Aircraft Plumes Research Articles

Mesoscale numerical investigations of air traffic emissions over the North Atlantic during SONEX flight 8: A case study

Chemical data from flight 8 of NASA's Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) exhibited signatures consistent with aircraft emissions, stratospheric air, and surface‐based pollution. These signatures are examined in detail, focusing on the broad aircraft emission signatures that are several hundred kilometers in length. A mesoscale meteorological model provides high‐resolution wind data that are used to calculate backward trajectories arriving at locations along the flight track. These trajectories are compared to aircraft locations in the North Atlantic Flight Corridor (NAFC) over a 27–33 hour period. Time series of flight level NO and the number of trajectory/aircraft encounters within the NAFC show excellent agreement. Trajectories arriving within the stratospheric and surface‐based pollution regions are found to experience very few aircraft encounters. Conversely, there are many trajectory/aircraft encounters within the two chemical signatures corresponding to aircraft emissions. Even many detailed fluctuations of NO within the two aircraft signature regions correspond to similar fluctuations in aircraft encountered. These NO spikes are due to the superposition of 14 to 25 aircraft plumes transported to the DC‐8 flight track during the previous 33 hours. Results confirm that aircraft emissions were responsible for two chemical signatures observed during SONEX flight 8. They also indicate that high‐resolution meteorological modeling, when coupled with detailed aircraft location data, is useful for understanding chemical signatures from aircraft emissions at scales of several hundred kilometers.

Modelling chemistry in aircraft plumes 1: comparison with observations and evaluation of a layered approach

An expanding plume model with chemistry has been used to study the chemical conversion of nitrogen oxides to reservoir species in aircraft plumes. The model represents the plume by several circular or cylindrical layers in order to give a more detailed description of the chemical evolution in the plume. Model simulations of plumes from two B747 aircraft monitored during the POLINAT measurement campaigns in winter 1994 and summer 1995 were performed. The NO x emission indices were 12.5 and 26.1 g kg −1, respectively. The chemical evolution was followed during the phase dominated by the aircraft-induced dispersion (vortex regime), and in the atmospheric-induced dispersion regime up to 15 h. The results were compared to observations of NO, HNO 3, HNO 2 and CO 2 taken within the trailing vortices. The modelled values were in broad agreement with the observations. With a NO x emission index of 26.1 g kg −1, 60% remained as NO x after 15 h at 50°N under summer conditions for emissions at 07 UT. This amount decreased to 40% when a one-layered plume was considered, suggesting that the distribution of NO x leads to different oxidising potentials which again influenced the plume chemistry.

Modelling chemistry in aircraft plumes 2: the chemical conversion of NO x to reservoir species under different conditions

An expanding plume model with chemistry has been used to study the chemical conversion of nitrogen oxides to reservoir species in aircraft plumes under various conditions. A plume from a B747 was studied, where the NO x emission index was 26.1 g kg −1. Several sensitivity tests were performed for emissions at 07 UT at 50°N under summer conditions. The amount of NO x remaining in the plume after 15 h was between 58 and 69%, assuming background concentrations of ozone and NO x ranging from 50 to 200 ppb and 50 to 200 ppt, respectively. A somewhat higher range in the chemical conversion of NO x was calculated when changing the temperature from 200 to 240 K. The remaining NO x in the plume increased to 90% in a study where the aircraft emissions were released in air masses already exposed to aircraft emissions some hours before. The rate of chemical conversion of NO x to reservoir species was highly dependent on the number of sunlit hours during the 15 h period. The diurnal, seasonal and latitudinal variations in the chemical conversion of NO x to reservoir species were studied. As a first approximation, the conversion rates can be used to modify the aircraft NO x emissions at cruise altitudes used in mesoscale models.

An assessment of aircraft as a source of particles to the upper troposphere

Condensation nuclei measurements are examined in conjunction with measurements of reactive nitrogen species (NOy) to identify aircraft plumes in primary air traffic corridors over the North Atlantic. Several hundred plumes exhibiting ≥100 pptv enhancements in NOy mixing ratio were observed. The plumes were typically a few hundred meters wide, exhibited high NO/NOy ratios, and ranged in age from ∼10 minutes to ∼10 hours. Assuming the sampled aircraft emitted ∼12 g NOx (as NO2) kg−1 fuel burned and that the loss of NOy to the particle phase was negligible, we calculate median aerosol emission indices in terms of number of particles kg−1 of fuel burned of ∼120×1015 for CN ≥8 nm in size; ∼50×1015 for CN ≥ 17 nm; and ∼3×1015 for the nonvolatile CN ≥ 17 nm. Using published fuel burn statistics, background aerosol concentrations, and a 10 day particle lifetime, we conclude that present aviation sources enhance global averaged upper‐tropospheric fine and nonvolatile aerosol number densities by ∼6% and ∼3%, respectively.

The present-day and future impact of NO<sub>x</sub> emissions from subsonic aircraft on the atmosphere in relation to the impact of NO<sub>x</sub> surface sources

Abstract. The effect of present-day and future NOx emissions from aircraft on the NOx and ozone concentrations in the atmosphere and the corresponding radiative forcing were studied using a three-dimensional chemistry transport model (CTM) and a radiative model. The effects of the aircraft emissions were compared with the effects of the three most important anthropogenic NOx surface sources: road traffic, electricity generation and industrial combustion. From the model results, NOx emissions from aircraft are seen to cause an increase in the NOx and ozone concentrations in the upper troposphere and lower stratosphere, and a positive radiative forcing. For the reference year 1990, the aircraft emissions result in an increase in the NOx concentration at 250 hPa of about 20 ppt in January and 50 ppt in July over the eastern USA, the North Atlantic Flight Corridor and Western Europe, corresponding to a relative increase of about 50%. The maximum increase in the ozone concentrations due to the aircraft emissions is about 3-4 ppb in July over the northern mid-latitudes, corresponding to a relative increase of about 3-4%. The aircraft-induced ozone changes cause a global average radiative forcing of 0.025 W/m2 in July. According to the ANCAT projection for the year 2015, the aircraft NOx emissions in that year will be 90% higher than in the year 1990. As a consequence of this, the calculated NOx perturbation by aircraft emissions increases by about 90% between 1990 and 2015, and the ozone perturbation by about 50-70%. The global average radiative forcing due to the aircraft-induced ozone changes increases by about 50% between 1990 and 2015. In the year 2015, the effects of the aircraft emissions on the ozone burden and radiative forcing are clearly larger than the individual effects of the NOx surface sources. Taking chemical conversion in the aircraft plume into account in the CTM explicitly, by means of modified aircraft NOx emissions, a significant reduction of the aircraft-induced NOx and ozone perturbations is realised. The NOx perturbation decreases by about 40% and the ozone perturbation by about 30% in July over Western Europe, the eastern USA and the North Atlantic Flight Corridor.Keywords. Atmospheric composition and structure (troposphere · composition and chemistry) · Meteorology and atmospheric dynamics (radiative processes)

Evolution of aircraft‐generated volatile particles in the far wake regime: Potential contributions to ambient CCN/IN

We apply a detailed aerosol microphysics model to study the evolution of aerosols in the far wake of an aircraft plume. Constrained by in‐situ measurements, our simulations reveal that the largest volatile particles—those most likely to contribute to the background abundance of condensation nuclei—are dominated by ion‐mode aerosols formed on chemiions. By tracking the evolution of the aircraft‐generated particles from the near wake to the far wake, we estimate the properties of the aircraft particles that are effectively injected into the upper troposphere. The entrainment of ambient vapors and aerosols into the aging plume is taken into account, which significantly affects the outcome. The model calculations reveal that a substantial number of the larger ion‐mode volatile particles may survive long enough to act as potential cloud nuclei, thus perturbing the background CCN population. This perturbation is significant during the winter season, and in other locales with low background aerosol concentrations, but is likely to be negligible during the summer season or at locations with heavy aerosol loading. The implications and uncertainties of the results are discussed.

Factors influencing ozone chemistry in subsonic aircraft plumes

This study investigates several factors that could influence ozone chemistry occurring in subsonic aircraft plumes in the upper troposphere. The study focuses on uncertainties in gas-phase rate parameters, but also examines the influence of selected heterogeneous reactions, the rate of expansion of the plume, ambient and initial plume concentrations, and the time of emissions. Monte Carlo analysis with Latin hypercube sampling was applied to an expanding box model of an aircraft plume, in order to estimate the sensitivities of O 3 perturbations (ΔO 3) to uncertainties in rate constants in the RADM2 chemical mechanism. The resulting coefficient of variation in ΔO 3 at the end of a 36 h simulation was about 50%. Influential uncertainties in gas-phase rate parameters include those for photolysis of NO 2 and HCHO, O 3+NO, HO 2+NO, and formation of PAN and HNO 3. With high background concentrations of non-methane hydrocarbons, uncertainties in rate parameters of reactions involving peroxy radicals from ethene and propene oxidation were also influential. The coefficient of variation for ΔO 3 due to uncertainties in emission indices of NO x , CO, and organic compounds was less than 15%. The effects of the heterogeneous reaction of N 2O 5 leading to HNO 3 formation, and hypothesized reactions of HNO 3 and NO 2 on soot, were also investigated. The results suggest that the latter two reactions could be influential for ΔO 3 if published estimates of reaction probabilities and high estimates of soot concentrations in plumes are realistic.

Aviation-Produced Aerosols and Contrails

Liquid and solid particles in the plumes of jet aircraft cruising in the upper troposphere and lower stratosphere lead to the formation of ice clouds (contrails), modify the microphysical properties of existing cirrus clouds, and provide sites for heterogeneous chemical reactions. Characterization of aviation-produced particles in terms of physico-chemical properties is an important step in assessing the global impact of aircraft emissions upon atmospheric chemistry and climate parameters. Chemistry and microphysics of the gas-aerosol system in aircraft plumes and its evolution in the atmosphere is a field of intense research. This paper reviews the current knowledge (mid-1998) and outlines possible atmospheric implications.

Perturbation of the aerosol layer by aviation‐produced aerosols: A parametrization of plume processes

The perturbation of the sulfate surface area density in the tropopause region and the lower stratosphere by subsonic and supersonic aircraft fleets is examined. The background aerosol surface area, the conversion of fuel sulfur into new sulfate particles in aircraft plumes, and the plume mixing with ambient air control this perturbation. Results from an analytic expression for the surface area changes are presented which contains the dependences on these parameters and can be employed in large‐scale atmospheric models.

Measurements of nitrogen oxides from aircraft in the northeast Atlantic flight corridor

We investigated the size and composition of exhaust plumes of commercial airliners in the northeast Atlantic flight corridor off the shores of Ireland and Scotland, an area with high air traffic density in the tropopause region. The primary objective was to measure the contribution of the NO and NOy emitted from aircraft to the nitrogen oxide background. We have made measurements in aircraft flight corridors by flying a research aircraft perpendicular to the routes of commercial transatlantic air traffic. Although previous studies had succeeded in identifying a few plumes, this study was the first systematic investigation of over 60 aircraft plumes through in situ NO and NOy measurements. These plumes were up to 1.5 km wide (along our flight paths) and showed NOy mixing ratios of up to 10 ppb above the background levels. The measurements showed that on larger scales the composition of NOy in the background air masses was very heterogeneous. On top of this natural variability of the NOy, we could not identify any influence of air traffic exhaust on air chemistry on scales larger than a few kilometers from our data. A secondary objective was to estimate the importance of the oxidation of NOχ to NOy in the relatively fresh plumes. The measured NO/NOy ratios were near the NO/NOχ ratios calculated from a simple photo‐stationary state assumption. This result was also consistent with calculations made with an expanding box model that included gas‐phase chemistry for the measured plume conditions. The model calculated NO/NOy and NC/NOχ ratios were almost equal, and these were consistent with the measured NO/NOy, ratios. These calculations showed that oxidation of NOχ to higher oxides played only a negligible role in our measured plumes of ages between 14 and 90 min.

On the mechanisms controlling the formation and properties of volatile particles in aircraft wakes

New observations taken in aircraft wakes, including the DLR ATTAS, provide strong constraints on models of aircraft plume aerosols. Using a comprehensive microphysics code, we have performed sensitivity studies to identify the key microphysical mechanisms acting in such plumes. Analysis of these simulations reveals that the largest volatile plume particles—those most likely to contribute to the background abundance of condensation nuclei—are dominated by ion‐mode particles when chemiions are included. Moreover, such modeling demonstrates that standard treatments of plume microphysics—in the absence of chemiions—fails to explain field measurements. The principal factor controlling the population of ultrafine plume particles is the number of chemiions emitted by the aircraft engines. Since the ions are a byproduct of the combustion itself, and their abundance in the exhaust stream is controlled by ion‐ion recombination, the initial ion concentrations—and so the eventual emission indices for ion‐mode particles—are expected to be relatively invariant. Our results indicate that reductions in fuel sulfur content, while not likely to lower the total number of volatile particles emitted, would decrease the size of the ion‐mode particles in fresh aircraft wakes, reducing their atmospheric lifetimes and potential environmental effects.

The formation and evolution of aerosols in stratospheric aircraft plumes: Numerical simulations and comparisons with observations

The formation and evolution of aerosols in jet aircraft plumes at high altitude are simulated using a detailed aerosol microphysics model that explicitly resolves particle size and composition. Two approaches are used to simulate the microphysics: a standard, or “classical,” approach in which new particle formation initially occurs via homogeneous binary nucleation, followed by condensation and coagulation; and a “kinetic” approach in which the entire course of particle evolution is calculated as a unified collisional mechanism. In the latter approach chemiions generated in the engine combustors can affect molecular cluster growth and ultrafine particle aggregation. Simulations with both approaches reveal that large numbers of volatile ultrafine sulfuric acid particles are generated in the near field behind the engines. The concentrations and subsequent evolution of these “volatile” particles are sensitive to the initial sulfuric acid vapor concentration, the plume dilution rate, and microphysical factors, especially the chemiion abundance, which is brought out through a sensitivity analysis. By contrasting predictions against available field data, it is demonstrated that the kinetic model provides a more realistic representation of particle formation and growth and hence their observable properties than the classical model. Moreover, the effects of chemiions during the early evolution of the plume are found to be crucial to the evolution of the largest volatile aerosols, which may play a role as cloud condensation nuclei. Major sources of uncertainty in the plume aerosol models are noted.

Ultrafine aerosol particles in aircraft plumes: Analysis of growth mechanisms

An analysis of in situ measurements of ultrafine volatile aerosols in the plume near field of the DLR aircraft ATTAS using low (0.02 g/kg) and high (2.7g/kg) fuel sulfur contents (FSCs) is presented. The observed growth of nanoparticles (diameter 5–10 nm) is reproduced in detail by a microphysical simulation with chemi‐ion emissions of 2.6×1017/kg fuel. Volatile aerosol dynamics is controlled by sulfuric acid (H2SO4) for high FSC, consistent with a S to H2SO4 conversion of 1.8%. The very high conversion for low FSC (55%) prescribed in the model to match the observations contradicts direct in situ measurements of H2SO4 and suggests that species other than H2SO4, likely exhaust hydrocarbons, control particle growth in such cases.

A modeling study of the formation of cloud condensation nuclei in the jet regime of aircraft plumes

The formation of cloud condensation nuclei in the jet regime of a B‐747 airliner at cruise has been investigated by modeling studies. Both the formation of H2O/H2SO4 clusters by homogeneous nucleation and the deposition of water vapor on soot particles activated by the adsorption of gaseous H2SO4, sulfuric acid hydrates, and H2O/H2SO4 clusters were taken into account. H2SO4 has been assumed to be formed only by OH oxidation of SO2 in the plume. Whereas at ambient temperatures between 219 and 224 K the heterogeneous condensation leads to ice particles with average diameters between 3.0 and 1.1 μm for soot emission indices of EI(soot) = (0.05 − 0.5) g/kg, respectively, no heterogeneous condensation occurs at higher temperatures. Homogeneously nucleated H2O/H2SO4 clusters, on the other hand, have diameters of less than 7 nm and do not contribute to visible contrail formation. Assuming different sulfur emission indices (0.1 g/kg≤EI(SO2)≤10 g/kg), we conclude that the contrail onset is essentially independent of this quantity and that a fractional H2SO4 surface coverage corresponding to a 0.1 monolayer (ML = 0.1) must be sufficient to activate the soot particles for H2O uptake, at least for EI(SO2)≥0.5 g/kg fuel. Calculations based on higher threshold values (i.e., 0.1<ML≤0.3) lead to results which are in disagreement with the onset of contrail formation as deduced by visual observations. Moreover, the present modeling study provides an estimate of the effect of mutual coupling of homogeneous and heterogeneous condensation pathways.

Physicochemistry of aircraft‐generated liquid aerosols, soot, and ice particles: 2. Comparison with observations and sensitivity studies

Results from a coupled microphysical‐chemical‐dynamical trajectory box model have been compared to recent in situ observations of particles generated in the wake of aircraft. Sulfur emissions mainly cause the formation of ultrafine volatile particles in young aircraft plumes (mean number radius ∼5 nm). Volatile particle numbers range between 1016 and 1017 per kg fuel burnt for average to high fuel sulfur levels, exceeding typical soot emission indices by a factor of 10–100. Model results come into closer agreement with observations when chemi‐ions from fuel combustion are included in the aerosol dynamics. Ice particles (mean number radius <1 μm) in young contrails mainly nucleate on water‐activated exhaust aerosols. Homogeneous freezing and soot‐induced heterogeneous freezing are competitive processes leading to ice formation, depending on the temperature and level of oxidized sulfur species. There is evidence that soot triggers freezing even for low fuel sulfur contents, suggesting a sulfur‐independent water activation pathway. Metal particles emitted by jet engines and entrained ambient aerosols may contribute to the formation of larger (>1 μm.) crystals. Contrails with larger crystals would also form without soot and sulfur emissions. The lifecycle of cirrus clouds can be modified by exhaust aerosols.

Aerosol size distribution in a coagulating plume: Analytical behavior and modeling applications

In a previous paper (Turco and Yu, 1997), a series of analytical solutions were derived for the problem of aerosol coagulation in an expanding plume, as from a jet engine. Those solutions were shown to depend on a single dimensionless time‐dependent number, NT, which is related to the particle coagulation kernel and the plume volume. Here, we derive a new analytical expression that describes the particle size distribution in an expanding plume in terms of NT. We show how this solution can be extended to include the effects of soot particles on the evolving volatile sulfuric acid aerosols in an aircraft wake. Our solutions apply primarily to cases where changes in the size distribution—beyond an initial period encompassing emission and prompt nucleation/condensation—is controlled mainly by coagulation. The analytical size distributions allow most of the important properties of an evolving aerosol population—mean size, number greater than a minimum size, surface area density, size dependent reactivities, and optical properties—to be estimated objectively. We have applied our analytical solution to evaluate errors associated with numerical diffusion in a detailed microphysical code, and demonstrate that, if care is not exercised in solving the coagulation equation, substantial errors can result in the predictions at large particle sizes. This effect is particularly important when comparisons between models and field observations are carried out. The analytical expressions derived here can also be employed to initialize models that do not resolve individual aircraft plumes, by providing a simple means for parameterizing the initial aerosol properties after an appropriate mixing time.

Contrail formation and impacts on aerosol properties in aircraft plumes: Effects of fuel sulfur content

The formation and evolution of fine particles and ice contrails in an aircraft exhaust plume containing varying amounts of fuel sulfur have been simulated using an advanced aerosol microphysics model. The “core” sulfate and soot particles are tracked during the contrail formation and dissipation phases. When ion electrostatic effects are incorporated into the microphysics, sulfuric acid vapor emitted by high‐sulfur‐content fuels generates water‐soluble particles that are large enough to be activated into contrails, improving the agreement between simulations and measurements. Our results also suggest that ice crystals formed in contrails efficiently scavenge vapors and particles, creating a sulfate aerosol accumulation mode that may contribute to cloud CCN/IN. The size distributions of aerosols produced both in the presence and absence of contrails agree reasonable well with the two characteristic types observed in the plumes of commercial aircraft.

The effects of the conversion of nitrogen oxides in aircraft exhaust plumes in global models

A parametrization is needed in global models to account for the sub‐grid chemical processes taking place in the plume of an aircraft, since these processes can cause the conversion of a considerable amount of the emitted NOx to reservoir species, such as HNO3. For this purpose, the chemical conversions of nitrogen oxides in the plume of an aircraft were investigated with a newly developed model. The calculated fractions of different nitrogen compounds formed within 24 hours in the exhaust plumes, differentiated for the global domain and season, were used to modify the original aircraft NOx emissions from the ANCAT emission inventory to emissions of various nitrogen compounds and we applied these to the global Chemistry Transport Model KNMI (CTMK). The results obtained imply that neglect of aircraft plume processes in global modeling leads to an overestimation of the NOx and O3 perturbations. Compared with a CTMK calculation with unmodified aircraft NOx emissions, the NOx perturbations in the North Atlantic flight corridor (NAFC) decreased by 15%–55%, due to conversions in the plumes. The resulting O3 perturbation decreased by 15%–25%.

The role of ions in the formation and evolution of particles in aircraft plumes

We consider the effects on aircraft plume microphysics of ions generated by chemiionization processes within the engine combustors. Ions provide centers around which molecular clusters rapidly coalesce, thus promoting the formation of electrically charged sulfuric acid/water aerosols. The resulting charged micro‐particles exhibit enhanced growth due to condensation and coagulation aided by electrostatic effects. Simulations with a plume microphysics code show that volatile particles observed behind aircraft may be explained by such processes, as long as initial ion concentrations in the exhaust exceed ∼108/cm³. This analysis also suggests that the primary emissions of sulfuric acid (plus sulfur trioxide) should amount to at least 20–30% of the fuel sulfur to explain the observed number of volatile particles >9 nm in diameter. Ionized plume simulations reveal a distinct bimodal aerosol distribution, in which an “ion” mode constitutes the larger “activated” volatile sulfuric acid particles, while a smaller “neutral” mode comprises the residual slowly‐growing neutral molecular clusters formed in the highly supersaturated region of the plume.

Heterogeneous chemistry in aircraft wakes: Constraints for uptake coefficients

Recent in situ measurements in subsonic and supersonic aircraft plumes show the presence of high aerosol abundances. Given the large initial surface areas of the exhaust particles (volatile aerosols, soot, and ice) of 103–105 μm2 cm−3 or more, heterogeneous processing can potentially become important. Based on an analytical model to predict the temporal evolution of the surface areas, the potential for heterogeneous chemistry during the lifetime of single aircraft wakes is investigated. The model surface areas are constrained by plume observations and compared to numerical simulations of aerosol formation and growth. Efficient heterogeneous processing on volatile aerosols and soot on timescales below 1 day generally requires uptake coefficients ≳0.003–0.007, depending on the specific surface area of soot. For low available surface areas and slow reactions, the lifetime of emitted exhaust species sensitively depends on the wake mixing properties. Shutting off uptake by volatile particles inhibits heterogeneous processing unless high soot surface areas and reaction probabilities are prescribed. Depending on the lifetime of ice contrails, uptake coefficients ≳0.1 are required for rapid uptake of exhaust species on the ice particles. This lower limit becomes relaxed if contrails are long‐lived or develop into persistent cirrus or polar stratospheric clouds, rendering activation of chlorine potentially important. The model is applied to investigate the uptake of gaseous HNO2 and SO2 by the observed particles in the plume of the Concorde in the lower stratosphere.