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.

中文翻译：

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首尔城市 CO2 通量随土地利用类型的时空变化

城市是大气二氧化碳的主要来源；然而，由于城市土地利用的高度异质性，了解决定每个城市排放的净 CO2 通量的地表 CO2 交换过程具有挑战性。因此，本研究使用具有不同城市土地利用类型（基线、住宅、旧城区住宅、商业、和植被区）。年 CO2 通量显着变化，从基线站点的 1.09 kg C m- 2 year- 1 到首尔首都地区老城区住宅区的 16.28 kg C m- 2 year- 1。每月 CO2 通量变化与所有地点的植被活动密切相关（r = - 0.61）；然而，每种土地利用类型与建筑能耗的相关性不同（住宅用地 r = 0.72，商业用地 r = 0.34）。昼夜 CO2 通量变化主要与所有站点的交通量相关（r = 0.8）；然而，它与流动人口的相关性在住宅（r = - 0.44）和商业（r = 0.80）地点相反。此外，每小时 CO2 通量与温度高度相关。在植被点，随着温度超过24 ℃，CO2 吸收对温度的敏感性比之前的温度增加了7.44 倍。相反，当温度低于或超过18 ℃基线时，非植被点的CO2通量增加，对低温的敏感性是高温的三倍。平均而言，非植被城市站点的排放量为 0。全年排放 45 g C m− 2 h− 1 的 CO2，无论温度如何。我们的结果表明，大多数城市地区在所有时区都是二氧化碳排放源；然而，即使在城市内部，CO2 通量特征也会因城市土地利用类型而有很大差异。因此，来自不同土地利用类型的多次观测对于确定每个城市的综合二氧化碳循环以制定有效的城市二氧化碳减排政策至关重要。