Anthropogenic and natural controls on atmospheric <i>?</i><sup>13</sup>C-CO<sub>2</sub> variations in the Yangtze River delta: insights from a carbon isotope modeling framework

Abstract. The atmospheric carbon dioxide (CO2) mixing ratio and its isotope (?13C-CO2) composition contain important CO2 sink source information spanning from ecosystem to global scales. observation simulation for both ?13C-CO2 can be used constrain regional emissions better understand the anthropogenic natural mechanisms that control variations. Such work remains rare urban environments, especially megacities. Here, we near-continuous measurements, September 2013 August 2015, inverse modeling budget investigate main factors dominated variations Yangtze River delta (YRD) region, one of largest hotspots densely populated regions in China. We WRF-STILT model framework with category-specified EDGAR v4.3.2 inventories simulate hourly ratios ?13C-CO2, evaluated these simulations observations, constrained total emission. show (1) top-down bottom-up estimates agreed well (bias < 6 %) on an annual basis, (2) generally reproduce observed diel seasonal variations, (3) played a much larger role than ecosystems controlling seasonality. When excluding respiration photosynthetic discrimination YRD area, seasonality increased 1.53 ‰ 1.66 ‰. (4) Atmospheric transport processes summer amplified cement enhancement proportions which monthly ?s (the mixture all end-members) These findings combination long-term observations provide powerful constraint cycle complex