In a large pot, add broth, onions, carrots, baking soda, salt, cinnamon, and pepper. Simmer uncovered for 10 to 15 minutes, until carrots are soft. Put vegetables and broth in a food processor or blender and blend until smooth (this step is optional). Return to pot. Add pumpkin and half and half (or evaporated milk). Simmer uncovered for 10 minutes. Garnish with sour cream.