Carsten Jahn

Influence of mixing layer height upon air pollution in urban and sub-urban areas

The mixing layer height is an important parameter characterising the potential of the atmospheric boundary layer to take up emitted air pollutants. During continuous measurements in Hanover, Germany, from 2001 until 2003 and around Munich, Germany, in summer and winter 2003 mixing layer heights (MLH) were determined by different remote sensing systems mainly from the thermal structure and turbulence of the air (SODAR), for some time from the aerosol layering of the air (ceilometer), and for a short period directly from the temperature profile (RASS). The temporal variations of the concentrations of PM 10 and PM 2.5 as well as of CO and NO x simultaneously measured near the surface were investigated and correlated with the MLH derived from SODAR data. The pollution measurements were performed inside a street canyon and at an urban background station close to Hanover and at three measurement locations inside and outside of Munich complementing the available monitoring networks. The analyses show that the correlations of pollutant concentrations with MLH are smallest inside street canyons. Correlations at the urban background stations are larger in winter than in summer, and they are larger for the urban stations than for the rural stations. It turns out further that the correlation of NO X concentrations with MLH is larger than the correlation of particles concentrations. Explanations for these findings must consider the varying emission source strengths for NO X and particles and the influence of gas-to-particle conversion within air masses especially during daytime in summer.

Multiple atmospheric layering and mixing-layer height in the Inn valley observed by remote sensing

Automatic mixing layer height monitoring was performed by continuous sodar and ceilometer measurements in the Inn valley east of Innsbruck, Austria during a winter measuring campaign on air and noise pollution. The ceilometer, an eye-safe commercial lidar originally designed to detect cloud base heights and vertical visibility for aviation safety purposes, was operated for about two months; the sodar was operated for more than four months. Special software for this ceilometer provides routine retrievals of multiple aerosol layers and mixing layer height from the optical backscatter data. An existing retrieval method for the mixing layer height from sodar data has also been enhanced in order to detect multiple atmospheric layering. Particular emphasis is given to the detection of thermally stable layers and inversions within the lower valley atmosphere and their temporal development. Such layers influence significantly the diurnal variations of air pollution and traffic noise impact. A comparison is performed with parallel mixing layer height retrievals from the sodar and the ceilometer. In clear and cold winter nights sometimes several layers one above the other can be detected with both instruments. These multiple layers form due to an interaction between the mountain wind and the down-slope winds. In the absence of low clouds and precipitation ceilometers can estimate the mixing-layer-height fairly well. Ceilometer and sodar partly complement each other.

Field measurements within a quarter of a city including a street canyon to produce a validation data set

Air pollutants and meteorological parameters were measured continuously by in situ instruments, path-averaging techniques (up to three DOAS systems), and SODAR inside a street canyon and in the surrounding area of 1 km x 1 km (Gottinger Strasse in Hanover, Germany) from 2001 until 2003 which are available in the data bank ValiData for validation of microscale models. During three IOPs tracer experiments with a SF6 line source, sampling techniques of up to 15 sites and path-averaging FTIR spectrometry were performed. Concentration measurement results at roof level were anti-correlated with SODAR mixing layer heights, while those inside the street canyon are not. Re-circulation flow patterns inside the street canyon were studied together with corresponding wind tunnel experiments.

Remote detection of methane by infrared spectrometry for airborne pipeline surveillance: first results of ground-based measurements

The total length of natural gas pipelines in Germany exceeds 350,000 km. Currently, inspections are performed using hand-held sensors such as flame ionization detectors. Moreover, transmission pipelines are inspected visually from helicopters. In this work, remote detection of methane by passive Fourier-transform infrared (FTIR) spectrometry for pipeline surveillance is investigated. The study focuses on fast measurements in order to enable methane detection from a helicopter during regular inspection flights. Two remote sensing systems are used for the detection of methane: a scanning infrared gas imaging system (SIGIS), which was originally developed for the visualization of pollutant clouds, and a new compact passive scanning remote sensing system. In order to achieve a high spectral rate, which is required due to the movement of the helicopter, measurements are performed at low spectral resolutions. This results in overlapping signatures of methane and other constituents of the atmosphere in the measured spectrum. The spectra are analyzed by a detection algorithm, which includes simultaneous least squares fitting of reference spectra of methane and other atmospheric species. The results of field measurements show that passive remote sensing by FTIR spectrometry is a feasible method for the remote detection of methane.

Remote sensing of aircraft exhaust temperature and composition by passive Fourier Transform Infrared (FTIR)

The scanning infrared gas imaging system (SIGIS-HR) and the quantitative gas analysis software MAPS (Multicomponent Air Pollution Software) are applied to investigate the spatial distribution of the temperature and gas concentrations (CO, NO) within the plume of aircraft engines at airports. The system integrates an infrared camera also. It is used for the localisation of the hot source that additionally suggests the best measurement position of the SIGIS-HR. The application of emission FTIR spectrometry for the measurement of temperature and gas emission index of CO and NO is presented for the exhaust of a small turbojet based on a helicopter turbine. In these measurements the emitted infrared radiation from the exhaust gas stream was collected by the SIGIS-HR at different spectral resolution (56 cm-1 and 0.2 cm-1). The software MAPS includes the Instrumental Line Shape (ILS) of the OPAG- 22 FTIR spectrometer obtained by active gas cell measurements and ILS modelling. The rough concept of the system will be presented and operational applications will be discussed. The results of the investigation of the temperature and gas concentrations (CO, NO) within the aircraft engine plumes will be shown. The limitations and of the systems will be discussed.

Remote measurement of the plume shape of aircraft exhausts at airports by passive FTIR spectrometry

Information about the interaction between the exhaust plume of an aircraft jet engine and ambient air is required for the application of small-scale chemistry-transport models to investigate airport air quality. This interaction is not well understood. In order to study the interaction, spatial information about the plume is required. FTIR emission spectroscopy may be applied to analyze the aircraft exhausts. In order to characterize the plumes spatially, a scanning imaging FTIR system (SIGIS) has been improved. SIGIS is comprised of an interferometer (Bruker OPAG), an azimuth-elevation-scanning mirror, a data acquisition and control system with digital signal processors (DSP), an infrared camera and a personal computer. With this instrumentation it is possible to visualise the plume and to obtain information about the temperature distribution within the plume. Measurements are performed at low spectral resolution, because the dynamic environment of these measurements limits the measurement time to about 2 minutes. Measurements of the plume shapes of an APU and of main engines were performed.

New results from continuous mixing layer height monitoring in urban atmosphere

Mixing layer height was continuously monitored by uninterrupted remote sensing measurements with ceilometer, sodar and RASS in Augsburg. The Vaisala ceilometers LD40 and CL31 were used which are eye-safe commercial lidar systems. Special software for these ceilometers provides routine retrievals of lower atmosphere layering from vertical profiles of laser backscatter data. These remote sensing instruments were operated at three different sites: at the northern edge (CL31 or LD40, RASS), in the middle (CL31) and at the southern edge of the town (sodar). A comparison of the different results during simultaneous measurements was performed. The information content of the different remote sensing instruments for mixing layer height was analysed further. The ceilometer measurements add information about the range-dependant aerosol concentration; gradient minima within this profile mark the borders of mixed layers. The sodar measurements detect the height of a turbulent layer characterized by high acoustic backscatter intensities due to thermal fluctuations and a high variance of the vertical velocity component. The RASS measurements provide the vertical temperature profile from the detection of acoustic signal propagation. Different measurement examples will be presented to demonstrate the information about the mixing layer height.

SIGIS HR: a system for measurement of aircraft exhaust gas under normal operating conditions of an airport

To gather information about the impact on the environment caused by airport operations, knowledge about the amount of gases such as CO or NOX emitted by aircraft engines on the ground is important. In order to avoid influences on airport operations an analysis system for this application has to enable measurements on the hot jet engine exhaust gas from a distance. The infrared radiation emitted by the hot gas can be analysed by Fourier-transform infrared spectroscopy to determine the composition of the gas. To fulfil this task, a new version of the scanning infrared gas imaging system (SIGIS HR), using relatively high spectral resolution (0.2 cm-1), has been developed. The period of time for measurements on the engine exhaust gas of an aircraft on the ground is short during normal airport operations. Hence the remote sensing system has to be aligned to the exhaust gas plume quickly. For this reason the system is equipped with a scanning mirror actuated by stepper motors in order to allow fast changes of the line of sight. An infrared camera combined with a DSP-system enables automatic alignment of the system to the hot exhaust gas and tracking of a moving engine via online analysis of the infrared image. Additionally fast scans with low spectral resolution of the area around the engine-outlet can be performed. On the basis of the low resolution data the optimal direction for the exhaust gas measurement can be found using several automatic evaluation- and positioning-algorithms. After the SIGIS HR-system has been positioned correctly it is operated in high- resolution-mode in order to quantify the target compounds.

Comparison of remote sensing techniques for measurements of aircraft emissions indices at airports

The emission indices of aircraft engine exhausts were measured at airports non-intrusively by FTIR emission spectrometry at the engine nozzle exit as well as by FTIR absorption spectrometry and DOAS (Differential Optical Absorption Spectrometry) behind the aircraft. Two measurement campaigns were performed to compare these different measurement methods. A kerosene powered burner was operated in that way that the different methods were applied for the exhaust gas investigations during the same time and at nearly the same exhaust gas volume. The burner was built with a nozzle exit diameter of 37 cm and a power of about 150 kW. Fresh air was pumped into the burner tube by a fan. Calibration gases as pure CO and NO were added in different amounts to vary the concentration of these gases in the exhaust. The sampling probe of an intrusive measurement system was installed in the centre of the exhaust stream near the exhaust exit for measurements of these gases and CO2 as well as NO2, UHC, SO2 and O2. An APU (GTCP36-300) in a test bed was used in the same way. CO was mixed into the exhausts near the nozzle exit. The passive FTIR instrument was operated in the test bed using special noise and vibration isolation. The open-path instruments were installed at the chimney exit on the roof of the test bed building. The deviations between the different measurement methods were in the order of ±10 up to ±20 %.

Fusion of air pollution data in the region of Munich, Germany, by the ICAROS NET platform

The air quality in Munich is monitored by the measurement network of the Bavarian Agency for Environmental Protection. Additional information can be provided from retrievals of optical thickness and corresponding particle concentrations from satellite images in an area of approximately 100 km x 100 km (depending on the satellite sensor used). The satellite measures the optical thickness of the entire atmosphere, which has to be attributed mainly to the mixing layer. The mixing layer height is determined either by remote sensing, by radiosondes, or by numerical models of the boundary layer. The corrected optical thickness of the satellite images can be interpreted as the particle concentration in the mixing layer. Data from the ground-based monitoring network and from satellite retrievals are fused in the ICAROS NET platform. This platform is applied to supply additional information on the air quality in the Munich region and it is tested as well as evaluated during field campaigns in summer and winter. The adaptation to the Munich region covers the development of routines for the collection of data, for example from the measuring network, and the disposal of information, which were defined by the Bavarian agency for environmental protection. During measurement campaigns in and around Munich PM 10, PM 2.5 and PM 1.0 concentrations and mixing layer heights by remote sensing (SODAR, ceilometer, WTR) were determined. Temporal variations of the concentration, the spatial distribution (3 measurement locations) and concentration conditions for selected particle sizes are presented.