Atmospheric Chemistry Program Seminar

Global Observations of Ammonium Balance and pH Indicate More Liquid Aerosol and Acidic Conditions than Current Models Predict by Benjamin Nault, CU ANYL Chem Postdoc, Jimenez group
"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 composition of the inorganic aerosols. Different models would predict very different properties such as aerosol liquid water concentration, aerosol acidity (but most models do not calculate this property), heterogeneous uptake of gases, aerosols direct and indirect impact on climate, et cetera. Here, I use airborne observations from campaigns conducted around the world to investigate how the inorganic composition, and one of its key parameters, aerosol acidity, changes from the polluted regions (Mexico City, Los Angeles, Northeastern US, and Seoul) to the most remote regions (the Atmospheric Tomography campaigns 1 and 2), to provide constraints for the chemical transport models. I find that the empirical ammonium balance (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 has a balance near 1. The data imply very low NH3 in the upper troposphere, contrary to predicts of some models. Real-world aerosols are less likely to be in the solid phase and more likely to be in a metastable liquid state. Next, I explore the aerosol acidity with the E- AIM model, constrained by observations, and find that the acidity increases from the most polluted (median = 2.3) to most remote regions (median = –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) and total amount of NH x present in the atmosphere (NH x = NH3 (g) + NH4 + (p)), which are currently being investigated and will be briefly discussed during this talk."
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Towards an improved representation of organic aerosol (OA) in the remote troposphere: Overall abundance, sources and physical and chemical removal by Pedro Campuzano Jost, CU ANYL Chem Research Scientist, Jimenez group
"Organic aerosol (OA) is one of the major contributors to the PM2.5 burden in the continental Northern Hemisphere (NH); understanding its sources and aging is central to current air quality control strategies. For the remote troposphere, sparse in-situ data to date results in highly under constrained OA prediction models, with model diversity of up to three orders of magnitude. As part of the recently concluded NASA Atmospheric Tomography (ATom) aircraft mission, we have acquired four unique global datasets of submicron aerosol concentration and composition over the remote Atlantic and Pacific Oceans. Overall, OA concentrations except for the cleanest regions were comparable to sulfate, as in the Northern Hemisphere, with OA, sulfate and seasalt being the main contributors to both CCN and submicron AOD. An evaluation of state-of-the-art models (CESM, GEOS-Chem) with ATom meteorology fields shows that while overall models reproduce remote OA concentrations fairly well, they mostly fail to reproduce the large ratio of secondary to primary observed in the measurements and use unrealistic OA/OC ratios for tracking OA. Improved model parameterizations that account for these factors overestimate OA in most remote regions, suggesting that an additional, slow loss channel for OA is needed. Based on a photochemical clock analysis of the Atom data, we find an OA lifetime of about 10 days for this process, consistent with recent estimations of the OA removal rate due to OH oxidation and photolysis."