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Transport and the Environment
Issues in Environmental Science and Technology
By R. E. Hester, R. M. Harrison
The Royal Society of ChemistryCopyright © 2004 The Royal Society of Chemistry
All rights reserved.
The Impact of Aviation on Climate
DAVID S. LEE
The atmospheric impact of aviation falls into two distinct categories: those upon the global atmosphere and those upon local air quality. Further, the impact upon the global atmosphere can be subdivided between climate change and stratospheric ozone (O) depletion. The latter has only been studied from a hypothetical point of view, since this is a potential effect that might arise from a fleet of supersonic aircraft flying in the mid stratosphere. This chapter will focus on the effects of aviation on the global atmosphere and, in particular, climate change. Whilst the scientific understanding of aviation's impacts on air quality is reasonably well understood, specific details that allow robust assessments of aviation impacts on local air quality — perhaps surprisingly — are only poorly quantified and the reader is directed elsewhere for a brief overview.
Interest in aviation's effects on climate has been provoked by the strong growth of the aviation industry, which has outstripped GDP, long-term growth rates of the order 5% per year being sustained. Particular events have been associated with set-backs to this growth: the Gulf conflict in the early 1990s slowed growth but it picked up quickly and returned to the long-term trend within a few years. More recently, the overall economic downturn of the industry, September 11th, and 'SARS' have taken their toll but there are indications that growth rates are recovering. The seasonal patterns of departures between 1997 and 2003 in Europe (Figure 1) provide evidence of this.
Many forecasts of aviation growth have been made, both by the industry and others; typical is that of the UK Department of Trade and Industry (Figure 2), which shows historical and future projected growth to 2020 in terms of the overall capacity.
In developing this overview, and what current research is telling us, it is worth considering some historical aspects — the origins of interest date back perhaps further than one might suspect. Local air quality was the original driver for the development of aircraft engine emissions regulations by the International Civil Aviation Organization (ICAO), first promulgated in 1981 (although earlier local rules to the USA were introduced by the US Environmental Protection Agency in 1973). However, one of the initial drivers of interest in aviation's atmospheric effects was concern in the late 1960s and early 1970s that emissions of nitrogen oxides (NOx, where NOx = NO + NO2) from a (proposed) fleet of supersonic aircraft flying in the stratosphere would significantly deplete stratospheric ozone (O3), resulting in increased exposure of harmful ultraviolet (UV) radiation at the Earth's surface.' The scientific research programmes that this concern initiated were of quite epic proportions and laid many of the modern foundations of our understanding of stratospheric chemistry and physics. A more detailed account of these research programmes, and their development, is given elsewhere. In fact, the US research programme proceeded after the decision had been taken in the US not to build a supersonic transport (or 'SST') and was partly in response to the intentions of the UK and France to build Concorde, and the USSR the Tupolev TU-144. During this early work it was conjectured that the current subsonic fleet may, in fact, impact upon tropospheric O3, following the proposal of Crutzen that in situ production dominated tropospheric O3.
Interest in the potential effects of subsonic aviation ensued in the 1980s and early 1990s. This interest arose because of the growing realization that the upper troposphere and lower stratosphere, where subsonic aircraft cruise, is a rather sensitive region of the atmosphere in terms of its chemistry. Initially, attention was focussed upon the effects of aircraft NOx emissions on tropospheric O3 production. Whereas O3 in the mid to upper stratosphere provides a protective 'shield' against harmful UV radiation, O3 in the upper troposphere and lower stratosphere acts as a powerful greenhouse gas, warming the Earth's surface. More recently, other effects such as those of contrails (condensation trails) have been studied intensively, although studies of contrails and climate can be traced back to the early 1970s.
Contrails are line-shaped ice clouds caused by the emission of water vapour and particles from the aircraft exhaust. Depending principally on the particular conditions of temperature and humidity (strictly, ice-supersaturation), contrails may be very short-lived or persistent, sometimes spreading by wind-shear, sedimentation and diffusion into cirrus-like clouds that are ultimately unrecognizable as having been caused by aircraft. Other effects on climate from associated particle emissions and the enhancement of cirrus clouds have also been suggested.
In 1996, the Intergovernmental Panel on Climate Change (IPCC), at the request of ICAO, announced its intention to assess aviation's effects on the global atmosphere; this was completed in 1999. However, the IPCC was not the first assessment: other previous assessments and syntheses include, e.g. Schumann (1994), Wahner et al. (1995), Friedl et al. (1997), Brasseur et al. (1998). This period saw tremendous activity originating from national/international research programmes and dedicated efforts for the IPCC report. Shortly before the completion of the IPCC assessment, Boeing announced that it no longer intended to pursue the development of an SST, largely on economic and environmental (noise) grounds. This, along with the overspending and overrunning NASA space station programme, precipitated the termination of NASA's Atmospheric Effects of Aviation Programme (AEAP). Subsequently, some activities were restarted in the US, albeit at a much lower budgetary level, primarily on global modelling and engine emissions. In Europe, however, the IPCC aviation report provided a springboard from which several research programmes into atmospheric science and technology were initiated under the European Commission's Fifth Framework Programme and included: PARTEMIS, NEPAIR, TRADEOFF, INCA, AERO2K, SCENIC and CRYOPLANE. The bulk of the efforts of these programmes were directed at subsonic effects/technology, with the exception of SCENIC and minor components of TRADEOFF, which addressed supersonic impacts.
In Section 2, the emissions from aircraft in terms of species and their global nature are described. Section 3 gives a brief description of the climate metric, radiative forcing, followed by specific aviation impact quantification (Section 4). In Section 5, some potential emissions reduction approaches are described and some brief conclusions drawn in Section 6.
2 Aircraft Emissions
Aircraft Engine Emissions
The civil subsonic fleet is dominated by aircraft equipped with turbofan gas turbine engines; the turboprop fleet being relatively small on a global scale. Gas turbine engines are technologically advanced systems that require stringent characteristics of safety and durability. Engine emissions are regulated through certification requirements of ICAO (ICAO, 1981; most recent update ICAO, 1995) for NOx, unburned hydrocarbons (HCs), carbon monoxide (CO), and smoke. It is worth reinforcing what these regulations are: they are manufacturing standards, not an in-service compliance regime. Thus, measurements are made using carefully prescribed methodologies on a limited number of engines for certification purposes.
In recent years, significant improvements in fuel efficiency have been achieved to reduce operating costs. Also, emissions of some pollutants, particularly smoke, have been reduced. Emissions of CO2 and H2O scale with fuel consumption depending on the specific fuel carbon to hydrogen ratio. Emissions of NOx and soot are highest at high power settings whilst CO and HCs are highest at low power settings as they are the result of incomplete combustion. In general, emissions of NOx, CO, HCs and particles are relevant to local air quality issues and CO2, H2O, NOx, SOx and particles are of most concern in terms of climate perturbation. The production and control of these emissions are described briefly below. For a more detailed account, the reader is referred to other reviews.
Oxides of Nitrogen (NOx). Emissions of NOx arise from the oxidation of atmospheric nitrogen in the high temperature conditions that exist in the engine's combustor, although a small amount comes from the nitrogen content of the fuel. Its production is a complex function of combustion temperature, pressure and combustor design. Although NOx emissions can be reduced, as overall engine pressure ratios have increased (to reduce fuel consumption), this has implied higher temperatures and pressures in the combustor, which tend to increase NOx production. Hence, to address NOx emissions, different combustor technologies have been developed. Nitrogen oxides at the engine exit plane consist primarily of NO. The percentage of NO2 to NO is estimated to be 1–10%, with an uncertainty of several percent. However, NO is quickly converted into NO2 in the atmosphere.
Particles. Particles emitted from aircraft can be categorized into volatile and non-volatile components; this being partially an operational measurement definition. Non-volatile particles primarily include carbonaceous material formed in the primary combustion zone arising from incomplete combustion of the fuel. A fleet average emission index (EI) for soot of 0.04 g per kg fuel burned has been estimated with a large uncertainty (at best a factor of 2). These soot particles are thought to have only a minor direct impact upon climate (see the Section on Sulfate and Soot Particles). However, soot and other particles emitted from aircraft engines play a role in contrail and cirrus cloud enhancement, as shown later. Soot particles per se are not regulated but rather the so-called 'Smoke Number',' which is an optical measurement of, effectively, the larger soot particles.
Volatile particles are primarily composed of sulfate, although recent research suggests that some smaller fraction of these particles is composed of organic material. Most of the sulfur in the fuel is expected to be emitted as sulfur dioxide (SO2). However, some oxidation through to SVI(e.g. SO3, H2SO4) is possible within the engine itself. The fraction of total gaseous sulfur in the engine exit plane is estimated to be up to 5% SVI: however, this estimate is highly uncertain. Emission of S species is thought to be important for volatile particle formation from sulfuric acid (H2SO4) and gas-phase H2SO4 has now been detected in the wake of aircraft.
Measurements of particle emissions from aircraft, engines and combustors have shown that they lie in the 3 nm to 4 µm aerodynamic diameter size range. The soot aerosol size distribution at the engine exit is log–normal, with number concentrations peaking in the 40–60 nm size range. Emission indices fall within the range of 10 soot aerosol particles per kg fuel for current advanced combustors and up to 10 for older engines.
Other Trace Species. Other trace species have not been as well characterized as, for example, NOx. Hydroxyl radicals (OH) are produced as a part of the combustion process and control the oxidation of NO and S species to their oxidized forms. Few measurements of OH have been made, despite its importance. Tremmel et al. (1998) used measurements of other odd N species in the plume to infer OH concentrations of 1 ppmv or less. However, recent static measurements using Laser Induced Fluorescence (LIF) indicated concentrations of 100 ppbv or less. Recent measurements from the PARTEMIS study indicate much lower OH concentrations, of the order 1 ppb. Clearly, given the importance of OH, more measurements are needed to provide upper limits for oxidation of various species within the engine and the plume.
Emissions of nitrous (HONO) and nitric (HNO3) acids relative to total NOy are estimated to be less than a few percent but emission indices are rather uncertain. More measurements are needed to better quantify this speciation.
The high temperatures involved in combustion of kerosene within aircraft gas turbines produces gaseous ions by chemiionisation of free radicals, often termed 'chemi-ions' (CIs). The first measurements of negative ions (HSO4-H2SO4, HSO4-HNO 3) were made behind an aircraft engine at ground level. Subsequently, other measurements have shown the presence of negative CIs in the plume of an aircraft exhaust, and also positive CIs. CIs may promote formation and growth of charged droplets.
Global Aircraft Emissions Characterization
Several estimations of global aviation emissions have been made over the past ten years or so have been summarized in the IPCC report.' A more recent discussion has been provided by Lee et al. (2002). Global inventories of aircraft emissions usually provide 3D gridded data and, by necessity, have simplifying assumptions. The essential components of an inventory include: an aircraft movements database; a representation of the global fleet in terms of aircraft and engines; a fuel-flow model; a method for calculation of emissions; and landing and take-off emissions data.
The emissions data in most common usage were calculated for the early 1990s, e.g. the ANCAT/EC2 dataset; the DLR-2 data set; and the NASA data set. More recently, Boeing have produced an estimate for 1999 and within the TRADEOFF project an estimate was made for 2000 emissions by scaling up the 1992 ANCAT/EC2 traffic data according to regional traffic statistics.
For future global emissions estimations, there are emission forecasts available for 2015 and scenarios for 2050.
Estimations of global fuel and NO for 1991/92,1999,2000,2015 and 2050 are given in Table 1. Emissions from the military fleet are much more uncertain than the civil estimates but are approximately 10% of the global total. For climate effects other than CO2, military aviation is less important as most emissions are at much lower altitudes than those from civil aircraft.
The horizontal and vertical structure of aircraft emissions is shown in Figures 3(a) and 3(b), respectively. These figures clearly depict how the overall pattern of emissions is concentrated in the Northern Hemisphere at altitudes between 8 and 12 km.
By 2015, it is forecast that emissions of NOx (as NO2) from civil aviation will have grown by a factor of nearly 2, to 3.53 TgN yr-1 with a fuel burn of 287 Tg yr-1. For 2050, the most commonly recognized data sets are those generated by the Forecasting and Economic Sub Group of the ICAO for the IPCC aviation Special Report. These scenarios were based upon a relationship between revenue passenger kilometres (RPK) and GDP. The GDP scenarios were the IPCC 'IS92a, c and e' scenarios. In addition, two technology scenarios for fuel and NOx were assumed, one ambitious, one less so, giving six scenarios in total. The nomenclature adopted was, for example, Fa1-FESG IS92a Technology Scenario 1. Fuel usages ranged from 268 to 772Tgyr-1 and NOx emissions by 3.1 to 11.4 Tg NO2 yr-1; these estimates equate to factors of approximately 2 to 6 on fuel and NOx emissions over early 1990s data. By 2050, assumptions in GDP growth are clearly critical to the overall emission, the technology assumptions having a second-order effect.
Updating these inventories is a major task, as revised global traffic data need to be incorporated. Moreover, the fuel and emissions factors used need to be calculated independently, either from manufacturer's data — which are usually proprietary — or from accepted engine models. There are a number of recent initiatives that will provide updated data in 2003–2004.
3 Radiative Forcing and Climate Change
Careful analyses of palaeoclimatological data have shown that climate has changed over long time-scales. However, more recent data (both direct and proxy) have revealed a remarkable rise in global average surface temperature since the industrial revolution. That such an increase exists is beyond doubt — nonetheless attribution of this temperature increase to greenhouse gas emissions remains a contentious issue and a challenging scientific problem. To assess climate change, the World Meteorological Organization and the United Nations Environment Programme jointly established the IPCC to provide authoritative international assessments of climate change. The IPCC published major reports on the science of climate change in 1990, 1995 and 2001. In addition, IPCC has also assessed impacts and mitigation.
Excerpted from Transport and the Environment by R. E. Hester, R. M. Harrison. Copyright © 2004 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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