J. Ovarlez

In Situ Observations of Air Traffic Emission Signatures in the North Atlantic Flight Corridor

Focussed aircraft measurements have been carried out over the eastern North Atlantic to search for signals of air traffic emissions in the flight corridor region. Observations include NO, NO2, HNO3, SO2, O3, H2O, total condensation nuclei (CN), and meteorological parameters. A flight pattern with constant-altitude north-south legs across the major North Atlantic air traffic tracks was flown. Signatures of air traffic emissions were clearly detected for NOx, SO2, and CN with peak concentrations of 2 ppbv, 0.25 ppbv, and 500 cm−3, respectively, exceeding background values by factors of 30 (NOx), 5 (SO2), and 3 (CN). The observed NOx, SO2, and CN peaks were attributed to aircraft plumes based on radar observations of the source air traffic and wind measurements. Major aircraft exhaust signatures are due to relatively fresh emissions, i.e., superpositions of 2 to 5 plumes with ages of about 15 min to 3 hs. The observed plume peak concentrations of NOx compare fairly well with concentrations computed with a Gaussian plume model using horizontal and vertical diffusivities as obtained by recent large-eddy simulations, measured vertical wind shear, and the corridor air traffic information. For the major emission signatures a mean CN/NOx abundance ratio of 300 cm−3ppbv−1 was measured corresponding to an emission index (EI) of about 1016 particles per 1 kg fuel burnt. This is higher than the expected soot particle EI of modern wide-bodied aircraft. For the most prominent plumes no increase of HNO3 concentrations exceeding variations of background values was observed. This indicates that only a small fraction of the emitted NOx is oxidized in the plumes within a timescale of about 3 hs for the conditions of the measurements.

Prevalence of ice-supersaturated regions in the upper troposphere: Implications for optically thin ice cloud formation

In situ measurements of water vapor and temperature from recent aircraft campaigns have provided evidence that the upper troposphere is frequently supersaturated with respect to ice. The peak relative humidities with respect to ice (RHI) occasionally approached water saturation at temperatures ranging from −40°C to −70°C in each of the campaigns. The occurrence frequency of ice supersaturation ranged from about 20% to 45%. Even on flight segments when no ice crystals were detected, ice supersaturation was measured about 5–20% of the time. A numerical cloud model is used to simulate the formation of optically thin, low ice number density cirrus clouds in these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less than about 5 × 10-3 to 2 × 10-2 cm-3 ice nuclei are present when these supersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturated layer, but the crystals sediment out too rapidly to prevent buildup of high supersaturations. If higher numbers of ice nuclei are present, then the clouds that form are visible and deposition growth of the ice crystals reduces the RHI down to near 100%. Even if no ice clouds form, increasing the RHI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoing longwave radiative flux at the top of the atmosphere of about 8 W m-2. If 0.02–0.1 cm-3 IN are present, the resulting cloud radiative forcing reduces the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick clouds in the upper troposphere, there is a potential for a climatically important class of optically thin cirrus with relatively low ice crystal number densities. The optical properties of these clouds will depend very strongly on the abundance of ice nuclei in the upper troposphere.

Water vapour measurements inside cirrus clouds in Northern and Southern hemispheres during INCA

Water vapour data inside cirrus clouds from in-situ measurements with an aircraft-borne frost-point hygrometer are analysed. These data have been obtained during two field campaigns, performed in the Southern and Northern hemisphere mid latitudes. There were many occurrences of ice supersaturation inside the investigated cirrus, with a higher frequency of occurrences in the Southern Hemisphere. The source of the differences in the humidity data from the two hemispheres is not clear, and it is speculated that these differences may be related to different levels of pollution. A distribution law for the relative humidity inside cirrus clouds is inferred.

Pollution from aircraft emissions in the North Atlantic flight corridor : overview on the POLINAT projects

The Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT) projects were undertaken to investigate the impact of aircraft engine exhaust emissions on the state of the atmosphere in the North Atlantic flight corridor. Changes in the composition of the lower stratosphere and upper troposphere from aircraft emissions are identified from combined measurements and model analyses. Measurements were performed using the Deutsches Zentrum fur Luft- und Raumfahrt Falcon research aircraft and a Swissair B-747 over the North Atlantic covering the altitude range 6 to 13 km in November 1994 and June/July 1995 and from August to November 1997. The measurements include those of nitrogen oxides, nitrous and nitric acids, sulfur dioxide, sulfuric acid, acetone, carbon dioxide, ozone, water vapor, carbon monoxide, aerosols, and meteorological parameters. The atmospheric composition was found to be highly variable, and emissions from sources at the surface or from lightning discharges also contribute strongly to the nitrogen oxides abundance and ozone formation. Contributions from aircraft emissions have been measured and identified in single and multiple plumes of several hours ages, and accumulation of such nitrogen oxides and particles emissions can be identified under certain conditions in and downstream of the flight corridor region. Acetone was found at high mixing ratios. The global and regional models predict ozone increases of 3 to 6% by current air traffic at the flight corridor altitude north of 30°N, in agreement with previous model analyses but too small to be measurable. In autumn, the upper troposphere is often humid with water vapor concentration far above ice saturation, providing conditions for persistent contrails.

Stratospheric water vapor content evolution during EASOE

During EASOE, 7 in situ stratospheric water vapor content measurements were performed, in the polar region, with a balloon-borne frost-point hygrometer, from November 1991 until March 1992. The comparison between the frost-point and the air temperature profiles shows on 11 December the occurrence of PSC formation conditions when a PSC was observed by lidars. Outside the vortex the mean water vapor content seems to be very stable, 4 to 5 ppmv from 16 to 25 km. But is very changeable below 16 km, due to the variability in the origin of the air masses where the measurements occur, as these air parcels come sometimes from the tropics, then bringing dry air. Inside the vortex, a vertical descent of the air masses is observed, and is estimated to be 4 km around 25 km at the end of March, before the vortex breakdown. The rehydration of the vortex induced by this descent, from February to March, is shown.

Comparison of water vapor measurements from POLINAT 2 with ECMWF analyses in high‐humidity conditions

During the Pollution From Aircraft in the North Atlantic Flight Corridor (POLINAT 2) experiment, water vapor content was measured during 12 flights of the Deutsches Zentrum fuer Luft und Raumfahrt Falcon research aircraft by means of a frost-point hygrometer. The last three flights were coordinated with the flights of the NASA DC8 aircraft, taking part in the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX). The measurements were generally performed in high-humidity and cloudy environments. Synthesized profiles of humidity and cloud cover along the flight tracks were retrieved from the European Centre for Medium-Range Weather Forecasts analyses. It is shown that the measurements of supersaturation are generally correlated to the occurrence of clouds as indicated by the model cloud cover product. However, there are large discrepancies between the mixing ratio from the analyses and the measurements: the model underestimates the water vapor content. Some reasons for this underestimation are put forward, and the consequences for the investigation of the occurrence of contrail formation and persistence, when using such analyses, are pointed out.

The total hydrogen budget in the Arctic winter stratosphere during the European Arctic Stratospheric Ozone Experiment

Vertical profiles of H2O and CH4 were measured using in situ techniques in the Arctic winter stratosphere during the European Arctic Stratospheric Ozone Experiment (EASOE) 1991/1992 in the pressure range between 12 and 80 hPa. The measurements were grouped according to the meteorological situation, i.e., their position relative to the polar vortex. The mixing ratio of the quantity 2·CH4 + H2O was determined in the altitude region between the hygropause and 30 km to be quasi constant at a mean mixing ratio of 6.91 ± 0.41 ppmv. No significant difference of 2·CH4 + H2O was observed inside and outside the vortex. From the correlation between CH4 and H2O ΔH2O/ΔCH4, i.e., the water vapor yield of the methane oxidation, was calculated to be −1.82 ± 0.21. The globally averaged H2O mixing ratio of air entering the stratosphere was found to be 3.61 ± 0.28 ppmv. The data are compared with studies on the total hydrogen budget over the past 20 years. Despite the variety of instrumental techniques used in these studies, an increase of total hydrogen is apparent over this period.

Some observations of gravity-wave-induced structure in ozone and water vapour during EASOE

During the EASOE campaign, measurements were made of temperature and different minor constituents, with the help of instrumented balloons. The vertical profiles nearly always show small scale structures. Here we show that on two different days a large part of these small scale structure can be explained by the transport induced by waves. The role of waves is supported by a good correlation found between the small scale structure of the minor constituents profiles and the temperature profile. Furthermore on 11 December 1991 lidar measurements show the trace of a wave on a polar stratospheric cloud (Godin et al. 1993).Comparison of some of its characteristics with what was found in water vapour and ozone measurement profiles carried out the same day, confirm the role of the wave in the minor constituent fluctuations.

Large nitric acid particles at the top of an Arctic stratospheric cloud

measurements approximately 200 km upwind of the in situ measurements indicate a similar vertical structure for the cloud. These in situ measurements represent, to our knowledge, the most comprehensive in situ observations of all phases of polar stratospheric cloud particles, while the large particles at cloud top have not been previously observed and may have implications for producing particles large enough to remove reactive nitrogen from the polar stratosphere. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric clouds, in situ stratospheric cloud measurements, Arctic stratospheric clouds, polar stratospheric cloud composition, balloon-borne aerosol measurements, large polar stratospheric cloud particles

Comparison of Water Vapor Measurements with Data Retrieved from ECMWF Analyses during the POLINAT Experiment

During the POLINAT (Pollution from Aircraft Emissions in the North Atlantic Flight Corridor) experiment, water vapor content was measured with a frost-point hygrometer on board the DLR (Deutsche Forschungsanstalt fur Luft-und-Raumfahrt) Falcon research aircraft. Model estimates of the water vapor mixing ratio along the flight track of the aircraft have been interpolated from 6-h ECMWF analyses. Comparisons are made between the in situ measurements and the model data for 16 POLINAT flights. In the high troposphere, the mixing ratio from the model is generally larger than the measured one, with differences amounting to a few percent up to 100%. But at mixing ratios below 20 ppmv and in the stratosphere, the mixing ratios from the model are generally smaller than the measured ones. The observed differences in the high troposphere can be explained by the low accuracy of radiosonde data at low temperatures and/or at small relative humidities. In the stratosphere, due to the technical limitations of radiosonde hygrometers, the water vapor concentration is set to a constant value at each ECMWF analysis step. Despite these discrepancies, many of the small-scale features are, at least in a qualitative sense, reproduced by the model.