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Ground-based remote sensing using multi-axis differential optical absorption spectroscopy (MAX-DOAS) was used to conduct continuous simultaneous observations of ozone (O 3 ), nitrogen dioxide (NO 2 ), and formaldehyde (HCHO) concentrations at Chiba (35.63° N, 140.10° E, 21 m a.s.l.) and Tsukuba (36.06° N, 140.13° E, 35 m a.s.l.), Japan, for 7 years from 2013 to 2019. These are urban and suburban sites, respectively, in the greater Tokyo metropolitan area. NO 2 and HCHO are considered to be proxies for nitrogen oxides (NOx) and volatile organic compounds (VOCs), respectively, both of which are major precursors of tropospheric O 3 . The mean concentrations below an altitude of 1 km were analyzed as planetary boundary layer (PBL) concentrations. For a more spatially representative analysis around the urban area of Chiba, four MAX-DOAS instruments directed at four different azimuth directions (north, east, west, and south) were operated simultaneously and their data were unified. During the 7-year period, the satellite observations indicated an abrupt decrease in the tropospheric NO 2 concentration over East Asia, including China. This suggested that the transboundary transport of O 3 originating from the Asian continent was likely suppressed or almost unchanged during the period. Over this time period, the MAX-DOAS observations revealed the presence of almost-constant annual variations in the PBL O 3 concentration, whereas reductions in NO 2 and HCHO concentrations occurred at rates of approximately 6–10%/year at Chiba. These changes provided clear observational evidence that a decreasing NOx concentration significantly reduced the amount of O 3 quenched through NO titration under VOC-limited conditions in the urban area. Under the dominant VOC-limited conditions, the MAX-DOAS-derived concentration ratio of HCHO/NO 2 was found to be below unity in all months. Thus, the multi-component observations from MAX-DOAS provided a unique data set of O 3 , NO 2 , and HCHO concentrations for analyzing PBL O 3 variations.

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Continuous multi-component MAX-DOAS observations for the planetary boundary layer ozone variation analysis at Chiba and Tsukuba, Japan, from 2013 to 2019