It has to be stressed that these simulations were made with a thermal contrast equal to zero. As NH 3 is only emitted at surface level and rapidly destroyed, most of the improvement is usually found between 0 and 2 km. For instance, a factor of 2 improvement in the spectral resolution leads to a 25% gain in detection sensitivity at the surface level (blue line), whereas a factor of 2 improvement in the radiometric noise leads to an improvement of 50% (red dashed line). It is well seen in Fig. 31 that an improvement of radiometric noise has a much higher impact than an improvement in the spectral resolution. Atmospheric Measurement Techniques 7, 4367–4385. Towards IASI-new generation (IASI-NG): Impact of improved spectral resolution and radiometric noise on the retrieval of thermodynamic, chemistry and climate variables. Adapted from Crevoisier, C., Clerbaux, C., Guidard, V., et al. Improvement of the spectral resolution by a factor of 2 compared to IASI is shown in full lines. Relative difference ratio (%) between the detection limit of NH 3 of each instrument configuration compared to the current spectral (Δ ν = 0.5 cm − 1) and noise ( N = 0.2 K in the 967.3 cm − 1 band) of IASI, as a function of altitude, for a case representative of a tropical atmospheric situation. A higher spectral resolution also induces a better vertical resolution thanks to thinner weighting functions and Jacobians (e.g., temperature, CO).įig. 31. In these regions, temperature signal is about 0.8–0.9 K, and the radiometric noise, which is much lower than the CH 4 signal, does not greatly impact the retrieval. However, having a better spectral resolution is a real asset since it allows finding spectral intervals where the CH 4 signal comes out of the H 2O signal: near 1300 cm − 1, as for IASI, but also near 1275 cm − 1 (with a slight contamination by N 2O), 1247 cm − 1 (with a slight contamination by the surface), or near 1340 cm − 1. The CH 4 signature for a 10% variation of the gas mixing ratio is similar for both instruments, even if a bit higher (0.2 K) for the latter, with values as high as 1.6 K. In the ν 4 absorption band, water vapor absorption largely dominates, with mean sensitivities of 1 K for a 20% variation of its mixing ratio. This is well illustrated by Fig. 30, which shows the typical channel sensitivities to CH 4 at the IASI or IASI-NG spectral resolution. Reducing the interferences thus leads to better accuracy. On the one hand, increasing the spectral resolution particularly matters when absorption lines of various gases located in the same spectral range interfere with each other (e.g., H 2O for the retrieval of CO and CH 4). The influence of the sampling interval on resolution and the resulting spectra are illustrated in Figure 10. However, diode array spectrometry does not cover applications requiring high-resolution spectral acquisition. For UV–Vis spectrometric applications on solutions, the NBW characterizing signals are generally higher than 20 nm, resulting in the acceptance of a 2 nm sampling interval. The basic requirement in spectrometry is that the sampling interval should be at least 10 times lower than the natural bandwidth (NBW) of the sample signal. However, a 2 nm resolution can be considered as the usual value in diode array spectrometry, which means significantly lower capabilities compared to resolutions of 0.5, 0.2, and even 0.1 nm, which is quite usual for scanning instruments. Spectrometers for the UV–Vis domain (180–820 nm) equipped with a 1024 photodiode array will consequently ensure a resolution of ∼0.6 nm. Higher resolution means not only more diodes composing the array but also smaller diodes. The distance, in terms of wavelength, between two adjacent diode centers is called sampling interval. The spectral resolution for diode array spectrometers is dependent upon the number of photosensitive diodes distributed in the focal plane of the concave diffraction grating.
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