<p>The outbreak of the 2019 novel coronavirus (COVID-19) has brought tremendous impact and influence on human health and social economy around the world. The lockdown implemented in China, starting on 23 January 2020, led to large reductions in human activities and the associated emissions. Sharp declines in primary pollution provided a unique chance to examine the relationships between anthropogenic emissions and air quality. Here, we report measurements of air pollutants and meteorological parameters at different heights on a tall tower in the Pearl River Delta, China, to investigate the response of the vertical scales of pollutants to reductions in human activities. Compared to the pre-lockdown period (starting from 16 December 2019), the observations showed that surface layer NO_x, PM_2.5 and mean values of the daily maximum 8&thinsp;h average O₃ (MDA8O₃) had significant reductions of 76.8&thinsp;%, 49.4&thinsp;%, and 18.6&thinsp;% respectively, but the average O₃ increased (9.7&thinsp;%) during lockdown period. The vertical profiles of NO_x and O₃ changed during the lockdown period, but not those of PM_2.5. The correlation between PM_2.5 and O₃ was statistically significant, but not that between PM_2.5 and NO_x for data collected at four different heights during the lockdown period. The significance of these correlations was the opposite during the pre-lockdown period, indicating that the main composition of PM_2.5 has changed dramatically since the lockdown, which is transited from primary aerosol dominating or nitrate dominating (affected by NO_x) before lockdown to secondary organic aerosol dominant dominating (affected by O₃) during the lockdown. We find weaker diurnal variation of O₃ during the lockdown period is similar to the case at background regions. O₃ concentrations were not sensitive to NO_x concentrations during lockdown, which implies that O₃ levels during the lockdown are more representative of the regional background, for which anthropogenic emissions are low and photochemical formation is not a significant ozone source. This evidence suggests that significant reductions of anthropogenic emissions are effective in simultaneous mitigation of PM_2.5 and O₃ levels.</p>