Abstract
The Environmental trace gas Monitoring Instrument (EMI) onboard the Chinese high-resolution remote sensing satellite GaoFen-5 is an ultraviolet–visible imaging spectrometer, aiming to quantify the global distribution of tropospheric and stratospheric trace gases and planned to be launched in spring 2018. The preflight calibration phase is essential to characterize the properties and performance of the EMI in order to provide information for data processing and trace gas retrievals. In this paper, we present the first EMI measurement of nitrogen dioxide (NO2) from a gas absorption cell using scattered sunlight as the light source by the differential optical absorption spectroscopy technique. The retrieved NO2 column densities in the UV and Vis wavelength ranges are consistent with the column density in the gas cell calculated from the NO2 mixing ratio and the length of the gas cell. Furthermore, the differences of the retrieved NO2 column densities among the adjoining spatial rows of the detector are less than 3%. This variation is similar to the well-known “stripes-pattern” of the Ozone Monitoring Instrument and is probably caused by remaining systematic effects like a nonperfect description of the individual instrument functions. Finally, the signal-to-noise ratios of EMI in-orbit measurements of NO2 are estimated on the basis of on-ground scattered sunlight measurements and radiative transfer model simulations. Based on our results, we conclude that the EMI is capable of measuring the global distribution of the NO2 column with the retrieval precision and accuracy better than 3% for the tested wavelength ranges and viewing angles.
Highlights
G LOBAL monitoring of atmospheric trace gas distributions from a number of ultraviolet–visible (UV–Vis) spaceborne spectrometers including GOME, SCIAMACHY, Ozone Monitoring Instrument (OMI), and GOME-2 [3]–[7] has been playing an important role in studies of atmospheric chemistry, air pollution, and climate change
The results indicate that the variability of the corrected NO2 slant column density (SCD) for different spatial rows is less than 3% in general, while after the geometric light-path correction, the same values should be obtained for all rows
Measurements of the NO2 absorption in a gas cell using scattered sunlight are performed with the Environmental trace gas Monitoring Instrument (EMI), which is designed for the Chinese high-resolution remote sensing satellite GaoFen-5
Summary
G LOBAL monitoring of atmospheric trace gas distributions from a number of ultraviolet–visible (UV–Vis) spaceborne spectrometers including GOME, SCIAMACHY, Ozone Monitoring Instrument (OMI), and GOME-2 [3]–[7] has been playing an important role in studies of atmospheric chemistry, air pollution, and climate change. Measurements of trace gases in a cell using a lamp and those in the atmosphere using scattered sunlight in the zenith view were performed for the OMI instrument with the DOAS retrieval approach [26]. By using a similar approach, scattered sunlight measurements of NO2 in a gas absorption cell are performed with the EMI to explicitly consider the solar Fraunhofer lines in the DOAS fit and to characterize the ability of EMI to retrieve NO2 SCD in orbit.
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