AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Using Nitric Acid- Nitrate Partitioning and Aerosol Composition Data to Constrain Gas-phase Ammonia Levels and Aerosol pH
IFAYOYINSOLA IBIKUNLE, Rodney J. Weber, Athanasios Nenes, Georgia Institute of Technology
Abstract Number: 474 Working Group: Aerosol Chemistry
Abstract Atmospheric aerosol acidity drives multi-phase/ heterogeneous chemistry, gas phase oxidant concentrations, secondary aerosol formation and particle composition. Aerosol acidity also drives the gas/particle partitioning of semi-volatile species (e.g., gas-phase ammonia - particle ammonium) which constitute a large fraction of particulate matter. At present, gas/particle partitioning and aerosol acidity (pH) are calculated most effectively through thermodynamic equilibrium models, among which include ISORROPIA II. To achieve the most accurate equilibrium model predictions, a full suite of ambient observations comprised of temperature, relative humidity, and total (gas + particle) concentrations are required as model input. However, a complete data set is often times unavailable. In-situ measurements of ammonia are challenging to acquire, and infrequent, whereas gas phase nitric acid and fine particle nitrate are more frequently measured. We have developed an algorithm based on ISORROPIA II that extends its functionality to infer ammonia concentrations using observed nitrate/nitric acid partitioning as a constraint for conditions where the inorganic aerosol is dominated by nitrate, sulfate, and ammonium. Given that observational data are uncertain, and ambient data does not always fully follow thermodynamic equilibrium, sensitivity tests using synthetic data show that ammonia inferences are most reliable under conditions where relative humidity is between 55-85%, and nitrate partitioning fractions are between 0.2-0.8. Preliminary findings from a field campaign in Korea during May- June 2016 resulted in ammonia estimations between 2-10 μg m-3 (3-15 ppbv), which were deemed plausible based on CrIS satellite observations. Predicted pH primarily fell within the range of 1-3. Future work will focus on applying this new thermodynamic analysis to existing ambient datasets for development of thermodynamically-based inferences of ammonia concentrations, pH predictions, and ultimately, the state of atmospheric (aerosol) acidity.