Monitoring the air pollution in China from geostationary satellites is explored

Air quality, particularly regional and urban air pollution, has become one of the most important environmental issues worldwide. The increasing of pollutants in the troposphere (e.g., O3, SO2, NO2 and HCHO) causes heavy health problems of human. Monitoring the air pollutants from satellite is an important approach to study the temporal and spatial distribution of pollutants and control air pollution around the world.

In previous years, measurements in the ultraviolet (UV), visible, and thermal infrared (TIR) bands from low-Earth-orbit (LEO) satellites are used widely to retrieve the concentration of tropospheric pollutants. Until now, some LEO satellites have been launched successfully by the United States, Europe and China aimed to observing tropospheric air pollutants.

However, the spatial and temporal resolution of LEO satellites are so low that limiting the retrieval precision of pollutants. This uncertainties of concentration of pollutants affect the monitoring of air quality and research about atmospheric physical and chemical process. Compared with LEO satellite, geostationary satellite has the ability of staring the observed target all the time, so that the observing intensity and the spatial and temporal resolution of measurements are improved.

As reported, some geostationary satellites are planned to be launched in North American, East Asia, and Europe between 2018 and 2020 to monitoring regional air pollution. Whereas, there is still not similar satellite plan in China.

The theoretical feasibility of measuring the atmospheric pollutants O3, SO2, NO2, and HCHO in China using geostationary satellites is explored in recent study. "we try to choose parameters of instrument onboard geostationary satellite for measuring air pollution in China," according to Zhaonan Cai and Xi Chen, scientists at the Institute of Atmospheric Physics, Chinese Academy of Sciences, in Beijing, China.

In an article coauthored with Fishman et al., researchers stated: "To measure O3 profiles in the boundary layer, the independent information content of O3 in the boundary layer should be greater than 2 ppb, and the retrieval uncertainty should be less than 10 ppb. NO2 pollution can be distinguished from background levels when the retrieval precision of tropospheric NO2 is greater than 1+ 10 15 molecules cm-2 and the spatial resolution of hourly observations is less than 8 km."

Based on these requirements of the retrieval precision of O3, SO2, NO2, and HCHO, four scholars revealed in the study, which was published in the Science China Earth Sciences the impacts of the sensor parameters (e.g., SNR and spectral resolution) on the retrieval sensitivity and errors of pollutants using the optimal estimation method.

Research aimed at finding appropriate parameters for the spectrometer used on geostationary satellite focusing on air pollutants measurements in China. To validate these sensor parameters, the three-dimension (3D) distributions of H2O, O3, NO2, SO2, and HCHO in China were simulated by the 3D atmospheric chemical transport model GEOS-Chem for the simulated retrieval experiments.

"As many as 90% of the experiments met the retrieval requirements for all target polluted gases. The retrieval uncertainties of total column O3 and stratospheric column O3 could be improved to 2%," the researchers wrote in an article titled "Monitoring air pollution in China from geostationary satellite: A synthetic study using simulated observations."

"These sensor parameters have the ability to meet the retrieval precision requirements of the target gases," they concluded.

Considering the polluted gases have strong absorption of solar radiation in UV and visible band, the absorption spectrum from geostationary satellites over China and the surrounding areas need to be simulated. A forward model consisting of a vector radiative transfer model, instrument model, surface model, and atmospheric model is used from simulation.

From the simulated spectrum, the change of instrument parameters (e.g., spectral resolution, retrieval bands, and signal-to-noise SNR) with the retrieval sensitivities and errors of the gases are shown according to the optimal estimation theory. Given the requirement of retrieval precision of polluted gases, the observing band, spectral resolution and SNR of the sensor are suggested 300~500 nm, 0.6 nm and 1000, respectively. This study firstly recommends the reasonable sensor parameters aimed to air pollutants observation over China from geostationary satellite.

"To monitor the air pollution focuses on China," wrote the four researchers, "this study offers a theoretical basis and simulation tool for improving the design of instruments onboard geostationary satellites."

Chen X, Cai Z, Liu Y, Yang D. 2018. Monitoring air pollution in China from geostationary satellite: A synthetic study using simulated observations. Science China Earth Sciences, 61

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