Utilizing Continuous Multi-Component MAX-DOAS Observations for the Near-Surface Ozone Sensitivity Diagnosis at Chiba and Tsukuba, Japan for 2013-2019

Abstract:

The ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) was utilized to conduct continuous simultaneous observations of near-surface ozone (O₃), nitrogen dioxide (NO₂), and formaldehyde (HCHO) concentrations at Chiba (35.63ºN, 140.10ºE, 21 m a.s.l.) and Tsukuba (36.06ºN, 140.13ºE, 35 m a.s.l.) located in urban and suburban sites of greater Tokyo metropolitan area, Japan for seven years from 2013 to 2019. NO₂ and HCHO are considered as proxies for nitrogen oxides (NOx) and volatile organic compounds (VOCs), respectively, which are major precursors of tropospheric O₃. The mean concentrations in the retrieved lowermost 1-km-thick layer (at altitudes of 0-1 km) during daytime were analyzed. For more spatially-representative analysis around Chiba situated in an urban area, four MAX-DOAS instruments directed to four different azimuth directions (north, east, west, and south) were operated simultaneously and their data were unified. The seven-year-long period was unique as satellite observations indicated an abrupt improvement in the tropospheric NO₂ concentration level over East Asia, including China. It is suggested that the transboundary transport of O₃ originating from the Asian continent was likely suppressed or almost unchanged during the period. Over such a unique time period, MAX-DOAS observations showed almost-constant annual variations or rather increasing tendency in the near-surface O₃ concentration at Chiba and Tsukuba, while reductions in NO₂ and HCHO concentrations occurred at rates of about 6-10%/year at Chiba. These changes provided clear observational evidence that decreasing NOx concentration significantly reduced the amount of O₃ quenched through the NO titration under the VOC-limited condition in the urban area. Under the VOC-limited condition, the MAX-DOAS-derived concentration ratio of [HCHO]/[NO₂] was found to indicate around 0.4-0.7. These new findings from MAX-DOAS observations are expected to constrain for more precise quantitative understanding of the processes leading to the near-surface O₃ variation.