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28 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.1.11 Applicability of light-emitting diodes as light sources for active DOAS measurements Holger Sihler (Christoph Kern, Ulrich Platt) Abstract The spectral stability of light-emitting diodes (LEDs) was studied in view of their applicability in Long Path Differential Optical A bsorption Spectroscopy (LP-DOAS). Beside a constant temperature also a highly accurate current source to drive the LEDs was found to be essential. intensity of LED [a.u.] 10 18 10 19 10 20 10 21 400 420 440 460 480 500 520 10 22 540 wavelength [nm] Figure 2.12: Spectral emission of a Luxeon LXHL-LR3C high power 3 W royal blue LED (at 700 mA, 10 ◦ C, blue line) in comparison with the absorption cross sections of two trace gases – NO2 (Voigt 2002, red line) and Glyoxal (Volkamer 2005, black line) – which have significant absoption structures in this spectral range. Background To date, high pressure xenon arc lamps have established themselves as the most common light sources for active DOAS instruments. However, these have several disadvantages including poor power efficiency in the required wavelength region and short lifetime resulting in high maintenance costs. Modern LEDs potentially represent a very advantageous alternative for both LP-DOAS [Kern et al. , 2006] and cavity enhanced absorption spectroscopy (CEAS) [Ball et al. , 2004; Langridge et al. , 2006]. Additionally LEDs are much easier to maintain considering the risk of explosions and interfering electromagnetic radiation. Methods and results As one may notice in the comparison between the emission of an LED and the trace gas absorption cross sections (Figure 2.12), the LED spectrum contains some narrowband structures considered to be etalon structures. Only a slight variation of these structures during the measurement process increases the residual of a DOAS evaluation by one order of magnitude or even more in comparison to Xenon arc lamps. Stabilising their emission spectrum is therefore the key to make LEDs competitive as DOAS light-sources. The spectral position of the etalon structure depends on the chip temperature. A non-zero heat resistance between chip and heat sink yields optical density of trace gas [cm2/molecule] a dependency on both the heat sink tempearture and the dissipation of electrical energy inside the chip. In the special case of the LXHL-LR3C an already attained temperature stabilisation within 0,1 K corresponds to 0,1 % of the current. Hence, a stabilisation of the LED drive current was found to be as important as controlling the temperature. A compact sealed LED housing with a peltiercooled heatsink and molecular sieve drying agent was designed to decouple the LED from the ambient temperature. Together with a standard PIDcontroller and a current source, the spectral noise of a LED could be reduced to a value lower than that of halogen lamps. The stability of an arc lamp has not been attained yet. Funding IUP Outlook/Future work Future experiments will concentrate on UV-LEDs to detect further trace gases (e.g., SO2, ClO, or CH2O). Also superluminescent LEDs (SLEDs) will be studied in respect to their applicability. The latter provide very high radiances but are only in the wavelength region above 650 nm available. Also the application of an even more precise temperature controller (TEC) is very promising. A newly customdesigned current source-TEC combination has to be tested and compared to industry standard components.
2.1. TROPOSPHERIC RESEARCH GROUP 29 2.1.12 Multi Axis Differential Optical Absorption Spectroscopy (MAX- DOAS) of Trace Gas and Aerosol Distributions Roman Sinreich (Rainer Volkamer, Thomas Wagner) Abstract MAX-DOAS measurements, i.e. DOAS measurements at different elevation angles from the ground, enable the detection of trace gases in the planetary boundary layer even at low concentrations due to their long light path at low elevation angles. Furthermore, MAX-DOAS values combined with radiative transfer modelling can yield distributions of trace gases and aerosols. Figure 2.13: HONO SCDs on March 6th, 2006, in Mexico City. Enhanced values in the morning were typical throughout the measurement campaign. Background For three decades DOAS has been applied to measure trace gases like e.g. NO2, SO2 or HCHO by means of sunlight [?]. Sunlight passing through the atmosphere is scattered and absorbed by gas molecules in the air whereby the absorption occurs at wavelengths which are characteristic for each trace gas. This absorption pattern is suitable to detect and quantify the according trace gases. In this study, ground-based DOAS measurements were performed and their spectra analysed. By using the measured results and modelled data from a radiative transfer model, it is possible to derive trace gas and aerosol distributions. Methods and results The retrieval of trace gases by means of DOAS yields Slant Column Densities (SCDs) which depend on the concentration of the according trace gas and the light path of the measurement. Since scattered sunlight is measured, the light path is not readily defined and has to be calculated by atmospheric radiation transport models. Furthermore, different elevation angles from the ground have to be performed in order to get information on the vertical profile. This method is called Multi-Axis-DOAS (MAX- DOAS) and allows to derive gas and aerosol distributions near the surface. Aerosol properties can be derived indirectly by measurements of gas species whose 3-dimensional distribution is already known and relatively constant. E.g. O4, whose concentration is proportional to the square of the O2 concentration, provides these qualities. So variations in measured O4-SCDs are dominated by the aerosol distribution, which causes different light paths in the atmosphere [Wagner et al. , 2004]. In March 2006, MAX-DOAS measurements were performed at several sites in Mexico City in the framework of MILAGRO (Megacity Initiative: Local and Global Research Observations, see also report of Merten et al. 2.1.7 ). Thereby, SCDs of NO2, CHOCHO, HCHO, O4 and SO2 could be retrieved. Furthermore, SCDs of HONO at daytime could be detected for the first time by means of MAX-DOAS (a typical diurnal cycle of HONO SCDs is shown in Figure 2.13). Outlook/Future work The MAX-DOAS values from Mexico City will be compared with insitu, active DOAS and satellite measurements. Moreover, improvements of the technique of deriving gas and aerosol distributions will allow to perform the inversion procedure more analytically. Main publication Sinreich et al. [2006]
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Overview Werner Aeschbach-Hertig Su
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180 CHAPTER 7. BIBLIOGRAPHY Butz, A
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182 CHAPTER 7. BIBLIOGRAPHY Leigh,
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184 CHAPTER 7. BIBLIOGRAPHY Wang, P
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186 CHAPTER 7. BIBLIOGRAPHY Kühl,
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7.5. INVITED TALKS 193 Invited Talk
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