Temporal characteristics of carbon dioxide and ozone over a rural-cropland area in the Yangtze River Delta of eastern China

In this study, rural atmospheric carbon dioxide (CO₂) and ozone (O₃) were measured from January 2015 to December 2018 to investigate characteristics of greenhouse gases in eastern China. Results showed that the annual average CO₂ (O₃) concentration in 2018 decreased by 2% (increased by 19%) when compared with that in 2015. CO₂ concentrations exhibited monthly variability, peaking in February (443.7 ppm) and reaching their lowest levels in July (363.0 ppm); whereas, monthly O₃ showed a bimodal pattern with peaks in June (51.3 ppb) and September (34.5 ppb). Regarding the diurnal variation, the maximum CO₂ (O₃) concentration occurred at nighttime (in the daytime) and a minimum CO₂ (O₃) in the daytime (at nighttime). As demonstrated by correlation analysis, CO₂ and O₃ variations were partly modulated by NOx and PM_2.5. Furthermore, CO₂ showed significant positive correlations with relative humidity in winter, while O₃ showed strong positive correlations with temperature in spring. CO₂ was accumulated from local sources under calm conditions (< 2 m s^-1) and derived from remote sources at high wind speeds (> 4 m s^-1), while O₃ concentrations were peaking at medium wind speeds of 2-4 m s^-1. CO₂ was found to derive from long-distance (short-distance transport) transport in spring (the other three seasons), whereas O₃ is mainly from long-distance (short-distance) transport in winter (the other three seasons). This work sheds light on the temporal characteristics of CO₂ and O₃, which has important implications for implementing practices to mitigate source emissions over cropland areas.