MAX-DOAS Network in China and Its Applications

<p>Entering the &#8220;14^th Five-Year Plan&#8221;, the coordinated control of PM_2.5 and O₃ has become a major issue of air pollution prevention and control in China. In order to achieve this goal, the stereoscopic monitoring of regional PM_2.5 and O₃ and their precursors (NO₂, HCHO etc.) is extremely necessary. Yet, current monitoring networks are inadequate to monitor the vertical profiles of all above atmospheric compositions simultaneously, and to support air quality control. We have established a nationwide ground-based hyperspectral stereoscopic remote sensing network based on MAX-DOAS since 2015. This monitoring network provides a significant opportunity for the regional coordinated control of PM_2.5 and O₃ in China. One-year vertical profiles of aerosol, NO₂, HCHO and O₃ monitored from four MAX-DOAS stations (CAMS, SH_XH, SUST and CQ) installed in four megacities (Beijing, Shanghai, Shenzhen and Chongqing) are used to characterize their vertical distribution differences in four key regions of Jing-Jin-Ji (JJJ), Yangtze River Delta (YRD), Pearl River Delta (PRD) and Sichuan Basin (SB), respectively. The normalized and yearly averaged aerosol vertical profiles in JJJ and PRD exhibit a box shape under 400 m and a Gaussian shape, respectively, and they all show exponential shapes in YRD and SB. The NO₂ vertical profiles in four regions all exhibit exponential shapes due to the obvious vehicle emissions. The shape of HCHO vertical profile in JJJ and PRD shows Gaussian, and it exhibits exponential shape in YRD and SB. The averaged O₃ vertical profiles in four regions all exhibit box shape and linear shape in pollution and non-pollution periods, respectively. Moreover, a regional transport event occurred at an altitude of 600-1000 m was monitored in the southwest-northeast pathway of North China Plain (NCP) by five MAX-DOAS stations (SJZ, WD, NC, CAMS and UCAS) belonging to above network. The aerosol optical depths (AOD) in these five stations varied in the order of SJZ > WD > NC > CAMS > UCAS. The short-distance regional transport of NO₂ in 800 m layer was monitored between WD and NC. As an important precursor of secondary aerosol, NO₂ air mass in WD and NC all occurred 1 hr earlier than aerosol. Similarly, the short-distance regional transport of HCHO in 800 m layer between NC and CAMS, and it potentially affected the O₃ concentration in Beijing. Finally, CAMS was selected as a typical site to learn O₃-NO_x-VOCs sensitivities in vertical space. We found the production of O₃ changed from predominantly VOC-limited condition to mainly mixed VOC-NO_x-limited condition from 0-100 m layer to 200-300 m layer. In addition, the downward transport of O₃ could make a contribute to the increase of ground surface O₃ concentration. ground-based hyperspectral stereoscopic remote sensing network provide a promising strategy to support PM_2.5 and O₃ and their precursors management and attribution of sources.</p>