AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Global Observations of Ammonium Balance and pH Indicate More Liquid Aerosol and Acidic Conditions than Current Models Predict
BENJAMIN A. NAULT, Pedro Campuzano-Jost, Douglas Day, Jason Schroder, Roya Bahreini, Huisheng Bian, Mian Chin, Simon Clegg, Peter Colarco, John Crounse, Jack Dibb, Michelle Kim, Jack Kodros, Felipe Lopez-Hilfiker, Eloise Marais, Ann M. Middlebrook, J. Andrew Neuman, John Nowak, Jeffrey R. Pierce, Eric Scheuer, Joel A. Thornton, Kostas Tsigaridis, Patrick Veres, Paul Wennberg, Jose-Luis Jimenez, CIRES, University of Colorado, Boulder
Abstract Number: 530 Working Group: Aerosol Chemistry
Abstract The inorganic composition of aerosol impacts numerous chemical and physical processes and properties. However, many chemical transport models show large variability in both the concentration of the inorganic aerosols and their precursors (up to 3 orders of magnitude differences) and the inorganic aerosol composition. Different models predict very different properties (e.g., aerosol liquid water concentration and aerosol acidity) and outcomes (e.g., heterogeneous uptake of gases or aerosols direct and indirect impacts on climate). Here, I use airborne observations from campaigns conducted around the world to investigate how the inorganic fine aerosol (PM1) composition, and one of its key parameters, aerosol acidity, changes from polluted regions (Mexico City, Los Angeles, Northeastern US, and Seoul) to remote ocean basins (the Atmospheric Tomography campaigns 1 and 2) in order to provide constraints for the chemical transport models. I find that the empirical ammonium balance with major ions (ammonium balance = mol NH4 / (2×mol SO4 + mol NO3)) rapidly decreases from 0.85 in polluted regions to less than 0.2 in remote regions, contradictory to some modeling studies that suggest most of the inorganic aerosol has a balance near 1. The data imply very low NH3 in the upper troposphere, contrary to predictions of some models, implying different physical properties than predicted in models. Comparison of 9 chemical transport models show large variability for the ammonium balance compared to observations and a general high bias in the ammonium balance, likely due to underestimation of sulfate aerosol. This would imply different chemical and physical properties in the models than observed. Next, I explore the aerosol acidity with the E-AIM model, constrained by the observations, and find that the acidity increases from the most polluted/urban (median pH = 2.3) to most remote regions (median pH = –0.5). The chemical transport models have difficulty reproducing the aerosol acidity, showing both over and underestimation in pH. Several causes likely lead to these measurement vs model differences in aerosol acidity, including the mixing state of sea salt (internal vs. external), total amount of NHx present in the atmosphere (NHx = NH3(g) + NH4+(p)), and an underestimation of sulfate.