Dieter Kley

Measurement of ozone and water vapor by Airbus in-service aircraft: The MOZAIC airborne program, an overview

Tentative estimates, using three-dimensional chemistry and transport models, have suggested small ozone increases in the upper troposphere resulting from current aircraft emissions, but have also concluded to significant deficiencies in todays models and to the need to improve them through comparison with extended data sets. The Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) program was initiated in 1993 by European scientists, aircraft manufacturers, and airlines to collect experimental data. Its goal is to help understand the atmosphere and how it is changing under the influence of human activity, with particular interest in the effects of aircraft. MOZAIC consists of automatic and regular measurements of ozone and water vapor by five long range passenger airliners flying all over the world. The aim is not to detect direct effects of aircraft emissions on the ozone budget inside the air traffic corridors but to build a large database of measurements to allow studies of chemical and physical processes in the atmosphere, and hence to validate global chemistry transport models. MOZAIC data provide, in particular, detailed ozone and water vapor climatologies at 9–12 km where subsonic aircraft emit most of their exhaust and which is a very critical domain (e.g., radiatively and stratosphere/troposphere exchanges) still imperfectly described in existing models. This will be valuable to improve knowledge about the processes occuring in the upper troposphere and the lowermost stratosphere, and the model treatment of near tropopause chemistry and transport. During MOZAIC I (January 1993–September 1996), fully automatic devices were developed, installed aboard five commercial Airbus A340s, and flown in normal airline service. A second phase, MOZAIC II, started in October 1996 with the aim of continuing the O3 and H2O measurements and doing a feasibility study of new airborne devices (CO, NOy). Between September 1994 and December 1997, 7500 flights, representing 54,000 flight hours, were made over the continents (Europe, North America, Asia, South America, and Africa) and the Atlantic Ocean. Most of the measurements (90%) correspond to cruise altitudes (9–12 km), the remaining being obtained during ascents and descents near the 50 cities frequented by MOZAIC operations. This paper reports the main characteristics of the program and the flights, with a brief summary of the general content and focus of papers already published and companion papers of this special issue. These deal with the following: description and validation of the ozone and water vapor measurement methods; presentation of an accurate ozone climatology at 9–12 km altitude, over the Northern Hemisphere (130°W–140°E; 0°–80°N), and down to 30°S over South America and Africa; comparison between a 2-year MOZAIC ozone climatology (1994–1996; 0–12 km) and a long series of older measurements made since the 1980s at 8 stations of the Ozone Sounding Network; study of ozone-rich transients, up to 500 ppbv on a horizontal scale of 5–80 km, in the upper tropical troposphere; and comparison between MOZAIC ozone data and output from the global chemistry and transport model (CTM) TOMCAT.

An improved fast-response vacuum-UV resonance fluorescence CO instrument

The fast-response resonance fluorescence instrument for the airborne measurement of carbon monoxide described by Gerbig et al. [1996] was modified by implementing an improved optical filter with more efficient optics and an optimized resonance lamp. Besides reductions in size and weight, the new instrument achieves a sensitivity 10 times higher, a lower background (65 ppb compared with 250 ppb), and a faster time response (<0.1 s) than the original instrument. The precision is ±1.5 ppb at an atmospheric mixing ratio of 100 ppb CO, and the detection limit is 3 ppb (2σ) for an integration time of 1 s. First results from the North Atlantic Regional Aerosol Characterization Experiment (ACE-2) campaign during July 1997, when the new instrument was deployed aboard the U.K. Meteorological Office C-130 aircraft, are used to demonstrate the performance of the new instrument.

Observations of Near-Zero Ozone Concentrations Over the Convective Pacific: Effects on Air Chemistry

A series of measurements over the equatorial Pacific in March 1993 showed that the volume mixing ratios of ozone were frequently well below 10 nanomoles per mole both in the marine boundary layer (MBL) and between 10 kilometers and the tropopause. These latter unexpected results emphasize the enormous variability of tropical tropospheric ozone and hydroxyl concentrations, which determine the oxidizing efficiency of the troposphere. They also imply a convective short circuit of marine gaseous emissions, such as dimethyl sulfide, between the MBL and the uppermost troposphere, leading, for instance, to sulfate particle formation.

Emission of NO from several higher plant species

Emission of nitric oxide (NO) from a variety of plant species was observed in a continuously stirred tank reactor. During daytime and at NO concentrations below 1 ppb in the chamber air, NO emissions were observed for all studied nitrate-nourished plant species. A relation was found between the NO emission rates during daytime and the uptake rates of CO 2 . The ratio of the NO emission rate to the CO 2 uptake was similar for all plants. Changes of the net rate of photosynthesis induced by variations of light intensity or changes of CO 2 concentrations changed the NO emission rates correspondingly. The link between NO emissions and CO 2 uptake during daytime allowed estimation of the potential of the vegetation to evolve NO on a global scale as 0.23 Tg N yr -1 . Strong NO emissions during nights were observed when the nitrate concentration in the nutrient solution was enhanced. Then NO emissions were observed with flux densities comparable to the highest emission rates found from soils.

On the spatial distribution and seasonal variation of lower-troposphere ozone over Europe

Surface ozone data from 25 Europeanlow-altitude sites and mountain sites located between79°N and 28°N were studied. The analysiscovered the time period March 1989–February 1993.Average summer and winter O3 concentrations inthe boundary layer over the continent gave rise togradients that were strongest in the north-west tosouth-east direction and west-east direction, respectively. WintertimeO3 ranged from 19 to 27 ppbover the continent, compared to about 32 ppb at thewestern border, while for summer the continentalO3 values ranged between 39 and 56 ppb and theoceanic mixing ratios were around 37 ppb. In the lowerfree troposphere average wintertime O3 mixingratios were around 38 ppb, with only an 8 ppbdifference between 28°N and 79°N. For summerthe average O3 levels decreased from about 55 ppbover Central Europe to 32 ppb at 79°N. Inaddition, O3 and Ox(= O3 + NO2)in polluted and clean air were compared. Theamplitudes of the seasonal ozone variations increasedin the north-west to south-east direction, while thetime of the annual maximum was shifted from spring (atthe northerly sites) to late summer (at sites inAustria and Hungary), which reflected the contributionof photochemical ozone production in the lower partsof the troposphere.

Calibration and performance of automatic compact instrumentation for the measurement of relative humidity from passenger aircraft

Compact airborne humidity sensing devices using capacitive sensors are employed on board in-service aircraft to measure water vapor concentrations in the troposphere up to 13 km altitude. The sensors are individually calibrated before onboard installation. After every 500 flight hours, each sensor is calibrated in an environmental simulation chamber under typical middle/upper tropospheric flight conditions. A Lyman-Alpha fluorescence hygrometer is used as reference instrument. Preflight and postflight calibration of each flown sensor agreed very well and showed good response. Typical overall uncertainties for the 1995 Measurement of Ozone by AIRBUS In-Service Aircraft (MOZAIC) relative humidity (RH) measurements are within ±4% RH in the middle troposphere, increasing to ±7% RH between 9 and 13 km. In-flight comparison of the MOZAIC humidity device with other water vapor measuring techniques showed agreement within ±(5–10)% RH and a time response of better than 10 s in the lower/middle troposphere, increasing to values of 1–3 min at 10–12 km altitude.

Water vapor and cloud water measurements over Darwin during the STEP 1987 tropical mission

Measurements of stratospheric and upper tropospheric cloud water plus water vapor (total water) and water vapor were made with two Lyman α hygrometers as part of the STEP tropical experiment. The in situ measurements were made in the Darwin, Australia, area in January and February of 1987 on an ER-2 aircraft. Average stratospheric water vapor at a potential temperature of 375 K (the average value of θ at the tropopause) was 2.4 parts per million by volume (ppmv). This water mixing ratio is below the 3.0 to 4.0 ppmv necessary to be consistent with the observed upper stratospheric dryness. Saturation with respect to ice and the potential for dehydration was observed up to θ = 402 K.

Measurements of alkyl nitrates in rural and polluted air masses

Abstract Gas chromatographic measurements of organic nitrates (C1C8) were made at Julich (52°N, 7°E) and at Schauinsland/Black Forest (48°N, 6°E), beginning in autumn 1988. Samples were collected cryogenically by passing 3l of ambient air through a sampling loop, filled with glass beads, at 77 K. Separation was performed by capillary gas chromatography. An optimized chemiluminescence NO-analyser operating at a pressure of 1 mbar in combination with a catalytic converter was used as a specific detector for NO-containing molecules with a detection limit on the order of 1 ppt for a 3l sample. The measurements show the presence of C1C5 nitrates at concentration levels between 1 and 230 ppt. Nitrates greater than C5 were only observed at Julich and at very low mixing ratios. Nitrates > C 8 were not observed. The combined concentrations of organic nitrates account, on the average, for 1–2% of the NOy. On a few occasions, the fraction could be as high as 7%. Therefore, organic nitrates are not likely to explain the missing fraction of NOy, first reported by Fahey et al. (1986). From the correlation between the concentrations of organic nitrates and ozone and from the absence of nitrates > C5 at Schauinsland it is concluded that ozone formation proceeds mainly via the smaller alkylperoxy radicals, HO2 and CH3C(O)O2.

Tropospheric ozone at elevated sites and precursor emissions in the United States and Europe

Abstract The emission of oxides of nitrogen (NOx) and of volatile organic compounds (VOC) from anthropogenic sources are compared between the United States of America, OECD-Europe and subregions thereof. Emission flux densities are larger in central Europe than in the eastern United States. Ozone levels, observed at elevated sites, are remarkably similar in these respective subregions provided that titration effects of NO are small. It is argued that ozone transport from the free troposphere exerts a strong influence on concentrations at elevated sites.

Fast response resonance fluorescence CO measurements aboard the C‐130: Instrument characterization and measurements made during North Atlantic Regional Experiment 1993

The resonance fluorescence instrument for the measurement of atmospheric CO described by Volz and Kley [1985] was characterized in the laboratory and adapted for use on aircraft. A major finding was that the background signal is largely due to continuum resonance Raman scattering by molecular oxygen and thus cannot be reduced by better design. The instrument was deployed on the United Kingdom Meteorological Office (UKMO) C-130 Hercules during August 1993 and in subsequent missions. The instrument achieved a detection limit (3σ) of 5 ppb at a time resolution of 30 s. For a typical CO concentration of 100 ppb, the signal-to-noise ratio (1σ) was 15 for an integration time of 2 s, which was the minimum time resolution that could be obtained during the flights because of limited pump capacity. Data collected over the North Atlantic show distinct layers of CO above the atmospheric boundary layer (ABL) that are well correlated with enhanced NOy mixing ratios and indicate transport of pollution from the American continent. Such layers, albeit much less pronounced, were encountered in westerly flow in the midtroposphere west of the coast of Portugal. Fairly high mixing ratios were observed in the lower troposphere associated with transport from southern Europe.