Tracer transport studies

When stratospheric aerosol injection (SAI) simulations across different Earth system models produce different aerosol distributions and lifetimes, it is difficult to know why. Some of the disagreement stems from differences in how each model represents stratospheric circulation, and some stems from differences in how each model handles aerosol microphysics (the physical and chemical processes that govern particle formation and growth) and size evolution. Disentangling these two sources of uncertainty is essential for interpreting model spread (the range of differing results across an ensemble of models) and building confidence in SAI projections.

Passive tracers (particles that move with the air but do not grow, settle, or interact chemically) isolate the contribution of circulation by removing aerosol size as a variable. Comparing the spatial distribution of a passive tracer against that of sulfate aerosols (the particles that form once injected sulfur dioxide reacts in the stratosphere) in the same model directly quantifies how much of the resulting distribution is driven by transport alone versus size-dependent processes.

Extending this approach across multiple Earth system models and a range of forced aerosol sizes will produce a systematic characterization of how circulation uncertainty and aerosol size distribution uncertainty each contribute to the range of outcomes seen across SAI simulations.

A minimum viable study will run passive tracer injection in place of sulfate aerosols in a single Earth system model (CESM or UKESM) for two injection scenarios: a hemispherically balanced subtropical scenario (30°N/S at 21 km altitude, G6-1.5K SAI) and a high-latitude, low-altitude scenario (60°N/S at approximately 13–15 km altitude). Comparing the resulting tracer distributions against sulfate aerosol distributions from equivalent simulations will reveal how much of the aerosol distribution is determined by atmospheric transport alone versus by size-dependent processes such as settling and growth.

The extended analysis has two components intended to run concurrently across modeling centers. The first repeats the passive tracer analysis across all participating G6 (or G7-1.5K) models — E3SM, CESM, UKESM, MIROC, and any additional models available at the time — enabling a systematic intercomparison of circulation-driven transport uncertainty across the full ensemble. The second develops a method for testing specified aerosol size distributions (three to four discrete sizes up to 500 nm) within Earth system models, providing the first direct comparison of how aerosol lifetime varies with particle size across different models and filling a significant gap in the existing literature.