Monitoring of Haze in Xi'an Based on Hyperspectral Remote Sensing Technology

With economic development and accelerated industrialization and urbanization, air pollution has become a serious problem affecting people's health and socio-economic development. In recent years, smog in my country has continued to spread and intensify, showing an explosive trend. In 2013, smog affected 25 provinces and more than 100 large and medium-sized cities, with the national average number of smoggy days reaching 29.9 days, the highest in 52 years. The occurrence of smog in my country has become a year-round phenomenon and may persist for decades to come.

Composition and monitoring of smog

Aerosols are mixed dispersion systems, consisting of small liquid particles, small solid particles, or a mixture of solids and liquids in a colloidal state. They are multiphase systems formed by gases and suspended in the atmospheric gaseous medium. Fog in smog is an aerosol system composed of tiny water droplets or ice crystals in the air.Haze is an aerosol system composed of non-aqueous substances in the air, such as dust, sulfuric acid, nitric acid, and organic hydrocarbons, which poses a serious threat to human health.Traditional meteorological monitoring methods often fail to accurately detect the formation and changes of haze, while satellite remote sensing technology has advantages such as high timeliness, wide coverage, and high resolution, making it possible to monitor aerosol conditions quickly and over large areas. Therefore, it has become an important tool for large-scale haze monitoring.

Principles and methods of remote sensing monitoring of haze

Current research and applications of remote sensing technology in haze mainly focus on aerosol optical depth (AOD), astigmatism coefficient, composition, vertical distribution, and haze image removal techniques during hazy weather. Among these, AOD is a physical quantity used to measure the most fundamental optical characteristic of aerosols, describing their attenuation effect on light. It is defined as the integral of the aerosol's extinction coefficient along its perpendicular path along the direction of radiative transmission.

Remote sensing monitoring of haze mainly utilizes remote sensing data to obtain data such as aerosol optical thickness and analyzes the relationship between aerosol optical thickness and haze concentration.Currently, the more mature aerosol optical thickness inversion algorithms mainly include: the dark target method based on the relationship between the mid-infrared channel (2.12μm) and the red (0.66μm) and blue (0.47μm) channels, the structure function method based on surface reflectance, the polarization aerosol inversion method (POLDER) based on polarization information, and the deep blue algorithm based on blue band reflectance, etc.

CAS Xi'an Aerospace - Xi'an Aerosol Concentration Inversion

In optical remote sensing, due to the shorter wavelength, the surface reflectivity in the blue band is generally lower, while the atmospheric reflection and scattering are relatively greater. Therefore, aerosol optical thickness is usually retrieved by inverting the blue band.

Recently, Xi'an has experienced frequent smog, significantly impacting the city's economic production and residents' lives. Based on the aforementioned remote sensing monitoring methods, the Xi'an Aerospace Team of the Chinese Academy of Sciences monitored and retrieved the aerosol thickness and concentration distribution in Xi'an, obtaining the following retrieval results;

CAS Xi'an Aerospace - Xi'an Aerosol Concentration Inversion

As can be seen from the figure,The aerosol thickness at three different times showed different trends, with the highest concentration on December 27 and a decrease in aerosol thickness on January 3, which is closely related to the actual situation in Xi'an.  

With continuous technological advancements and widespread adoption, remote sensing monitoring technology will receive increasing attention in the field of air pollution. Specifically, the application prospects of remote sensing technology for monitoring haze will become increasingly broad. Using remote sensing data and models, the spatiotemporal changes and impact range of haze pollution can be comprehensively assessed, providing a basis for relevant decision-making.