Spatiotemporal variations in urban CO₂ flux with land-use types in Seoul

Cities are a major source of atmospheric CO₂; however, understanding the surface CO₂ exchange processes that determine the net CO₂ flux emitted from each city is challenging owing to the high heterogeneity of urban land use. Therefore, this study investigates the spatiotemporal variations of urban CO₂ flux over the Seoul Capital Area, South Korea from 2017 to 2018, using CO₂ flux measurements at nine sites with different urban land-use types (baseline, residential, old town residential, commercial, and vegetation areas).

Annual CO₂ flux significantly varied from 1.09 kg C m^- 2 year^- 1 at the baseline site to 16.28 kg C m^- 2 year^- 1 at the old town residential site in the Seoul Capital Area. Monthly CO₂ flux variations were closely correlated with the vegetation activity (r = - 0.61) at all sites; however, its correlation with building energy usage differed for each land-use type (r = 0.72 at residential sites and r = 0.34 at commercial sites). Diurnal CO₂ flux variations were mostly correlated with traffic volume at all sites (r = 0.8); however, its correlation with the floating population was the opposite at residential (r = - 0.44) and commercial (r = 0.80) sites. Additionally, the hourly CO₂ flux was highly related to temperature. At the vegetation site, as the temperature exceeded 24 ℃, the sensitivity of CO₂ absorption to temperature increased 7.44-fold than that at the previous temperature. Conversely, the CO₂ flux of non-vegetation sites increased when the temperature was less than or exceeded the 18 ℃ baseline, being three-times more sensitive to cold temperatures than hot ones. On average, non-vegetation urban sites emitted 0.45 g C m^- 2 h^- 1 of CO₂ throughout the year, regardless of the temperature.

Our results demonstrated that most urban areas acted as CO₂ emission sources in all time zones; however, the CO₂ flux characteristics varied extensively based on urban land-use types, even within cities. Therefore, multiple observations from various land-use types are essential for identifying the comprehensive CO₂ cycle of each city to develop effective urban CO₂ reduction policies.