The Acidity of Atmospheric Particles and Clouds

Area of Science:

• Atmospheric chemistry
• Aqueous chemistry

Background:

• Acidity (pH) is vital in atmospheric aqueous chemistry, influencing gas partitioning and reaction rates.
• Particle acidity affects pollutant lifetime, deposition, and human health.
• Studies on particle acidity are growing, but direct measurements are lacking, requiring thermodynamic models for pH estimation.

Purpose of the Study:

• To review and synthesize current knowledge on atmospheric condensed phase acidity (particles, cloud, fog).
• To provide recommendations for acidity/pH estimation and standard nomenclature.
• To present new model calculations for local and global acidity.

Main Methods:

• Literature review and synthesis of existing studies.
• Thermodynamic model calculations for pH estimation.
• Analysis of historical and current observational data.

Main Results:

• Acidic fine particles are widespread, but observational pH data has limited spatial and temporal coverage.
• Cloud and fog droplets are generally acidic, with pH sensitive to sulfur/nitrogen oxides and ammonia emissions.
• Cloud/fog pH has shifted with emission controls; aerosol acidity appears stable due to buffering.

Conclusions:

• Atmospheric acidity is a key factor in atmospheric processes with significant environmental and health implications.
• Further research and improved measurement techniques are needed to better understand and constrain atmospheric acidity.
• This review provides a foundation for future research and policy decisions regarding air quality and climate.