Air quality changes in cities during the COVID-19 lockdown: A critical review

Abstract

In response to the rapid spread of coronavirus disease-2019 (COVID-19) within and across countries and the need to protect public health, governments worldwide introduced unprecedented measures such as restricted road and air travel and reduced human mobility in 2020. The curtailment of personal travel and economic activity provided a unique opportunity for researchers to assess the interplay between anthropogenic emissions of primary air pollutants, their physical transport, chemical transformation, ultimate fate and potential health impacts. In general, reductions in the atmospheric levels of outdoor air pollutants such as particulate matter (PM), nitrogen dioxide (NO 2 ), carbon monoxide (CO), sulfur dioxide (SO 2 ), and volatile organic compounds (VOCs) were observed in many countries during the lockdowns. However, the levels of ozone (O 3 ), a secondary air pollutant linked to asthma and respiratory ailments, and secondary PM were frequently reported to remain unchanged or even increase. An increase in O 3 can enhance the formation of secondary PM 2.5 , especially secondary organic aerosols, through the atmospheric oxidation of VOCs. Given that the gaseous precursors of O 3 (VOCs and NO x ) are also involved in the formation of secondary PM 2.5, an integrated control strategy should focus on reducing the emission of the common precursors for the co-mitigation of PM 2.5 and O 3 with an emphasis on their complex photochemical interactions. Compared to outdoor air quality, comprehensive investigations of indoor air quality (IAQ) are relatively sparse. People spend more than 80% of their time indoors with exposure to air pollutants of both outdoor and indoor origins. Consequently, an integrated assessment of exposure to air pollutants in both outdoor and indoor microenvironments is needed for effective urban air quality management and for mitigation of health risk. To provide further insights into air quality, we provide a critical review of scientific articles, published from January 2020 to December 2020 across the globe. Finally, we discuss policy implications of our review in the context of global air quality improvement.

Impact of COVID-19 on Extremely Polluted Air Quality and Trend Forecast in Seven Provinces and Three Cities of China

“…During the COVID-19 lockdown, air quality has been significantly improved (Pandey et al, 2021;Raffaele et al, 2021). The reduction in personal travel and economic activities provides researchers with an opportunity to objectively evaluate air pollutants (Adam et al, 2021). The main pollutants of air quality can be determined by the changes in the concentration of pollutants before and after the epidemic is blocked (Xu et al, 2021).…”

The Multi-Time Scale Changes in Air Pollutant Concentrations and Its Mechanism before and during the COVID-19 Periods: A Case Study from Guiyang, Guizhou Province

“…The lockdown measures had greatly reduced the emission of nitrogen o and significantly reduced the NO2 concentration in the air. The NO concentration u changes synergistically with the NO2 concentration [15]. Decreased NO concent weakened the "titration effect", resulting in a significant increase in the O3 concent Compared with the same lunar periods in 2019, 2018, and 2017, the mean concentration of NO 2 during the Fir-Lockdown period in 2020 decreased by 4.90 µg•m −3 (32.92%), 9.54 µg•m −3 (48.83%), and 16.27 µg•m −3 (61.94%), respectively (Figure 2b), again indicating that the COVID-19 lockdown was very effective in reducing transportation activities.…”

“…The lockdown measures had greatly reduced the emission of nitrogen oxides and significantly reduced the NO 2 concentration in the air. The NO concentration usually changes synergistically with the NO 2 concentration [15]. Decreased NO concentration weakened the "titration effect", resulting in a significant increase in the O 3 concentration by 19.59 µg•m −3 (35.5%) during the Fir-Lockdown period in 2020 compared with the same lunar calendar in 2019 [6].…”

“…As for the source of particulate matter PM 2.5 (diameter ≤ 2.5 µm) and PM 10 (diameter ≤ 10 µm), it includes not only the primary direct emissions from coal combustion, vehicle exhaust, and road dust, but also the secondary aerosols generated by particle conversion, showing some significant regional differences in their response to the lockdown measures [6,14]. Due to the differences in various regions caused by factors such as pollutant emission intensity, meteorological conditions, and atmospheric oxidizability, the improvement of air quality failed to show any consistent spatiotemporal trend between the cities in the same country and in different countries [15]. For example, the highly humid weather in the North China Plain during the COVID-19 pandemic, coupled with the persistent emissions from power plants and the petrochemical industry, still caused serious haze events [6,16].…”

Spatiotemporal Change of Air-Quality Patterns in Hubei Province—A Pre- to Post-COVID-19 Analysis Using Path Analysis and Regression

“…Therefore, the period of epidemic prevention and control is a good point in time for the further exploration of the main factors affecting urban air quality changes. [13]. In this study, we examine the AQI and the levels of six ambient air pollutants, namely NO 2 , O 3 , sulphur dioxide (SO 2 ), carbon monoxide (CO), PM 10 , and PM 2.5 , before, during and after the 2020 COVID-19 lockdown in the four cities of Hubei province in China, namely Huangshi, Yichang, Jingzhou, and Wuhan.…”