Investigating the Impacts of the COVID-19 Lockdown on Trace Gases Using Ground-Based MAX-DOAS Observations in Nanjing, China

Abstract

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.

Ground-Based MAX-DOAS Observations of Tropospheric NO2 and HCHO During COVID-19 Lockdown and Spring Festival Over Shanghai, China

“…Due to the specific nature of events, different studies reported the effects of COVID-19 lockdowns on air quality and trace gas concentrations. Our results comply with recent studies depicting an overall decline in trace gas concentrations and an improvement in air quality [23][24][25][26][27][28][53][54][55][56][57].…”

Quantifying the Impacts of COVID-19 Lockdown and Spring Festival on Air Quality over Yangtze River Delta Region

“…The level of O 3 in the atmosphere was not impacted by the lockdown or change in anthropogenic activity and kept on rising during the study period while the solar radiation intensified for late winter and early spring days [5,6]. O 3 measurements show notable rise in concentration during lockdown compared to pre lockdown days depicting 45% and 60% for Shanghai, 100% and 136 for Nanjing, 108% and 113% for Hefei, and 33% and 42% for Hangzhou for Level-1 and Level-2 of the COVID-19 lockdown, respectively.…”

“…It is quite evident from the data that the average concentration of NO 2 over these cities dropped during end of January 2020 compared to preceding and succeeding years. Several studies have reported the decrease in tropospheric NO 2 emissions over different parts of the world during the COVID-19 lockdown [5][6][7][8][9][10][11][12]. Table 2 shows percentage change in NO2 over different cities for Level-1 and Level-2 compared to the pre-lockdown days from 2018 to 2021 where negative sign indicates reduction while positive shows increase in concentration.…”

“…On average, the concentration is lower during the period preceding the lockdown. Several studies have reported an increase in O 3 concentration with reduction in anthropogenic activity during the lockdown period [5,6,11,12].…”

“…A sharp decline of 29% and 32% in industrial electricity consumption was observed in Anhui and Zhejiang provinces during the COVID-19 lockdown. Various studies conducted across the globe to explain the impact of COVID-19 lockdowns reported reductions in the levels of atmospheric trace gas species during the lockdown periods [5][6][7][8][9][10][11][12].…”