PNNL

The Science                                 

Sulfur dioxide (SO₂) is a key air pollutant responsible for acid rain and human health impacts, and has a substantial impact on climate change through the formation of cooling aerosols. Earth system and chemistry models are important tools for studying the impact of such emissions on humans and the Earth. However, these models do not consistently or accurately define some key emission characteristics. This study found that model assumptions about the effective emission height—the height at which SO₂ emission plumes disperse into the atmosphere—can have a large impact on model results, including surface SO₂ concentrations and the amount of sunlight reflected into space.

The Impact

Assumptions on emission height are not standardized across models and can have a significant impact on model results. These assumptions are a "hidden” source of inter-model variability and may be leading to bias in some climate model results, particularly in radiative forcing responses, surface gas concentrations, and aerosol dynamics. This points to a need for clear reporting of model assumptions, standardized guidance on how to incorporate emission height into models, and better regional and global data, particularly for stack height and emission thermal plume rise. The study also found that the fraction of SO₂ converted to sulfate within emission plumes, another uncertain characteristic, also had an impact on model results.

Emissions height also serves as a novel diagnostic to probe model responses to fundamental changes in emission characteristics. This work found that models varied substantially in their response to emissions higher in the atmosphere, which indicates a need for improved fundamental understanding of SO₂ chemistry, transformation, and transport within the atmosphere.

Summary

Earth system models are essential tools for studying the effects of human-made emissions on the Earth–atmosphere system. While overall magnitudes of emissions will affect model results, it is not well quantified how sensitive models are to other aspects of emissions data. The Emissions Model Intercomparison Project, coordinated by researchers at Pacific Northwest National Laboratory with contributions from modeling teams around the world, examined how selected emissions-related properties affected results across 11 global chemistry and Earth-system models, including emission seasonality, emission height, and assumed fraction of SO₂ emitted as sulfate aerosol.

The work found that the assumed height of SO₂ injection had the largest overall impact, particularly on global mean net radiative flux, the balance between incoming and outgoing energy at the top of the atmosphere (up to 0.35 W m⁻² of additional cooling), and the concentration of SO₂ at the Earth’s surface (up to 59% reduction). These findings point to a need for better information on emission height, which includes both smokestack height and subsequent thermal plume rise. While regional models often include plume-rise parametrizations, this is not the case in global models.

Resources at the National Energy Research Scientific Computing Center, a Department of Energy (DOE), Office of Science user facility, were used to perform model runs from DOE’s Energy Exascale Earth System Model (E3SM) and the Community Earth System Model.

Contact

Hamza Ahsan, Pacific Northwest National Laboratory, hamza.ahsan@pnnl.gov, corresponding author (primary contact); Steven J Smith, Pacific Northwest National Laboratory, ssmith@pnnl.gov, project lead

Funding

This work was funded by the DOE Office of Science, Biological and Environmental Research program, Earth and Environmental Systems Modeling.