Develop plume-tracking instruments

Tracking a small stratospheric plume over weeks requires more than meteorological forecasting alone. Weather forecast models inevitably drift — they accumulate errors over time and become unreliable beyond a few days, especially at the small scales being considered for an aerosol microphysics experiment. Direct observations of the plume are essential to correct these forecast errors and keep tracking accurate over the full 2–4 weeks of the aerosol microphysics experiment.

However, tracking presents two distinct technical challenges that emerge over the plume's lifetime. Soon after any release, SO₂ concentrations will be high enough to detect using satellites like TROPOMI (a satellite-borne atmospheric monitoring instrument) or balloon-based ultraviolet (UV) cameras — instruments that measure the UV light absorbed by SO₂ gas. As the plume becomes more dilute, the concentration will be too low to track easily through this approach. Researchers must then use different methods and instruments, such as a Differential Optical Absorption Spectrometer (DOAS), that can detect trace gases such as SO₂ (Davis and McLaren, 2020). Additionally, the mini-SASP (Scanning Aerosol Sun Photometer) offers a useful starting point for instrumentation that could track a plume after the SO₂ converts into aerosol particles (Murphy et al, 2015). 

These instruments have not been tested or validated specifically for tracking stratospheric plumes. UV cameras with appropriate filters and lightweight aerosol instruments exist, but they have not been deployed together on balloon or aircraft platforms, nor tested in the stratospheric environment where the plume will evolve. Before committing to an aerosol microphysics experiment, researchers should validate these instruments under realistic conditions.

Field testing at natural SO₂ sources — active volcanoes or coal plants without sulfur-scrubbing equipment — allows researchers to validate that the instruments can detect and track real plumes under operational conditions; instrument sensitivity at the much lower concentrations expected in Phase 1 will be characterized separately through laboratory testing and signal-to-noise modeling. Additionally, ground environmental testing will ensure the instruments can handle expected vibrational and thermal loads without damage or noise.

This activity encompasses instrument development, signal characterization, and field validation for at least two instruments to track the evolving plume throughout an aerosol microphysics experiment. The first series of tests validates the ability to track an SO₂ plume; the second validates that the instrumentation can withstand stratospheric flight conditions. Atmospheric and microphysical model simulations will provide realistic plume scenarios to inform instrument configuration requirements and establish target detection thresholds.

Researchers will modify existing UV cameras for stratospheric SO₂ detection by selecting filters that isolate SO₂ absorption while rejecting interference from ozone, which also absorbs ultraviolet light. Similarly, the mini-SASP or mini-DOAS may require modification to optimize detection at SO₂ absorption wavelengths. Signal-to-noise modeling and lab testing with a gas cell will characterize the detection sensitivity and spatial resolution achievable by any given instrument.

Where useful, these instruments will be field-tested near or over a natural SO₂ source, such as an active volcano or unscrubbed coal power plant. Systems that prove promising will be integrated into a coordinated instrument suite compatible with balloon or aircraft deployment. The integrated suite will then undergo ground environmental testing, including vibration table and thermal vacuum chamber testing in flight configurations. Based on instrumentation specifications and validation test results, researchers will develop a detailed measurement plan specifying instrument configuration, sampling cadence, platform requirements, and deployment procedures for plume tracking in an aerosol microphysics experiment.