In highly complex terrain path-averaged (or even volume-averaged measurements by scanning with remote sensing techniques) can be advisable. Compared with single point measuring techniques path-averaging measurement results are a more adequate tool for the comparison with modelling results, especially if the spatial resolution of the models does not match the spatial representativity of the measurements (Schäfer et al., 2005).
The trace gas concentrations are measured by FTIR absorption spectrometry at an open path (Kayser-Threde GmbH K300). The IR radiation source (glow bar) is located at the opposite edge of the measuring path. The telescope of the spectrometer has a diameter of 15 cm and a field of view of 3 mrad. The detection limits of the K300 spectrometers at a 100 m open path are 5 ppb or 10 µg m-3 for N2O, 50 ppb or 30 µg m-3 for CH4, 5 ppm or 10 mg m-3 for CO2 and 5 ppb or 6 µg m-3 for CO. The measurement errors are about 5 % (Haus et al., 1994; Briz et al., 2006).
K. Schäfer contributed to the development of the guidelines EN 15383 “Ambient air quality – Atmospheric measurements near ground with FTIR spectroscopy” and VDI 4211, part 1 “Remote sensing - Atmospheric measurements near ground with FTIR spectroscopy. Measurement of gaseous emissions and immissions. Fundamentals”.
The measurement method is used for the detection of greenhouse gases and CO in ambient air (Schäfer et al., 2005) as well as for the detection of emission indices of aircraft exhausts (Schäfer et al., 2003). Furthermore, the system is applicable for tracer gas experiments with SF6 (detection limit 1 ppb or 6 µg m-3) as described in Schäfer et al., 2005. The detections limits and measurement accuracies are appropriate to detect increases of concentrations of more than 1 % within a measuring tunnel during a time frame of up to 10 hours (Schäfer et al., 2008).
One example for studies in urban air are the continuous field measurements in a street canyon (Goettinger Strasse, Hannover) from beginning 2001 until end of 2003 to generate validation data sets for micro-scale models. Air pollutants and meteorological parameters were measured at five in situ stations. The path-averaging optical FTIR and DOAS measurements at the ground extended the measurements (see Figure 1).
Figure 1: Location of measurement systems inside and around of the street canyon Göttinger Strasse, Hanover. View from the roof of the NLÖ building (location of the in situ monitoring station HRSW): in situ measurement stations (container) HRV1 (westerly side walk – left), HRV2 (Deisterplatz – above) and HRV3 (easterly side walk – right), open paths of DOAS system (dotted lines), open paths of FTIR (dashed lines), SF6 line source (full line) and SF6 sampling sites (stars).
Three successful intensive measurement campaigns were performed during different seasons. Path-integrated CO and tracer SF6 concentrations at both side walks of the street were measured. On the basis of these measurement results micro-scale processes in the street canyon were investigated. The horizontal and vertical concentration variations show the influence of wind direction upon the circulation in the street canyon and a rotor during perpendicular wind directions. The measurement results of one day are shown in the Figure 2.
Figure 2: FTIR (two spectrometers K300-1 and K300-2) and in situ measurement results of CO on 11 April 2003 at the westerly (stations HRT10, HRV1, HRVS) and easterly sidewalk (HRT11, HRV3) in the upper part (horizontal variations) as well as at the ground (HRT10, HRV1, HRVS) and at the roof (HRSW) at the westerly side of the street in the middle part (vertical variations) together with wind speed, wind direction and global radiance at the station HRSW in the lower part of the figure.
For an up-scaling of in situ soil emission measurements of N2O and other greenhouse gases a non-intrusive emission rate measurement method was developed, installed and tested using the FTIR concentration measurements and open-path acoustic tomographic wind measurements for the application of the flux-gradient method. The concentrations of N2O, CO2 and CH4 as well of humidity were measured by the multi-component FTIR absorption spectrometry at 99 m length near the surface of grassland. Figure 3 shows the results during a three days measurement campaign. At the third day a measuring tunnel for controlled enrichment of released soil gases was installed at the soil surface over an area of about 495 m2 so that the N2O and CO2 concentrations increased immediately.
Figure 3: Concentrations of N2O, CO2 and CH4 as well of humidity measured by the path-averaging, multi-component FTIR absorption spectrometry at an open path of 99 m length in about 50 cm altitude above soil surface at a campaign from 15 until 17 October 2007.
The altitude gradients measured by two open-path FTIR spectrometer measurements in 50 and 270 cm altitude were investigated. The results from a further campaign are shown in Figure 4. There are stable concentration gradients of N2O during night and day. The gradients of CO2 show emissions during the night but an up-take during the later daytime if the sun is still shining but the near surface air becomes stable.
Figure 4: Concentration gradients of N2O (above) and CO2 (below) measured by the path-averaging, multi-component FTIR absorption spectrometry at an open path of 99 m length in about 50 cm and 270 cm altitude above soil surface at a campaign from 17 until 18 June 2008.
Contact: Klaus Schäfer, Carsten Jahn