Friday, September 15, 2017 -
12:00pm to 1:00pm
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Pierce 100F, 29 Oxford St., Cambridge

Atmospheric & Environmental Chemistry Seminar

"Constraining Aerosol Acidity and Its Impacts on Air Quality, Public Health and Ecosystem Nutrient Deposition" by Athanasios Nenes.

Knowledge of the levels and drivers of aerosol acidity (pH) is critical for understanding many aerosol processes that determine aerosol concentrations, chemical composition, toxicity and nutrient bioavailability. Aerosol acidity, however, remains poorly constrained because its direct measurement is currently not possible. This talk will focus on recent advances in constraining in-situ particle pH from the thermodynamic analysis of aerosol and gas-phase composition. Using this approach, we show that simple “pH proxies” (cation-anion molar ratios, ion balances) often used are too uncertain to be reliable indicators of aerosol acidity, with important implications for studies that heavily rely on their use. A summary thermodynamic analysis of comprehensive ambient observations then follows, revealing that acidic aerosol (with pH levels that range between 0 and 2) in the fine mode is a preferred state of aerosol. The strong acidity is caused by the large difference in volatility between sulfate (the main acidic compound, which resides completely in the aerosol phase), and ammonia (the main neutralizing agent, which partitions between aerosol and gas-phase). This counterintuitive, but thermodynamically consistent result, explains why aerosol acidity in the southeastern United States has not decreased over the past few decades, despite a 70% reduction in sulfates and a constant ammonia background, or, why high levels of NH3 in SE Asia cannot approach “neutral” pH levels (with important implications for the dominant sulfate production mechanism in the region). Strong acidity, however, can be reduced from the presence of non-volatile cations (NVC; Na, K, Ca, Mg from seasalt and mineral dust) to the point where nitrate aerosol formation is readily promoted, and, the solubility of trace metals (e.g., Fe, Cu) is highly affected. Because of this, incorrect treatment of NVC in models can bias pH enough to fundamentally alter model behavior (e.g., the expectation that nitrate aerosol will increase as sulfate levels drop may be incorrect for many regions of the globe). We conclude by emphasizing the need to evaluate models for their predicted pH, and, to revisit the conceptual model used by the aerosol community and policymakers to rationalize the drivers of aerosol acidity.  

Contact Name: 

Shaojie Song

Research Areas: 

Harvard University
Center for the Environment

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