Tower observed vertical distribution of PM_2.5, O₃ and NO_x in the Pearl River Delta

The vertical structure characteristics of the concentrations of PM_2.5, O₃ and NO_x are analyzed in conjunction with the meteorological data collected in December 2017 from a new meteorology tower in the Pearl River Delta region. The results show that: (1) The concentration of PM_2.5 generally decreased with height, while the magnitude of the decrease differed between the polluted and the clean situations. The concentration of O₃ increased with height, and it increased more drastically during the polluted days than during the clean days. The vertical distribution of NO_x concentration was quite similar to that of PM_2.5, while the gradient of NO_x concentration was much steeper below 220 m and was more gentle over 220 m. (2) The diurnal cycles of PM_2.5 were different at different heights, while the diurnal cycles of O₃ and NO_x were similar at all heights. A diurnal pattern with a single peak appearing during 14:00 to 16:00 Local Standard Time was observed for PM_2.5 at higher altitudes (220 m and 335 m), hinting that the secondary formation contributed significantly to PM_2.5 at higher layers. (3) The correlation coefficients between PM_2.5 and O₃ were weak near the ground but increased with height and maintained their positive values. O₃ and NO_x were generally negatively correlated, and the correlation coefficients between the two pollutants decreased with height. The correlation between PM_2.5 and NO_x was always significant at all heights of the observation.