Spatiotemporal variations in urban CO flux with land-use types in Seoul.

Background: Cities are a major source of atmospheric CO; however, understanding the surface CO exchange processes that determine the net CO 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 CO flux over the Seoul Capital Area, South Korea from 2017 to 2018, using CO flux measurements at nine sites with different urban land-use types (baseline, residential, old town residential, commercial, and vegetation areas).

Results: Annual CO flux significantly varied from 1.09 kg C m year at the baseline site to 16.28 kg C m year at the old town residential site in the Seoul Capital Area. Monthly CO 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 CO 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 CO flux was highly related to temperature. At the vegetation site, as the temperature exceeded 24 ℃, the sensitivity of CO absorption to temperature increased 7.44-fold than that at the previous temperature. Conversely, the CO 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 h of CO throughout the year, regardless of the temperature.

Conclusions: Our results demonstrated that most urban areas acted as CO emission sources in all time zones; however, the CO 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 CO cycle of each city to develop effective urban CO reduction policies.