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A unique illness, the coronavirus disease 2019 (COVID-19), emerged in Wuhan, People’s Republic of China, in December 2019. To reduce the spread of the virus, strict lockdown policies and control measures were put in place all over the world. Due to these enforced limitations, a drastic drop in air pollution and an improvement in air quality were observed. The present study used six air pollutants (PM₁₀, PM_2.5, SO₂, NO₂, CO and O₃) to observe trends before, during and after the COVID-19 lockdown period in Nanjing, China. The data were divided into six phases: P1–P3, pre-lockdown (1 October–31 December 2019), lockdown (1 January–31 March 2020), after lockdown (1 April–30 June 2020), P4–P6: the same dates as the lockdown but during 2017, 2018 and 2019. The results indicate that compared with the pre-lockdown phase, the PM₁₀ and PM_2.5 average concentrations decreased by –27.71% and –5.09%. Compared with the previous three years, 2017–2019, the reductions in PM₁₀ and PM_2.5 were –37.99% and –33.56%, respectively. Among other pollutants, concentrations of SO₂ (–32.90%), NO₂ (–34.66%) and CO (–16.85%) also decreased during the lockdown, while the concentration of O₃ increased by approximately 25.45%. Moreover, compared with the pre- and during lockdown phases, PM₁₀, PM_2.5 and NO₂ showed decreasing trends while SO₂, CO and O₃ concentrations increased. These findings present a road map for upcoming studies and provide a new path for policymakers to create policies to improve air quality.