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. The atmospheric carbon dioxide (CO2) mixing ratio and its carbonisotope (δ13C-CO2) composition contain important CO2 sinkand source information spanning from ecosystem to global scales. Theobservation and simulation for both CO2 and δ13C-CO2 can beused to constrain regional emissions and better understand the anthropogenicand 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, fromSeptember 2013 to August 2015, and inverse modeling to constrain theCO2 budget and investigate the main factors that dominatedδ13C-CO2 variations for the Yangtze River delta (YRD) region, one of the largest anthropogenic CO2 hotspots and densely populated regionsin China. We used the WRF-STILT model framework with category-specifiedEDGAR v4.3.2 CO2 inventories to simulate hourly CO2 mixing ratiosand δ13C-CO2, evaluated these simulations with observations, andconstrained the total anthropogenic CO2 emission. We show that (1) top-down and bottom-up estimates of anthropogenic CO2 emissions agreed well (bias &lt; 6 %) on an annual basis, (2) the WRF-STILT model cangenerally reproduce the observed diel and seasonal atmosphericδ13C-CO2 variations, and (3) anthropogenic CO2 emissions played a much larger role than ecosystems in controlling the δ13C-CO2seasonality. When excluding ecosystem respiration and photosyntheticdiscrimination in the YRD area, δ13C-CO2 seasonality increasedfrom 1.53 ‰ to 1.66 ‰. (4) Atmospheric transport processes in summer amplified the cement CO2 enhancement proportions in the YRD area, which dominated monthly δs (themixture of δ13C-CO2 from all regional end-members)variations. These findings show that the combination of long-term atmospheric carbon isotope observations and inverse modeling can provide apowerful constraint on the carbon cycle of these complex megacities.

atmospheric chemistry and physics

Anthropogenic and natural controls on atmospheric &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C-CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; variations in the Yangtze River delta: insights from a carbon isotope modeling framework