Retrieving tropospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; vertical column densities around the city of Beijing and estimating NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; emissions based on carMAX-DOAS measurements