Abstract
In the context of the outbreak of coronavirus disease 2019 (COVID-19), strict lockdown policies were implemented to control nonessential human activities in Xi’an, northwest China, which greatly limited the spread of the pandemic and affected air quality. Compared with pre-lockdown, the air quality index and concentrations of PM2.5, PM10, SO2, and CO during the lockdown reduced, but the reductions were not very significant. NO2 levels exhibited the largest decrease (52%) during lockdown, owing to the remarkable decreased motor vehicle emissions. The highest K+ and lowest Ca2+ concentrations in PM2.5 samples could be attributed to the increase in household biomass fuel consumption in suburbs and rural areas around Xi’an and the decrease in human physical activities in Xi’an (e.g., human travel, vehicle emissions, construction activities), respectively, during the lockdown period. Secondary chemical reactions in the atmosphere increased in the lockdown period, as evidenced by the increased O3 level (increased by 160%) and OC/EC ratios in PM2.5 (increased by 26%), compared with pre-lockdown levels. The results, based on a natural experiment in this study, can be used as a reference for studying the formation and source of air pollution in Xi’an and provide evidence for establishing future long-term air pollution control policies.
Highlights
Since the Industrial Revolution in the 18th century, industrialization has transformed production patterns and lifestyles in society
Our study focused on air quality variation during the COVID-19 lockdown in the city of Xi’an, China
The air quality index (AQI) decreased in sequence before, during, and after the COVID-19 lockdown with statistical difference between periods of post-lockdown and during lockdown, and air quality gradually improved from moderately polluted (AQI: 151–200) to mildly polluted (AQI: 101–150) and further, to good air quality (AQI: 51–100; Figure 3a)
Summary
Since the Industrial Revolution in the 18th century, industrialization has transformed production patterns and lifestyles in society. Industrialization and modernization have led to major environmental problems [1–3]. Anthropogenic emissions, including vehicle and industrial exhaust emissions, fossil fuel combustion, resident smoking, and household heating, are an important cause of deteriorating air quality [4–6]. Ghaffarpasend et al [7] concluded that motor vehicle emissions accounted for an average of 45% of the air pollutants in Tehran, Iran. Of particulate matter with aerodynamic diameter equal to or less than 2.5 μm (PM2.5 ), and fossil fuel combustion contributed 15.8% of particulate matter with aerodynamic diameter equal to or less than 10 μm (PM10 ) emissions in Shandong Province of China [3]. Zhang et al [2] reported that industrial processes were the main source of organic carbon (OC) emissions in total suspended particulate (TSP), accounting for 23.6%
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