Detlev Sprung

Chemical characterization of pollution layers over the tropical Indian Ocean: Signatures of emissions from biomass and fossil fuel burning

We have performed airborne measurements of atmospheric trace gases and aerosol composition on the National Center for Atmospheric Research C-130 research aircraft over the tropical Indian Ocean during the Indian Ocean Experiment (INDOEX) intensive field phase in February and March 1999. Gases measured included acetone, acetonitrile, sulfur dioxide, and carbon monoxide. The aerosol composition was analyzed for water-soluble ions, and black and organic carbon. South of the Intertropical Convergence Zone (ITCZ), we sampled pristine air originating from the remote southern Indian Ocean. North of the ITCZ, signatures of heavy pollution were evident over large areas of the Indian Ocean. Heavy pollution was present in the marine boundary layer as well as in the free troposphere at altitudes up to almost 4000 m. Outflow from the Indian subcontinent as well as from other source regions (Arabian Sea, Southeast Asia) could be identified by back trajectory calculations using the Hybrid Single Particle Lagrangian Integrated Trajectory model. The highest pollutant concentrations were observed in a free tropospheric pollution layer (“residual layer”), which originated from the Indian continental boundary layer. High mixing ratios of acetonitrile (up to 0.8 ppb) and submicron aerosol potassium (up to 0.6 ppb) indicate an important contribution from biomass or biofuel burning sources. On the other hand, high mixing ratios of sulfiir dioxide (up to 1.5 ppb) and aerosol sulfate (up to 3 ppb) indicate the influence of fossil fuel burning. During most flights the contributions from these two sources were well mixed within the same air mass, suggesting that the sources on the ground are also close to each other. This is consistent with the assumption that biomass is mainly burnt as biofuel for domestic use in populated areas, where fossil fuel is also used. The ratios dX/dCQ (X=acetone, acetonitrile, sulfur dioxide, potassium, or sulfate) measured during the flights indicate that most of the CO in the continental outflow is due to biomass or biofuel burning, whereas the majority of the aerosols results from fossil fuel burning.

STAAARTE-MED 1998 summer airborne measurements over the Aegean Sea 2. Aerosol scattering and absorption, and radiative calculations

descent over a ground-based site in northeastern Greece (40� 24 0 N, 23� 57 0 E; 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm � 1 , and the particle absorption coefficient was around 15 Mm � 1 , giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6–9% of the total accumulation mode particle mass (<1 mm diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude-dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short-wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground-based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short-wave radiative forcing (daytime average) between 280 nm and 4 m mi s up to � 64 W m � 2 at the surface and up to � 22 W m � 2 at the TOA. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 9335 Information Related to Geographic Region: Europe; KEYWORDS: aerosols, Aegean Sea, optical properties, vertical profiles, direct radiative forcing

Acetone and acetonitrile in the tropical Indian Ocean boundary layer and free troposphere: Aircraft‐based intercomparison of AP‐CIMS and PTR‐MS measurements

We have performed aircraft-borne intercomparison measurements of acetone and acetonitrile using two different mass spectrometric techniques: A Proton Transfer Reaction Mass Spectrometer (PTR-MS) operated and developed by the Institute of Ion Physics, University of Innsbruck, and an Atmospheric Pressure Chemical lonization Mass Spectrometer (AP-CIMS) operated and developed by the Max Planck Institute for Chemistry, Mainz. The two devices were employed for atmospheric trace gas measurements over the Indian Ocean on board the National Center for Atmospheric Research C-130 research aircraft during the Indian Ocean Experiment (INDOEX) intensive field phase in February and March 1999. Both instruments use reactions of ions with atmospheric trace gas molecules and mass spectrometric detection techniques for the determination of trace gas concentrations. For the detection of acetone and acetonitrile, both mass spectrometers operate in the positive ion mode and detect the protonated species of these compounds. One difference of the two systems is the operating pressure of the reaction chamber, where the ion molecule reactions take place. While the PTR-MS uses a low-pressure (2.2 hPa) drift tube, a much higher pressure of 300 hPa is used by the AP-CIMS. Other differences are the determination of background concentrations and the calibration procedure. The AP-CIMS uses a charcoal filter, and the PTR-MS uses a Pt catalyst. For the AP-CIMS an on-line calibration is necessary, which is performed during the flights. The intercomparison measurements described here were carried out on two research flights along a north-south transect along 73°E between 6°N and 8°S. Altitudes between 100 m and 6500 m above sea level were covered. Backward trajectory analyses indicate that the sampled air was taken from polluted air masses originating from India and the Bay of Bengal as well as from remote southern hemispheric air. During the two flights on February 20 and 24 the position of the Intertropical Convergence Zone was located around 1.50S in the investigated area. The measurements of both instruments of acetone and acetonitrile indicate a strong gradient in the boundary layer and a weaker one in the free troposphere. The values measured by the PTR-MS are higher than the ones by the AP-CIMS, but are within the combined error range of ±20–30% for each instrument. Acetone mixing ratios up to 2.5 parts per billion (ppb) and acetonitrile up to 0.4 ppb were measured in the northern hemispheric boundary layer. The measurements in the free troposphere give values between 0.3 ppb and 1.0 ppb for acetone and 0.12 ppb and 0.3 ppb for acetonitrile.

On-line measurements of ozone surface fluxes: Part II. Surface-level ozone fluxes onto the Sahara desert

Surface-level ozone concentrations, the vertical turbulent ozone flux as well as the fluxes of sensible and latent heat were continuously monitored by the eddy covariance method in the Lybian desert, 30 km south of the Dakhla Oasis in Egypt, from 23 March until 9 April 1993. An automatic station powered by a photovoltaics generator system was used to measure the vertical turbulent ozone flux to the desert ecosystem. Fairly high ozone volume fractions up to 60 ppb were recorded when northerly winds prevailed. When southerly winds were blowing, the ozone volume fractions were lower and reached maximum values slightly above 40 ppb. On-line eddy correlation measurements of the vertical turbulent ozone flux to the desert were performed with a novel fast-response ozone sensor. The fairly small ozone fluxes were corrected for effects of micro-turbulent density fluctuations caused by the concomitant fluxes of heat and water vapour in the air volume (Webb correction). While ozone fluxes to the desert ecosystem are below 2 ppb cm s− in the night, maximum daytime ozone fluxes of 20 ppb cm s−1 were measured which yielded a maximum daily dry deposition velocity of 0.15 cm s−1. During the whole measurement campaign of 16 d a mean deposition velocity of Vd = 0.065 cm s−1 for ozone is calculated. For global numerical models in which the sources and sinks of ozone in the troposphere are taken into account, a daytime Vd of 0.1 cm s−1 and a nighttime value of 0.04 cm s−1 are recommended for the desert ecosystem.

Comparative analysis of sodar and ozone profile measurements in a complex structured boundary layer and implications for mixing height estimation

Profile data from simultaneous sodar and tethered balloon measurements have been analyzed with respect to the complex structure of the atmospheric boundary layer in the Upper Rhine Valley. Special attention was focused on ozone concentration profiles measured with a novel lightweight ozone sensor at the balloon. In general, good agreement was found between the signature of the ozone concentration profiles and special features of the backscattered sound intensity profiles. This allows reliable estimation of the mixing height from the sodar data even in a complex stable ABL, except for very shallow mixing layers (below about 75 m), which could not be detected by the sodar.

Analyzing atmospheric trace gases and aerosols using passenger aircraft

CARIBIC (Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container) resumed regular measurement flights with an extended scientific payload in December 2004. After an automated measurement container was successfully deployed on intercontinental flights using a Boeing 767 from 1997 to 2002, a far more powerful package is deployed using a new Airbus A340-600 made available by Lufthansa German Airlines (Star Alliance). The new CARIBIC system will help address a range of current atmospheric science questions during its projected lifetime of 10 years. European and Japanese scientists are developing a variety of atmospheric chemistry research and monitoring projects based on the use of passenger aircraft. This is a logical approach with a main advantage being that near-global coverage is obtained, in contrast to limited coverage through research aircraft-based expeditions. Moreover, highly detailed and consistent data sets can be acquired, as compared to satellite observations in general. In addition, even compared to land-based observatories, operational costs are moderate.

Measuring non-Kolmogorov turbulence

We have performed a series of experiments aiming at understanding the statistics of deep turbulence over cities. The experimental setup consisted of a Shack-Hartmann wavefront sensor and an imaging camera that simultaneously recorded wavefront-, and focal-plane data, respectively. At the same time, measurements of deep optical turbulence were performed at the urban area of interest using two large-aperture scintillometer systems to get an impression of the strength of Cn2 above the rooftops of Ettlingen. Our focus is “urban” turbulence because we are interested in the usefulness of adaptive optics for free-space optical communications over urban areas. We discuss methods of determining departure from Kolmogorov turbulence. Our “last mile problem” is that urban turbulence can be significantly stronger, in the sense of flatter power spectrum, compared to the classic Kolmogorov turbulence. This could pose a significant challenge for adaptive optics systems.

Stability and height dependant variations of the structure function parameters in the lower atmospheric boundary layer investigated from measurements of the long-term experiment VERTURM (vertical turbulence measurements)

Operation and design of electro-optical systems are affected by atmospheric optical turbulence quantified by the refractive index parameter Cn2. Regarding wave propagation in the visible and infrared (IR), Cn2 is a function of height, dependant on temperature, pressure, and the structure temperature function parameter Cn2. The long-term experiment VerTurM (vertical turbulence measurements) was designed to characterize the vertical variations of optical turbulence up to 250 m in the lower atmospheric boundary layer for a moderate typical central European climate. Since May 2009 three independent measurement systems have been operated in a flat pasture site in north-western Germany. In the atmospheric surface layer at a tall tower sonic anemometer measurements are performed on four discrete heights between 4 and 64 m providing information about atmospheric stability and turbulence. Cn2 is derived. From 30 to 250 m a SODAR-RASS system (Sound Detection and Ranging - Radio acoustic sounding system) yields every half an hour profiles of Cn2. Additional direct measurements of Cn2 have been performed near the ground using a scintillometer. First results of the three measurement systems are presented. Vertical profiles and stability dependence are analysed in respect of Monin- Obukhov-similarity theory (MOST). Differences in the measurement systems and the expected height variations are discussed.

Comparison of slant-path scintillometry, sonic anemometry and high-speed videography for vertical profiling of turbulence in the atmospheric surface layer

The optical effect of atmospheric turbulence greatly inhibits the achievable range of Detection, Recognition and Identification (DRI) of targets when using imaging sensors within the surface layer. Since turbulence tends to be worst near the ground and decays with height, the question often arises as to how much DRI range could be gained by elevating the sensor. Because this potential DRI gain depends on the rate of decay of turbulence strength with height in any particular environment, there is a need to measure the strength profile of turbulence with respect to height in various environments under different atmospheric and meteorological conditions. Various techniques exist to measure turbulence strength, including scintillometry, sonic anemometry, Sound Detection and Ranging (SODAR) and the analysis of point source imagery. These techniques vary in absolute sensitivity, sensitivity to range profile, temporal and spatial response, making comparison and interpretation challenging. We describe a field experiment using multiple scintillometers, sonic anemometers and point source videography to collect statistics on atmospheric turbulence strength at different heights above ground. The environment is a relatively flat, temperate to sub-tropical grassland area on the interior plateau of Southern Africa near Pretoria. The site in question, Rietvlei Nature Reserve, offers good spatial homogeneity over a substantial area and low average wind speed. Rietvlei was therefore chosen to simplify comparison of techniques as well as to obtain representative turbulence profile data for temperate grassland. A key element of the experimental layout is to place a sonic anemometer 15 m above ground at the centre of a 1 km slant-path extending from ground level to a height of 30 m. An optical scintillometer is operated along the slant-path. The experiment layout and practical implementation are described in detail and initial results are presented.

Characterization of optical turbulence at the GREGOR solar telescope: temporal and local behavior and its influence on the solar observations

Local atmospheric turbulence at the telescope level is regarded as a major reason for affecting the performance of the adaptive optics systems using wavelengths in the visible and infrared for solar observations. During the day the air masses around the telescope dome are influenced by flow distortions. Additionally heating of the infrastructure close to telescope causes thermal turbulence. Thereby optical turbulence is produced and leads to quality changes in the local seeing throughout the day. Image degradation will be yielded affecting the performance of adaptive optical systems. The spatial resolution of the solar observations will be reduced. For this study measurements of the optical turbulence, represented by the structure function parameter of the refractive index Cn2 were performed on several locations at the GREGOR telescope at the Teide observatory at Tenerife at the Canary Islands / Spain. Since September 2012 measurements of Cn2 were carried out between the towers of the Vacuum Tower Telescope (VTT) and of GREGOR with a laser-scintillometer. The horizontal distance of the measurement path was about 75 m. Additional from May 2015 up to March 2016 the optical turbulence was determined at three additional locations close to the solar telescope GREGOR. The optical turbulence is derived from sonic anemometer measurements. Time series of the sonic temperature are analyzed and compared to the direct measurements of the laser scintillometer. Meteorological conditions are investigated, especially the influence of the wind direction. Turbulence of upper atmospheric layers is not regarded. The measured local turbulence is compared to the system performance of the GREGOR telescopes. It appears that the mountain ridge effects on turbulence are more relevant than any local causes of seeing close to the telescope. Results of these analyses and comparison of nearly one year of measurements are presented and discussed.