Anthropogenic and natural controls on atmospheric δ 13 C-CO 2 variations in the Yangtze River Delta: Insights from a carbon isotope modeling framework

Abstract. The atmospheric CO2 mixing ratio and its δ13C-CO2 composition contain important CO2 sink and source information spanning from ecosystem to global scales. The observation and simulation for both CO2 and its carbon isotope ratio (δ13C-CO2) can be used to constrain regional emissions and better understand the anthropogenic and natural mechanisms that control δ13C-CO2 variations. Such work remains rare for urban environments, especially megacities. Here, we used near-continuous CO2 and δ13C-CO2 measurements, from September 2013 to August 2015, and inverse modeling to constrain the CO2 budget and investigate the main factors that dominated δ13C-CO2 variations for the Yangtze River Delta (YRD) region, one of the largest anthropogenic CO2 hotspots and densely populated regions in China. We used the WRF-STILT model framework with category-specified EDGAR v432 CO2 inventories to simulate hourly CO2 mixing ratios and δ13C-CO2, evaluated these simulations with observations, and constrained the anthropogenic CO2 emission categories. Our study shows that: (1) Top-down and bottom-up estimates of anthropogenic CO2 emissions agreed well (bias