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