992; 50% Kasibhatla, 1993; 25% Brasseur et al., 1996). These differences can partly be ascribed to differences in the amount of NO~. emitted by aviation, but is mainly the result of the lower background NO~ mixing ratios at cruise altitudes in northern mid-latitudes in January. The resulting increase in ozone calculated with CTMK, which is maximally 3 ppbv (2%) in January and 9 ppbv (4%) in July, is, however, in good agreement with the 3D calculations of Brasseur et al. (1996). An ozone perturbation of maximally about 9 ppbv is also obtained by Stevenson et al. (1996), but they find that the ozone perturbation has almost no seasonal dependence. The perturbation of NOx by the aircraft emissions at cruise altitudes in northern mid-latitudes is large compared to the standard deviation (20-100pptv compared to 10-20 pptv). In fact, the relatively low variability at 200 hPa in northern mid-latitudes in January is probably the result of the large contribution of continuous aircraft emissions. Therefore, it is expected that the effect of aviation on NOx is distinguishable from the contribution from the other sources. As the standard deviation of ozone is about 30%, it is doubtful whether the modifications due to aircraft emissions are observable from measurements. Especially, since in situ observations will show even larger fluctuations due to the presence of small-scale meteorological structures that are not resolved by the

1834

W.M.F. WAUBEN et al.

model. However, the effect of aviation on ozone may become significant in observations under special meteorological situations or in the future if the aircraft NOx emissions continue to increase more rapidly than the other NOx sources.

5. CONCLUSIONSAND OUTLOOK The impact of present day aircraft emissions of nitrogen oxides on atmospheric NOx and ozone concentrations is investigated with the global three-dimensional chemistry transport model CTMK. C T M K seems to give realistic values for the concentrations of NOx and 03 in the free troposphere and reproduces observed characteristic features of these species. The variability of the NOx background concentrations is large, especially in the lower stratosphere polar region during winter (up to 70%) and in the lower and middle troposphere winter polar region (90%). The standard deviation of NOx is typically about 30% at cruise altitudes in northern mid-latitudes. The standard deviation of the monthly mean ozone background concentrations is typically 20% near the surface and about 30% at cruise altitudes. Aviation contributes to 20-100 pptv of the NOx at cruise altitudes in northern mid-latitudes, i.e. 30-80% and 20-50% for January and July, respectively. The increase in NOx occurs mainly in the North Atlantic Flight Corridor and is transported eastwards from the emission area. The effect of aircraft emissions is found to be small (i.e. less than 5 pptv) in the southern hemisphere. The resulting increase in upper tropospheric ozone poleward from 30°N is 2-3 ppbv (1-2%) in January and 5-10 pptv (2-3%) in July. The ozone perturbation is almost zonally symmetric and attains maximum values at northern mid-latitudes in January and in the polar region in July. The perturbation exhibits a strong seasonal dependence. The perturbation of NOx by the aircraft emissions at cruise altitudes in northern mid-latitudes is most likely distinguishable from the contributions from the other NOx sources. As the standard deviation of background ozone is already about 30%, it is doubtful whether the ozone perturbation caused by aviation is detectable in measurements except under special meteorological conditions. Many improvements/extensions could be made to the present study. However, this field of research is relatively new and basic input such as the emission data, especially from other sources than aircraft, still have large uncertainties. The chemical and physical processes in the atmosphere are also not completely understood. However, chemistry transport models are continuously being improved. Specifically, the resolution of C T M K will be improved together with better representations of sub scale processes and stratosphere-troposphere exchange; more detailed chemical modules will be developed, including e.g. non-methane hydrocarbons and PAN, which can also be used

in the polluted planetary boundary layer and in the stratosphere; photolysis rates will be calculated online to allow for a feedback of ozone changes via photolysis. In addition, improved emission inventories will be developed and observations of trace gas distributions will be performed. Currently, C T M K is used in the framework of the European Union P O L I N A T and STREAM projects, which address some of these aspects. All this will improve the simulation of the atmospheric effects of aircraft emissions. However, the results obtained with C T M K are consistent with those obtained by others using different models and emission inventories. This supports our results of the perturbation of NOx and ozone due to aircraft emissions. The ozone perturbation resulting from aircraft emissions should be evaluated in terms of its impact on climate and the environment. Here, only dynamical and chemical aspects are considered, but the impact of a given ozone perturbation strongly depends on its location. The biosphere is only directly affected by an ozone perturbation because of its toxicity by changes near the surface. In contrast, the climate sensitivity to ozone perturbations is largest near the tropopause, whereas the amount of damaging ultraviolet radiation reaching the surface is mainly influenced by the change in the total ozone column density. A first investigation of the direct and indirect effects of aircraft on radiative forcing as an indicator for climate impact has been made by Fortuin et al. (1995) using literature estimates for the changes in the relevant atmospheric constituents due to aircraft emissions. These calculations showed that the climatic impact of ozone changes due to aircraft emissions are relatively small, but since air traffic will increase in the future and with it their ozone perturbation this may become significant. In addition, aviation introduces other components into the atmosphere that affect radiative forcing and may have an even larger impact on climate. Acknowledgements--This work has been performed in the

framework of the European Union AERONOX project under CEC contract EV 5V-CT 91-0044. We thank Martin Heimann and Ralf Hein from the Max-Planck-lnstitut fiir Meteorologie in Hamburg for making the TM2 model available to us and Paul Crutzen and Peter Zimmermann from the Max-Planck-Institut fiir Chemie in Mainz for providing the chemistry module. Stimulating discussions and cooperation within the AERONOX project is also gratefully acknowledged.

REFERENCES

AERONOX (1995) In The Impact of NOx Emissions from Aircraft upon the Atmosphere at Flight Altitudes 8-15 km (edited by Schumann U.), EC-DLR Publication on Research Related to Aeronautics and Environment, Brussels. Atkinson R., Baulch D. L., Cox R. A., Hampson R. F., Kerr J. A. and Troe J. (1992) Evaluated kinetic and photochemical data for atmospheric chemistry supplement IV. J. phys. Chem. Ref Data 21, 1125-1568.

3D Chemistry transport model study of aircraft emissions Beck J. P., Reeves C. E., de Leeuw F. A. A. M. and Penkett S. A. (1992) The effect of aircraft emissions on tropospheric ozone in the Northern Hemisphere. Atmospheric Environment 26A, 17-29. Brasseur G. P., Miiller J. F. and Granier C. (1996) Atmospheric impact of NO~ emissions by subsonic aircraft: a three-dimensional study. J. geophys. Res. 101, 1423-1428. Bregman B. and Lelieveld J. (1997) Evaluation of H2 and ~b1~" N20 oxidation by O 2~ /~,+ )' in the lowermost stratosphere during summer. Geophys. Res. Lett. (submitted). Briihl C. and Crutzen P. J. (1989) On the disproportionate role of tropospheric ozone as a filter against solar UV-B radiation. Geophys. Res. Lett. 16, 703 706. Briihl C. and Crutzen P. J. (1993) MPIC two-dimensional model. In The Atmospheric Effects of Stratospheric Aircraft: Report of the 1992 Models and Measurements Workshop (edited by Prather M. J. and Remsberg E. E.). NASA Reference Publication 1292, Washington, District of Columbia. Chatfield R. B. (1994) Anomalous HNO3/NO, ratio of remote tropospheric air: conversion of nitric acid to formic acid and NO~? Geophys. Res. Lett. 21, 2705 2708. Crutzen P. J. (1979) The role of NO and NO2 in the chemistry of the troposphere and stratosphere. Ann. Rev. Earth Planet. Sci. 7, 443--472. Crutzen P. J. (1995) Ozone in the troposphere. In Composition, Chemistry, and Climate of the Atmosphere (edited by Singh. H. B.). Von Nostrand Reinhold, New York. Crutzen P. J. and Zimmermann P. H. (1991) The changing photochemistry of the troposphere. Tellus 43AB, 136 151. Danielsen E. F. (1968) Stratospheric tropospheric exchange based on radioactivity, ozone and potential vorticity. J. atmos. Sci. 25, 502 518. De More W. B., Sander S. P., Golden D. M., Hampson R. F., Kurylo M. J., Howard C. J., Ravishankara A. R., Kolb C. E. and Molina M. J. (1992) Chemical Kinetics and Photochemical Data .]'or use in Stratospheric Modelling. J.P.L. Publ. 92-20, Pasadena. Dentener F. (1993) Heterogeneous chemistry in the troposphere. Ph.D. dissertation, Utrecht University, Utrecht. Dentener F. and Crutzen P. J. (1993) Reaction of NzO5 on tropospheric aerosols: impact on the global distributions of NO.,, 03 and OH. J. geophys. Res. 98, 7149 7163. Dentener F., Hein R. and Roelofs G. J. (1995) A comparison of background ozone concentrations calculated by the three dimensional models ECHAM, MOGUNTIA and TM2 with measurements. Report CM-88, International Meteorological Institute in Stockholm, Department of Meteorology, Stockholm University, Stockholm. Derwent R. G (1982) Two-dimensional model studies of the impact of aircraft exhaust emissions on tropospheric ozone. Atmospheric Environment 16, 1997-2007. Dianov-Klokov V. I. and Yurganov L. N. (1981) A spectroscopic study of the global space-time distribution of atmospheric CO. Tellus 33, 262 273. ECAC/ANCAT and EC Working Group (1995) A Global Inventory ~" Aircraft NO~ Emissions. In AERONOX, (edited by Schumann U.). EC-DLR Publication on Research Related to Aeronautics and Environment, Brussels. Ehhalt D. H. and Drummond J. W. (1988) NO~ sources and the tropospheric distribution of NO~ during STRATOZ llI. In Tropospheric Ozone (edited by Isaksen I. S. A.). D. Reidel, Dordrecht. Ehhalt D.H. and Rohrer F. (1994) The impact of commercial aircraft on tropospheric ozone. Proc. 7th BOC Priestly Conf. Lewisburg. Ehhalt D. H., Rohrer F. and Wahner A. (1992) Sources and distribution of NO~ in the upper troposphere at northern mid-latitudes. J. geophys. Res. 97, 3725-3738.

1835

Emmons L. K., Carroll, M. A. et al. (1997) Climatologies of NO~ and NOr: a comparison of data and models. Atmospheric Environment 31, 1851-1904. Fehsenfeld F. C. and Liu S. C. (1994) Tropospheric ozone: distribution and sources. In Global Atmospheric Chemistry Change (edited by Hewitt C. H. and Sturges W. T.). Elsevier, London. Fortuin J. P. F and Langematz U. (1995) An update on the global ozone climatology and on concurrent ozone and temperature trends. Proc. SPIE Atmospheric Sensing and Modeling, Vol. 2311, pp. 207-216. Fortuin J. P. F., van Dorland R., Wauben W. M. F. and Kelder H. (1995) Greenhouse effects of aircraft emissions as calculated by a radiative transfer model. Ann. Geophys. 13, 413-418. Fung I., John J., Lerner J, Matthews E., Prather M., Steele, L. P. and Fraser P. J. (1991) Three-dimensional model synthesis of the global methane cycle. J. geophys. Res. 96,

13,003-13,065. Gardner R., Adams K. et al. (1997) The ANCAT/EC global inventory of NOx emissions from aircraft. Atmospheric Environment 31, 1751 1766. Granier C. and Brasseur G. (1992) Impact of heterogeneous chemistry on model predictions of ozone changes. J. geophys. Res. 97, 18,015 18,033. Hauglustaine D. A., Granier C., Brasseur G. P. and M6gie G. (1994) lmpact of present aircraft emissions of nitrogen oxides on tropospheric ozone and climate forcing. Geophys. Res. Lett. 21, 2031 2034. Heimann M. (1995) The global atmospheric tracer model TM2: DKRZ TM2 model documentation. Technical Report No. 10, Max-Planck-lnstitut ffir Meteorologic, Hamburg. Hein R. (1994) Inverse Modellierung des Atmosph~irischen Methan-Kreislaufs unter Verwendung eines Drei-Dimensionalen Modells des Transport und der Chemic der Troposph~ire. Examensarbeit No. 25, Max-Planck-Institut ffir Meteorologic, Hamburg. Hidalgo H. and Crutzen P. J. (1977) The tropospheric and stratospheric composition perturbed by NOx emissions of high altitude aircraft. J. geophys. Res. 82, 5833 5866. Holton J. R. (1990) On the global exchange of mass between the stratosphere and troposphere. J. atmos. Sci. 47, 392 395. Hough A. M. (1991) Development of a two-dimensional global tropospheric model: model chemistry. J. geophys. Res. 96, 7325-7362. IPPC (1995) Climate Change 1994: Radiative Forcing o/ Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios. lntergovernmental Panel on Climate Change, (edited by Houghton J. T. et aLL Cambridge University Press, Cambridge. Jacob D. J.. Prather M. J. et al. (1997) lntercomparison and evaluation of global atmospheric transport models using 222Rn and other short-lived tracers. J. geophys. Res. (in press). Johnson C., Henshaw J. and Mclnnes G. (1992) Impact of aircraft and surface emissions of nitrogen oxides on tropospheric ozone and global warming. Nature 355, 69 71. Johnston H. S. (1971) Reduction of stratospheric ozone by nitrogen oxide catalysts from supersonic transport exhaust. Science 173, 517 522. Johnston H. S., Kinnison D. S. and Wuebbles D..1. 11989) Nitrogen oxide from high altitude aircraft: an update of potential effects on ozone. J. ~teophvs. Res. 94, 16,351 16,363. Kanakidou M., Singh H. B., Valentin K. M. and Crutzen P. J. (1991) A two-dimensional study of ethane and propane oxidation in the troposphere. J. geophys. Re.v 96, 15,395 15,413. Kasibhatla P. S. (1993) NOy from sub-sonic aircraft emissions: a global three-dimensional model study. Geophys. Res. Lett. 21, 1707 1710.

1836

W . M . F . WAUBEN et al.

Kasibhatla P. S., Levy II H., Moxim W. J. and Chameides W. L. (1991) The relative impact of stratospheric photochemical production on tropospheric NO r levels: a model study. J. geophys. Res. 96, 18,631-18,646. Kasibhatla P. S., Levy II H. and Moxim W. J. (1993) Global NOx, HNO3, PAN, and NOy distributions from fossil fuel combustion emissions: a model study. J. geophys. Res. 98, 7165-7180. K/Shler I., Sausen R. and Reinberger R. (1997) Contributions of aircraft emissions to the atmospheric NO~ content. Atmospheric Environment 31, 1801-1818. Komhyr W. D., Oltmans S. J., Franchois P. R., Evans W. F. J. and Matthews W. A. (1989) The latitudinal distribution of ozone to 35 km altitude from ECC ozonesonde observations, 1985-1987. In Ozone in the Atmosphere (edited by Bojkov R. D. and Fabian F.). A. Deepak, Hampton. Kraus A. B., Rohrer F., Grobler E. S. and Ehhalt D. H. (1997) The global tropospheric distribution of NO~ estimated by a 3-D chemical tracer model. J. geophys. Res. (submitted). Lacis A. A., Wuebbles D. J. and Logan J. A. (1990) Radiative forcing of climate by changes in the vertical distribution of ozone. J. geophys. Res. 95, 9971-9981. Lee D. S., K/~hler I., Grobler E., Rohrer F., Sausen R., Gallardo Klenner L., Olivier J. J. G., Dentener F. J. and Bouwman A. F. (1996) Estimates of global NO~ emission and their uncertainties. Atmospheric Environment 31, 1735-1749. Louis J. F. (1979) A parametric model of vertical eddy fluxes in the atmosphere. Boundary-Layer Met. 17, 187-202. Mahowald N. M., Rasch P. J. and Prinn R. G. (1995) Cumulus parameterizations in chemical transport models. J. geophys. Res. 100, 26,173-26,189. NASA (1976) U.S. Standard Atmosphere. U.S. Government Printing Office, Washington, District of Columbia. Newell R. W., Kidson J. W., Vincent D. G. and Boer G. J. (1974) The General Circulation of the Tropical Atmosphere and Interactions with Extratropical Latitudes, Vol. 2. MIT Press, Cambridge. Oltmans S. J., Komhyr W. D., Franchois P. R., Evans W. F. J. and Matthews W. A. (1989) Tropospheric ozone: variations from surface and ECC ozonesonde observations. In Ozone in the Atmosphere (edited by Bojkov R. D. and Fabian F.). A. Deepak, Hampton. Prather M. J. (1986) Numerical advection by conservation of second-order moments. J. geophys. Res. 91, 6671-6681. Ramaroson R. and Louisnard N. (1994) Potential effects of ozone of future supersonic aircraft/2D simulation. Ann. Geophys. 12, 986-995. Roelofs G. J. and Lelieveld J. (1995) Distribution and budget of 03 in the troposphere calculated with a chemistry general circulation model. J. geophys. Res. 100, 20,983-20,998. Russell G. L and Lerner J. A. (1981) A new finite-differencing scheme for the tracer transport equation. J. appl. Met. 20, 1483-1498. Schumann U. (1994) On the effect of emissions from aircraft engines on the state of the atmosphere. Ann. Geophys. 12, 365-384.

Singh H. B., Herlth D., O'Hara D., Zahnle K., Bradshaw B. Sandholm S. T.. Talbot R., Crutzen P. J. and Kanakidou M. (1992) Relationship of peroxyacetyl nitrate to active and total odd nitrogen at northern high latitudes: influence of reservoir species on NOx and 03. J. geophys. Res. 97, 16,523-16,530. Stevenson D. S., Collins W. J., Johnson C. E. and Derwent R. G. (1997) The impact of aircraft nitrogen oxide emissions on tropospheric ozone studied with a 3-D Lagrangian model including fully diurnal chemistry. Atmospheric Environment 31, 1837-1850. Strand A. and Hov O. (1996) The impact of man-made and natural NOx emissions on upper tropospheric ozone: a two-dimensional model study. Atmospheric Environment 30, 1291-1303. Tie X. X., Brasseur G., Ling X., Friedlingstein P., Granier C. and Rasch P. (1994) The impact of high altitude aircraft on the ozone layer in the stratosphere. J. atmos. Chem. 18, 103-128. Tiedtke M. (1989) A comprehensive mass flux scheme for cumulus parameterization in large-scale models. Mon. Weath Rev. 117, 1779-1800. Van Velthoven P. F. J. and Kelder H. (1996) Estimates of stratosphere-troposphere exchange: sensitivity to model formulation and horizontal resolution. J. geophys. Res. 101, 1429-1434. Van Velthoven P. F. J., Sausen R. Johnson C. E. Kelder H., Kingdon R., Krhler I., Kraus A., Ramaroson R., Rohrer F., Strand A. and Wauben W. M. F. (1997) The passive transport of NO~ emissions from aircraft with a hierarchy of models. Atmospheric Environment 31, 1783-1799. Velders G. J. M., Heijboer L. C. and Kelder H. (1994) The simulation of the transport of aircraft emissions by a three-dimensional global model. Ann. Geophys. 12, 385-393. Wauben W. M. F., van Velthoven P. F. J. and Kelder H. (1994) Chemistry and transport of NOx aircraft emissions in a global 3-D chemical transport model. In International Scientific Colloquium on the Impact of Emissions from Aircraft and Spacecraft upon the Atmosphere (edited by Schumann U. and Wurzel D.). DLR Mitteilungen 94-06, Cologne. Wauben W. M. F., van Velthoven P. F. J. and Kelder H. (1995) Changes in tropospheric NOx and O3 due to subsonic aircraft emissions. Scientific Report, WR 95-04, KNMI, De Bilt. Wauben W. M. F., Fortuin J. P. F., van Velthoven P. F. J. and Kelder H. M. (1997) Validation of modelled ozone distributions with sonde and satellite observations. J. geophys. Res. (submitted). Weinheimer A. J., Walega J. G. et al. (1994) Meridional distribution of NOx, NO r and other species in the lower stratosphere and upper troposphere during AASE II. Geophys. Res. Lett. 21, 2027-2030. Zimmermann P. H. (1988) MOGUNTIA: a handy global tracer model. In Air Pollution Modeling and its Applications VI (edited by van Dop H.). NATO/CCMS, Plenum, New York.