code: def _to_DOM(self):
          
          root_node = ET.Element("no2index")
          reference_time_node = ET.SubElement(root_node, "reference_time")
          reference_time_node.text = str(self._reference_time)
          reception_time_node = ET.SubElement(root_node, "reception_time")
          reception_time_node.text = str(self._reception_time)
          interval_node = ET.SubElement(root_node, "interval")
          interval_node.text = str(self._interval)
          no2_samples_node = ET.SubElement(root_node, "no2_samples")
          for smpl in self._no2_samples:
              s = smpl.copy()
              # turn values to 12 decimal digits-formatted strings
              s['label'] = s['label']
              s['value'] = '{:.12e}'.format(s['value'])
              s['precision'] = '{:.12e}'.format(s['precision'])
              xmlutils.create_DOM_node_from_dict(s, "no2_sample",
                                                 no2_samples_node)
          root_node.append(self._location._to_DOM())
          return root_node
docs: Dumps object data to a fully traversable DOM representation of the
          object.
  
          :returns: a ``xml.etree.Element`` object