{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,15]],"date-time":"2026-04-15T07:52:40Z","timestamp":1776239560819,"version":"3.50.1"},"reference-count":43,"publisher":"MDPI AG","issue":"23","license":[{"start":{"date-parts":[[2020,12,1]],"date-time":"2020-12-01T00:00:00Z","timestamp":1606780800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["NSFC, 41701551, 41605117, 41771291"],"award-info":[{"award-number":["NSFC, 41701551, 41605117, 41771291"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>The spread of the COVID-19 pandemic and consequent lockdowns all over the world have had various impacts on atmospheric quality. This study aimed to investigate the impact of the lockdown on the air quality of Nanjing, China. The off-axis measurements from state-of-the-art remote-sensing Multi-Axis Differential Optical Absorption Spectroscope (MAX-DOAS) were used to observe the trace gases, i.e., Formaldehyde (HCHO), Nitrogen Dioxide (NO2), and Sulfur Dioxide (SO2), along with the in-situ time series of NO2, SO2 and Ozone (O3). The total dataset covers the span of five months, from 1 December 2019, to 10 May 2020, which comprises of four phases, i.e., the pre lockdown phase (1 December 2019, to 23 January 2020), Phase-1 lockdown (24 January 2020, to 26 February 2020), Phase-2 lockdown (27 February 2020, to 31 March 2020), and post lockdown (1 April 2020, to 10 May 2020). The observed results clearly showed that the concentrations of selected pollutants were lower along with improved air quality during the lockdown periods (Phase-1 and Phase-2) with only the exception of O3, which showed an increasing trend during lockdown. The study concluded that limited anthropogenic activities during the spring festival and lockdown phases improved air quality with a significant reduction of selected trace gases, i.e., NO2 59%, HCHO 38%, and SO2 33%. We also compared our results with 2019 data for available gases. Our results imply that the air pollutants concentration reduction in 2019 during Phase-2 was insignificant, which was due to the business as usual conditions after the Spring Festival (Phase-1) in 2019. In contrast, a significant contamination reduction was observed during Phase-2 in 2020 with the enforcement of a Level-II response in lockdown conditions i.e., the easing of the lockdown situation in some sectors during a specific interval of time. The observed ratio of HCHO to NO2 showed that tropospheric ozone production involved Volatile Organic Compounds (VOC) limited scenarios.<\/jats:p>","DOI":"10.3390\/rs12233939","type":"journal-article","created":{"date-parts":[[2020,12,1]],"date-time":"2020-12-01T20:06:09Z","timestamp":1606853169000},"page":"3939","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":18,"title":["Investigating the Impacts of the COVID-19 Lockdown on Trace Gases Using Ground-Based MAX-DOAS Observations in Nanjing, China"],"prefix":"10.3390","volume":"12","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-0684-1970","authenticated-orcid":false,"given":"Zeeshan","family":"Javed","sequence":"first","affiliation":[{"name":"Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7290-2551","authenticated-orcid":false,"given":"Yuhang","family":"Wang","sequence":"additional","affiliation":[{"name":"School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2717-7557","authenticated-orcid":false,"given":"Mingjie","family":"Xie","sequence":"additional","affiliation":[{"name":"Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China"}]},{"given":"Aimon","family":"Tanvir","sequence":"additional","affiliation":[{"name":"Department of Environmental Science &amp; Engineering, Fudan University, Shanghai 200433, China"}]},{"given":"Abdul","family":"Rehman","sequence":"additional","affiliation":[{"name":"CAS-Key Laboratory of Crust-Mantle Materials and the Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China"}]},{"given":"Xiangguang","family":"Ji","sequence":"additional","affiliation":[{"name":"School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei 230026, China"},{"name":"Key Lab of Environmental Optics &amp; Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China"}]},{"given":"Chengzhi","family":"Xing","sequence":"additional","affiliation":[{"name":"Key Lab of Environmental Optics &amp; Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9273-4500","authenticated-orcid":false,"given":"Awais","family":"Shakoor","sequence":"additional","affiliation":[{"name":"Department of Environment and Soil Sciences, University of Lleida, Avinguda Alcalde Rovira Roure 191, 25198 Lleida, Spain"}]},{"given":"Cheng","family":"Liu","sequence":"additional","affiliation":[{"name":"Key Lab of Environmental Optics &amp; Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China"},{"name":"Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China"},{"name":"Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230026, China"},{"name":"Anhui Province Key Laboratory of Polar Environment and Global Change, University of Science and Technology of China, Hefei 230026, China"}]}],"member":"1968","published-online":{"date-parts":[[2020,12,1]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1775","DOI":"10.5194\/acp-17-1775-2017","article-title":"Cleaning up the air: Effectiveness of air quality policy for SO2 and NOx emissions in China","volume":"17","author":"Mijling","year":"2017","journal-title":"Atmos. 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