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• Reductions of PM, NO 2 , SO 2 and CO under lockdown differed in urban agglomerations. • The mitigation of PM2.5 concentrations due to lockdown was considerable. • Although the lockdown did not reverse uptrend of O 3 , it retarded increment of O 3. • BTH, YRD and PRD suffered from more polluted air masses of regional transport. To investigate the air quality change during the COVID-19 pandemic, we analyzed spatiotemporal variations of six criteria pollutants in nine typical urban agglomerations in China using ground-based data and examined meteorological influences through correlation analysis and backward trajectory analysis under different responses. Concentrations of PM 2.5 , PM 10 , NO 2 , SO 2 and CO in urban agglomerations respectively decreased by 18%–45% (30%–62%), 17%–53% (22%–39%), 47%-64% (14%–41%), 9%–34% (0%–53%) and 16%-52% (23%–56%) during Lockdown (Post-lockdown) period relative to Pre-lockdown period. PM 2.5 pollution events occurred during Lockdown in Beijing-Tianjin-Hebe (BTH) and Middle and South Liaoning (MSL), and daily O 3 concentration rose to grade Ⅱ standard in Post-lockdown period. Distinct from the nationwide slump of NO 2 during Lockdown period, a rebound (∼40%) in Post-lockdown period was observed in Cheng-Yu (CY), Yangtze River Middle-Reach (YRMR), Yangtze River Delta (YRD) and Pearl River Delta (PRD). With slightly higher wind speed compared with 2019, the reduction of PM 2.5 (51%–62%) in Post-lockdown period is more than 2019 (15%–46%) in HC (Harbin-Changchun), MSL, BTH, CP (Central Plain) and SP (Shandong-Peninsula), suggesting lockdown measures are effective to PM 2.5 alleviation. Although O 3 concentrations generally increased during the lockdown, its increment rate declined compared with 2019 under similar sunlight duration and temperature. Additionally, unlike HC, MSL and BTH, which suffered from additional (> 30%) air masses from surrounding areas after the lockdown, the polluted air masses reaching YRD and PRD mostly originated from the long-distance transport, highlighting the importance of joint regional governance. [Display omitted] [ABSTRACT FROM AUTHOR]