impact of aircraft emissions on stratospheric ozone: a research strategy

Pergamon Phys. Chem. Earth, Vol. 20, No. 1, pp. 123-131, 1995. Copyright © 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0079-1946(95)00014-3 0079-1946/95 $9.50 + 0.00

Impact of Aircraft Emissions on Stratospheric Ozone: A Research Strategy

B. Kiircher I and T. Peter 2

1 Lehrstuhl fOr Bioklimatologie und Immissionsforschung, Universit~t M0nchen, Hohenbachernstr. 22, D-85354 Freising, Germany 2 Max-Planck-Instittit for Chemie, Posffach 3060, D-55020 Mainz, Germany

ABSTRACT

We briefly summarize what is currently known about the impact of aircraft emissions on the chemical composition of the upper troposphere and lower stratosphere, with special emphasis on the ozone balance. We report on recent research work concerning the chemical transformation and particle formation processes in young aircraft exhaust plumes and outline a possible research strategy to investigate the present and future impact of air traffic on the stratospheric ozone layer by means of process-related numerical modeling and experimental near-field studies.

KEYWORDS

Aircraft emissions; stratospheric ozone; aerosol microphysics; heterogeneous reactions; process- related studies.

SCIENTIFIC BACKGROUND

In view of the rapidly growing commercial air traffic, the investigation of possible chemical and climatic effects of aviation on the atmosphere received a renewed interest in this decade and led to several international research programs [e.g., Schumann, 1994: Stolarski and Wesoky, 1995; World Meteorological Organization (WMO), 1995]. Besides the greenhouse gases carbon dioxide and water vapor, cruising aircraft primarily release nitrogen oxides, sulfur dioxide, non-methane hydrocarbons, and soot into a region of our atmosphere which is characterized by a high sensitivity with regard to changes of its radiative properties and long lifetimes of trace gases. On average 45 % of the total cruising time of civil airliners above the North Atlantic takes place in the lower stratosphere, with a maximum of 70 % in February and a minimum of 25 % in September [Hoinka ct al., 1993].

Model studies suggest that part of the nitrogen oxides (and therefore ozone) in the upper troposphere is due to past and present air traffic [Ehhalt et al., 1992]. However, it has not yet been conclusively demonstrated that subsonic aircraft emissions have a negative impact on stratospheric ozone. Concerning the threat of the stratospheric ozone layer by planned high-flying aircraft, models predict a considerable increase of the formation frequency of polar stratospheric clouds (PSCs) [Peter et al., 1991; Considine et al., 1994] and a related ozone loss of several per- cent, but such estimatcs suffer from existing uncertainties concerning the detailed mechanisms of aircraft-induced PSC formation and the long-term diffusive transport propertics of the lowermost stratosphere.

Yet another factor rcle~rant for ozone are the sulfur cmissions caused by air traffic, injecting more sulfur into the lower stratosphere than necessary to sustain the natural background aerosol

P ~ ZO:I-I 123

124 B. K~cher and T. Peter

layer. Measurements of Hofmann and Rosen [1978] suggest that new particles are formed by binary nucleation of sulfuric acid and water vapor in jet aircraft plumes. The observed increase of the background aerosol mass of about 5 %/yr in the past 20 years [Hofmann, 1990; Hofmann, 1991; J£ger, 1994] and the measured ozone loss at midlatitudes at altitudes between 10 and 25 km [Stolarski et al., 1991; Logan, 1994] could therefore be partly induced by aircraft. Ozone depletion at midlatitudes is very pronounced and observations reveal that air layers with low ozone abundances are often correlated with aerosol-rich layers (D.J. Hofmann, personal communication, 1995). In view of the importance placed on heterogeneous chemistry and its possible role in declining ozone levels [Peter and Crutzen, 1993], this issue should be a major research priority.

IMPACT ON GLOBAL ATMOSPHERIC CHEMISTRY

Upper Troposphere

Aircraft primarily interact with the tropospheric HOx cycle through the emission of nitrogen oxides. In a nitrogen oxide-free atmosphere, ozone is destroyed, whereas in the presence of NO~, ozone can be produced photochemicaily. The transition from ozone loss to ozone production occurs at a ratio of [NO]/[O3] ~ 2 x 10 -4 or at NO~ mixing ratios of roughly 20pptv for typical ozone concentrations of 100 ppbv [Ehhalt and Rohrer, 1994]. Including the emissions from subsonic aircraft, upper tropospheric NO mixing ratios in the northern hemisphere (40o-60 ° N) range from 10-400pptv [WMO, 1995]. Hence, cruising airliners favor ozone production due to their NOz emissions and shift the HO~ balance towards higher OH levels. The impact of emitted non-methane hydrocarbons resulting from incomplete jet fuel combustion on ozone production in the airlanes has not been satisfactorily explored.

Similarly, the impact of sulfur emissions at cruise altitudes is not weU understood, although there is now a growing body of experimental [Arnold et al., 1994; Baumgardner and Cooper, 1994; Hagen et al., 1994; Schumann et al., 1995] and theoretical [Miake-Lye et al., 1994; K£rcher et al., 1995; Zhao and Turco, 1995] evidence that aircraft are a strong source of new particles, most probably due to binary H2SO4/H20 gas-to-particle conversion. Through a perturbed nighttime chemistry, heterogeneous reactions of N2Os and NO3 on the enhanced aerosol surface area may reduce active NOx [Dentener and Crutzen, 1993] and in turn counteract the pure gas phase effect of the primary NO~ emissions.

Lower Stratosphere

Emissions from high-flying aircraft result in perturbations of the gas phase catalytic ozone destruction cycles involving NO~, HOx, CIO=, and BrO= species by causing a repartitioning between the chemical families. Nitrogen oxides emitted at lower stratospheric altitudes increase the relative importance of the corresponding catalytic destruction cycle, but at the same time the influence of the HO~ cycle becomes reduced due to deactivation of OH and HO2 by the newly formed NO~. Likewise, the influence of the CIOz cycle diminishes because the emitted NO2 tends to shift the daytime balance between C1ONO2 and C10 towards chlorine nitrate and the smaller OH levels prevent HC1 from forming atomic chlorine. Hence, more chlorine remains in inactive reservoirs as compared to an unperturbed situation.

In -s i tu observations point towards the potential importance of the heterogeneous reaction N~Os (gas) + H20 (liquid) --~ 2HNO3 (liquid) on the background sulfate aerosols [e.g., Fahey et al., 1993]. Compared to the pure gas phase chemistry, this leads to a reduction of gaseous active nitrogen levels, stronger C10 production under sunlit conditions, and reduced loss of HO= into HNO3. In summary, taking the heterogeneous processes into account, current global assessment models predict that the injection of NO= and water vapor by stratospheric aircraft leads to a small ozone decrease [WMO, 1995].

Figure 1 illustrates the change in model predictions of the ozone column due to changes in the understanding of basic chemical processes applied to an assumed scenario with NOz and H20 emissions at 20km from a fleet of 500 supersonic aircraft. The calculations up to 1988 were performed with the Lawrence Livermore National Laboratories 1D model [Wuebbles and

Impact of Aircraft Emissions on Stratospheric Ozone 125

I0

0

_ -10 o Y -15

~ -20 ©

-25 1974

_ _ ' - " ~ " - - - gasphas¢ on ly j

N O x e m i s s i o n s , 20kin

I f 1 I I I i i i t t i i t i t i i

1976 1978 1980 1982 1984 1986 1988 1990 1992

Year in which calculation was made

Fig.1. Calculated ozone column change for a supersonic aircraft scenario obtained by the LLNL 1D model (up to 1988) and by the Mainz 2D model (1992). Both models consider heterogeneous chemistry and microphysics only by means of rough parametrizations. Adapted from Peter and Crutzen [1994].

Kinnison, 1990] and the 1992 values are based on results of the Mainz 2D model [Grooss et al., 1994]. In the early 1970s the catalytic NOx and C1Oz ozone destruction cycles were treated separately, which led to an overestimation of the aircraft effects. The model even predicted net ozone production in the late 1970s due to a revised understanding of the HOz chemistry. By 1988 the calculated ozone losses again reached the 1975 level due to recognition of the enhanced importance of HO=-destroying catalytic cycles. Finally, in the 1990s, it was recognized that the global background aerosol is capable of removing part of the emitted active nitrogen species via the heterogeneous reaction N2Os (gas) + H20 (liquid) --* 2 HNO3 (liquid). However, on the basis of global models we cannot decide whether the predicted strong enhancement of PSC occurrence due to aircraft [Peter et al., 1991] may lead to much higher destruction of polar ozone than indicated in Fig.l, because such studies require the modeling of detailed microphysics and heterogeneous chemistry together with high spatial resolution.

NEAR-FIELD PLUME PROCESSING

In the early, aircraft-dominated phase of plume dispersion background conditions become strongly perturbed. Already on this local scale, in the so-called jet regime, secondary exhaust species, new aerosols, and ice~ crystals are generated within seconds after emission. The aircraft-induced components are then concentrated in the airlanes and become transported along synoptic-scale trajectories. Depending on the location of the emissions, the chemically processed air parcels are finally transported to lower or higher altitudes through tropopause folds or by eddy diffusion.

A variety of global models exist to investigate chemical effects of aviation [cf., Prather, 1994; Stolarski and Wesoky, 1995], but only very few theoretical studies are dedicated to the problem of physico-chemical transformations in the jet regime, although the processes at work in young aircraft plumes set the stage for the subsequent chemical and microphysical evolution of the aircraft exhaust [Miake-Lye et aL, 1993; K~cher and Fabian, 1994; K£rcher, 1994]. In the following, we describe two key issues of our recent research work on this topic.

Chemical Conversion

Among the most important phenomena occurring at high plume temperatures in the jet regime are the evolution of the ttO= radical balance and the conversion of primary NO, NO2, and SO2 into their respective acids HNO2, ttNOs, and H2SO~, which have been detected in-situ by Arnold et al. [1992, 1994] and Fahey et al. [1995]. Fast gas phase chemical reactions driven by high levels of emitted OH in the parts-per-million range cause the partial conversion of emitted species into secondary components on time scales of 10 milliseconds. For instance, the emitted SO2 becomes oxidized by OH, the lifetime of which is determined by reactions with NOx and partly by self

126 B. Kiircher and T. Peter , , " , v , , , i , , , , , , ,

~ NO2 inlo HNOs 10 NO into HN02

~'c° >" . . . . . S O z ~ ~

._u

I n -T " , , , , , , , , r , . . . . . . . 1 1 . 0 . 1 0 2

OH exit plane mJxang ratio (ppmv)

Fig.2. Maximum rates for the conversion of NO, NO2, and S02 into HNO2, HNO3, and H~SO4, respectively, in a B 747 jet exhaust plume versus the OH exit plane mixing ratio.

reactions. The resulting SO3 reacts rapidly with water vapor to give sulfuric acid, leading to H2SO4 mixing ratios which largely exceed the background abundances. The transformation of the nitrogen species evolves similarly. As a result of a sensitivity study, we depict the maximum conversion rates for a cruising B 747 as a function of the OH mixing ratio at the exit plane of the jet engine in Fig.2. Combustion simulations of the internal engine flow suggest [OH]0 -~ 10 ppmv, but this value is subject to considerable uncertainties. The data are taken from a gas phase chemical calculation which is fully coupled to the two-dimensional turbulent dynamics of jet expansion (B. K~rcher et al., Small-scale chemical evolution of aircraft exhaust species in the upper troposphere, manuscript submitted to the Journal of Geophysical Research, 1995). The results indicate linear dependences, that is, the conversion efficiencies grow in proportion to [OH]0 for low OH levels and show a tendency to saturate for high OH mixing ratios up to 100 ppmv, which is due to the non-linear HOz chemistry and HOz-NOz coupling reactions in the jet plume. In summary, for [OH]0 around 10ppmv, we find conversion rates of 2 - 4 % for NO= and around 0.5 % for SO2, in agreement with the recent experimental findings.

Aerosol Formation

As the jet plume cools down to ambient temperatures binary homogeneous H2SO4/H20 nucleation rapidly depletes the sulfuric acid molecules and produces sub-nanometer liquid solution droplets. They initially grow quickly due to bimolecular condensation. As soon as the gaseous H2SO4 is depleted the droplets take up additional water vapor as long as the temperature is decreasing. This results in an effective radial growth and a reduction of the sulfuric acid weight fraction of the largest droplets, whereas the smallest particles stay highly acidic. In addition, coagulation rapidly reduces the number of small droplets, causing a slow increase of their radii. Figure 3 illustrates these microphysical processes in a weight percentage-temperature-phase diagram (thick solid lines) for the B 747 plume under threshold contrail formation conditions, where the jet plume never reaches water saturation during cooling [K~rcher et al., 1995]. Only the largest droplets enter the region of high homogeneous freezing rates (dashed lines) and nucleate ice crystals. In this simulation up to a plume age of 103 s, the vast majority of new particles stays below radii of 10 nm and constitute supercooled, long-lived solution droplets.

The importance of particle formation processes in aircraft plumes has recently been demonstrated by Busen and Schumann [1995], who investigated contrail formation using fuels with different sulfur contents during one single flight. According to the common notion, homogeneous nucleation of binary H2SO4/H20 droplets should constitute a necessary precursor for the generation of ice

Impact of Aircraft Emissions on Stratospheric Ozone

=t . . . . . . ///0 t .4nm 0. ! 5

S'IAA >lOOnm 0.25

1 260I ~ ~ / / / / / / t :

i -j~0: " " 1 V l 0

~'("1(} I . , L I ~ ' ~ ¥ I I I I I I I

---0 0.2 0.4 0.6 0.8 1.0 sulfuric acid weight fraction

Fig.3. Phase diagram of homogeneously nucleated sulfuric acid droplets in a B 747 jet plume under cruising conditions. The thick lines show the evolution of the H2SO4 weight fraction as a function of temperature and plume age for droplets with different radii. The thin lines are melting curves for several hydrates. The dashed lines indicate homogeneous freezing rates per cm ~ droplet volume per second.

127

crystals in contrails. This would result in a very strong sensitivity of the threshold formation conditions on the fuel sulfur content. With their experiment they could show that the build- up of visible contrails does not depend on the sulfur level for emission indices below 0.25 g S per kg fuel. This surprising result is in accord with the calculations performed by Kircher et al. [1995] who predicted that binary homogeneous droplet formation cannot be the dominant mechanism for visible contrail formation. In addition, we find strong evidence that activation of the combustion aerosols (soot) emitted by the jet engines was largely responsible for the creation of the ice crystals that made up the observed contrail (B. Kixcher et al., The initial composition of jet condensation trails, manuscript in preparation, 1995). However, the detailed mechanisms that lead to soot activation are still unresolved.

RESEARCH STRATEGY

Two- and three-dimensional models currently employed to investigate stratospheric transport and chemistry issues rely on very simplified parametrizations of heterogeneous processes, which often mask chemical or microphysical details. As an example relevant for aircraft studies the kinetics of particle formation and growth (for both liquid aerosols and solid PSCs) axe omitted and one tries to cover the gross effects using equilibrium assumptions and prescribed particle properties. The reasons for using parametrizations in global-scale modeling are threefold. First, details of the underlying physical mechanisms may still be unknown. Second, the processes under consideration evolve on spatial or temporal scales too small to be resolved by global models. Third, the numerical complexity of the processes render their implementation into large-scale models ineffective due to large demands on computational storage and CPU time.

To account for important near-field phenomena in future investigations, we propose to comple- ment these efforts by process-related modeling. Such work should focus specifically on physico- chemical interactions between gases and particles and on heterogeneous chemical reactions relevant to the fate of stratospheric ozone in aging aircraft plumes, which are depicted schemati- cally in Figs.4 and 5. The primary aerosol is composed of freshly nucleated solution droplets,


activated soot particles, and entrained background aerosol, which interact by condensation and coagulation in the cooling plume. Part of the aerosol may freeze and create the visible ice contrails. The resulting secondary aerosol spectrum contains both liquid and solid components (with and without soot inclusions) and interacts on synoptic timescales with the background aerosol layer. A proper description of these interactions forms the basis for the evaluation of possible warm and cold heterogeneous chemical reactions that may occur on the surfaces or in the bulk of the aircraft-induced aerosols as well as of the potential for activating chlorine and inducing catalytic reaction chains that affect ozone. The approx- imate timescales over which such processes will be operative are also indicated in Fig.5, ranging from fast radical chemistry and nucleation phenomena (10 -4 s) up to typical transport times of air parcels (10 ~ s).

A suitable tool for these investigations are trajectory box models. Whereas the initial, aircraft-dominated evolution of the box is governed by intense

heteromoiaculsr gee-to-particle

convemion homogeneous I heterogeneous nucleation i nucleaUon on

primary volatile . soot ~ background aerosol droplets particles aerosol

t t

IraNa- {orma- lion (in iltu)

secondary aerosol

trarte- forma tion atmo

sphere)

modlfled background aerosol

heleromolecular condensation coagulation end collection

freezing nucleation

l lquld llquld aerosol Ice

aerosol Inclusions

I J evaporation

contrail cycle

cloud condeneatlon nuclel heterogeneous reactions

Fig.4. Illustration of the physico-chemical processes that generate and modify the types of aerosol particles in aging aircraft plumes.

entrainment processes and high cooling rates [K/ixcher, 1995], it later follows synoptic-scale trajectories which can be derived from meteorological data or general circulation models. Referring to Figs.4 and 5, key questions which have to be resolved include:

What is the composition of the primary aerosols generated in the young jet plume ? Do the strongly perturbed gas phase conditions favor the build up of ternary mixtures ?

What are the lifetimes and the microphysicai properties of ice contrails at midlatitudes in the polar regions under stratospheric conditions ? Do these ice crystals trigger PSC I-formation ?

How do we describe heterogeneous chemistry in or on internally mixed aerosol particles ? What is the importance of soot inclusions in liquid or solid particles ?

How does the perturbed gas phase chemistry in aging aircraft plumes control the efficiency of heterogeneous reactions ? Could additional nitric acid production from excess exhaust NO~ enhance particle formation and growth rates ?

The trajectories and entrainment rates as drivers of the box models can be varied for different aircraft scenarios (sub- or supersonic, emission indices) as well as for different meteorological conditions (up- or downward transport, lee-wave activity) and should take into account small- and mesoscale temperature fluctuations that can greatly enhance cooling or heating rates over synoptical values [Murphy and Gary, 1995]. This type of study renders detailed investigations and sensitivity studies of all single underlying mechanisms possible, thus allowing relevant and insignificant processes to be discriminated and eventually helping to place bounds on the range of uncertainties of unknown parameters. Research in this direction will certainly bene- fit from earlier investigations of heterogeneous chemical processing of air parcels, for example during the EASOE campaign [Peter et al., 1992; Mfiller et al., 1994] in the Arctic stratosphere.

primary aerosol

secondary aerosol

modif ied background aeroso|

Impact of Aixcraft Emissions on Stratospheric Ozone

gu-to-p~tickm conversion -4

:.10

!10 "3

~10 "2

I -1 : 1 0

warm heterogeneous reactlolls

NOx ~ HONO > HNO~

SOx '> H.~SO4

cooling and freezing mixing with background air

cold hcterogeneaus reactions

N20s + H~O > 2 HNO]

HCI + CIONO2 > CI2 + HNO~

H20 • CIONO2 > HOC[ + HNO3

HCl + HOCI ) CI 2 + H20

further dilution ClOx/NOx i ~ with O] long-lived cirrus-like clouds

!1

1 LIO

i 2

3 .10

4

S ~_10

6 [_10 i i t/sec

Fig.5. Possible heterogeneous chemical reactions involving the particles present in aging aircraft plumes.

129

Of course, exchange of results with global modelers will be equally important, either for the development of improved, accurate parametrization schemes or for the assessment of the global relevance of discovered phenomena.

The theoretical efforts should also be coordinated with process-related laboratory and field experiments. Although the most important gas phase chemical reactions relevant for jet plume chemistry are known, they have to be sub- stantiated by additional measurements, as illustrated by changes in the rate coefficient of the reaction SO3 + H~O

M -'-'* H2SO4 discovered by Reiner and Arnold [1993]. Furthermore, precise near-field measurements of emission indices of key exhaust products like OH, NO~ (especially NO2), and CO under cruising conditions are still pending. Results from experiments dedicated to resolve open questions concerning the physico-chemicai state and freezing be-

haviour of stratospheric aerosols are strongly related to droplet evolution and contrail formation in aircraft plumes and can be directly applied to aircraft-related research issues. In view of the complexity of the particle formation processes and of basic uncertainties concerning the underlying nucleation theory, we certainly need more experimental near-field contrail studies simultaneously detecting meteorological conditions, relevant gas phase chemical budgets, as well as composition, size, and morphology of in-situ generated particles to further constrain the model results.

CONCLUSIONS AND OUTLOOK

The heterogeneous chemistry of ice crystals, liquid aerosols, and soot particles in aircraft contrails generated at tropopanse altitudes is virtually unexplored. It is conceivable that the same reaction channels are operative in the flight corridors at midlatitudes, which so effectively destroy ozone in the polar stratosphere. Moreover, because aging aircraft plumes may provide a strongly perturbed chemical environment for days after release of the primary exhaust and in view of the pronounced nonlinearity of heterogeneous chemical and microphysical processes, we can probably expect surprising results from aircraft-related investigations.

For this reason, we strongly emphasize the need for coordinated research projects, both experimental and theoretical, which address these issues. This is certainly an area which requires a great deal of effort, but we believe that such detailed work becomes essential in future assessments. The resulting scientific progress forms a sound basis for a judgment of the chemical influence of air traffic on the global stratospheric ozone layer and supports efforts to develop suitable and reliable parametrizations of important sub-grid processes for use in large-scale chemical transport models. Furthermore, we expect a far-reaching applicability of such results to the heterogeneous chemistry of aerosols and ice crystals in natural cirrus clouds in the upper troposphere.

ACKNOWLEDGEMENT

We gratefully acknowledge support by the German Environmental Agency, Berlin.


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impact of aircraft emissions on stratospheric ozone: a research strategy

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