Whenever I blow air into the the Atmotube, the AQS number goes down. Why is that?

Because exhaled air contains VOCs.

The gas exhaled is 4% to 5% by volume of carbon dioxide, about a 100 fold increase over the inhaled amount. The volume of oxygen is reduced by a small amount, 4% to 5%, compared to the oxygen inhaled.

The typical composition is:

• 5.0–6.3% water vapor
• 74.4% nitrogen
• 13.6–16.0% oxygen
• 4.0–5.3% carbon dioxide
• 1% argon and several parts per million (ppm) of hydrogen and carbon monoxide, 1 ppm of ammonia and less than 1 ppm of acetone, methanol, ethanol and other volatile organic compounds

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