Role of Carbon Dioxide, Ammonia, and Organic Acids in Buffering Atmospheric Acidity: The Distinct Contribution in Clouds and Aerosols.

Acidity is one central parameter in atmospheric multiphase reactions, influencing aerosol formation and its effects on climate, health, and ecosystems. Weak acids and bases, mainly CO 2 , NH 3 , and organic acids, are long considered to play a role in regulating atmospheric acidity. However, unlike strong acids and bases, their importance and influencing mechanisms in a given aerosol or cloud droplet system remain to be clarified. Here, we investigate this issue with new insights provided by recent advances in the field, in particular, the multiphase buffer theory. We show that, in general, aerosol acidity is primarily buffered by NH 3 , with a negligible contribution from CO 2 and a potential contribution from organic acids under certain conditions. For fogs, clouds, and rains, CO 2 , organic acids, and NH 3 may all provide certain buffering under higher pH levels (pH > ∼4). Despite the 10 4 to 10 7 lower abundance of NH 3 and organic weak acids, their buffering effect can still be comparable to that of CO 2 . This is because the cloud pH is at the very far end of the CO 2 multiphase buffering range. This Perspective highlights the need for more comprehensive field observations under different conditions and further studies in the interactions among organic acids, acidity, and cloud chemistry.