Aerosols are tiny solid or liquid particles suspended in the air. They influence the climate by absorbing or scattering sunlight and serving as cloud condensation nuclei. Moreover, they can impact human well-being through adverse health effects of fine particulate matter.
A large fraction of particulate matter consists of nitrate, sulfate, and ammonium ions. The formation of these major aerosol components is strongly influenced by aerosol acidity, which varies widely between different regions with aerosol pH values ranging from ~1 to ~6. The drivers of these large variations, however, are not clear.
Researchers have now discovered how important the water content and total mass concentration of aerosol particles are for their acidity. A team led by Yafang Cheng and Hang Su from the Max Planck Institute for Chemistry discovered that these factors can be even more important than the dry particle composition. For populated continental areas with high anthropogenic emissions of ammonia from agriculture, traffic, and industry, they found that aerosol pH can be efficiently buffered and stabilized at different levels by the conjugate acid-base pair of ammonium ions and ammonia (NH4+/NH3).
The investigations now published in the interdisciplinary research journal `Science´ started with the question if and how the pH of aerosols is buffered in different continental regions. To address this issue, the scientists from Mainz developed a new theory of multiphase buffering in aerosols, analyzed atmospheric measurement data and performed global model simulations of aerosol composition and acidity.
“It turned out that the acid-base pair NH4+/NH3 is buffering the aerosol pH over most populated continental areas, even though the acidity may vary by multiple pH units,” says Yafang Cheng, Minerva Research Group leader at the Max Planck Institute for Chemistry.