Unusual response of O₃ and CH₄ to NO₂ emissions reduction in Japan during the COVID-19 pandemic

ABSTRACT

We investigate the impact of NO₂ emissions reduction on O₃ and CH₄ levels in 14 metropolitan areas of Japan in 2020. To account for meteorological variations, we employ business-as-usual air quality time series generated by machine learning models. Additionally, we use satellite observations and biogeochemical model simulations to analyse air quality changes. During the lockdown period from April 7 to May 25 in 2020, we observed a NO₂ reduction that equated to a decrease equivalent to 3.4 and 5 years of the corresponding trends in roadside and ambient air quality recorded from 2010 to 2019. After meteorological normalization, NO₂ decreased by 14.5% at ambient air stations and 19.1% at roadside stations. Surprisingly, the NO₂ reduction did not immediately lead to increased O₃. Instead, O₃ levels rose after the lockdown, specifically in August due to favorable sunny conditions. This finding is important for Japan and has not been reported in previous studies. We found that changes in NO₂ and CO marginally contributed to variations in CH₄ levels across the study areas. It is recommended to simultaneously reduce NO_x as well as non-methane volatile organic compounds to mitigate their adverse effects on future policies.

KEYWORDS:

• NO₂
• O₃
• CH₄
• Japan
• COVID-19
• weather-normalization

Acknowledgements

We express our gratitude to National Institute for Environmental Studies for providing ground observation data for NO₂, O₃, CO, and CH₄ levels. We would like to thank the Japan Meteorological Agency for providing meteorological ground observation data. We acknowledge the Copernicus Climate Change Service for archiving ERA5 reanalysis data and making it accessible to the public free of charge. We extend our sincere appreciation to the European Space Agency (ESA) and Google Earth Engine for granting us free access to satellite derived NO₂ and HCHO data from Sentinel 5P. We would like to thank NIES for the distribution of CH₄ emission simulations from the VISIT model.

Lastly, we would also like to thank the two anonymous reviewers and the editor for their valuable feedback to enhance the quality of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The availability of the data used in the study is summarized as follows:

• Ground observation for air pollution: https://tenbou.nies.go.jp/download/ (Last access: 17 May 2023)

• Meteorological ground observation data: https://www.data.jma.go.jp/gmd/risk/obsdl/index.php (Last access: 4 May 2023)

• Sentinel 5P NO₂ data: https://developers.google.com/earth-engine/datasets/catalog/COPERNICUS_S5P_OFFL_L3_NO2 (Last access: 24 April 2023)

• Sentinel 5P HCHO data: https://developers.google.com/earth-engine/datasets/catalog/COPERNICUS_S5P_OFFL_L3_HCHO (Last access: 24 April 2023)

• ERA5 data reanalysis: https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels (Last access: 23 March 2023)

• CH₄ emission simulations from VISIT model: 10.17595/20210521.001 (Last access: 7 June 2022)

Additional information

Funding

This work was supported by Ministry of Education, Culture, Sports, Science and Technology - Japan.