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