Observations of Stratospheric Aerosols from Heliotrope Solar Hot Air Balloons

Abstract:

The injection of aerosols into the stratosphere, a form of solar climate intervention, has become increasingly popular as a method of reducing average global temperatures. Our understanding of the climate risks associated with aerosol injections are almost entirely model-based, though there are programs that propose to conduct real-world experimental assessments. Using Sandia’s 'heliotrope' solar hot air balloon system, we are developing and fielding an inexpensive and lightweight system that can be used to collect stratospheric measurements (aerosol, gas, and state variables) in order to establish background metrics for this under-sampled and difficult to measure region. The heliotrope envelope is composed of thin plastic and lined with charcoal dust. Instead of lifting gas, the heliotrope is filled with ambient air and lift is created when incoming solar radiation heats the envelope and induces positive buoyancy. The flight is terminated with the sunset. This balloon system has enabled our team to make repeat measurements of the stratosphere and test the performance of instrumentation not originally envisioned for use in a stratospheric environment. Throughout 2020, we launched heliotropes from New Mexico with payloads that included aerosol Optical Particle Counters (OPC), gas sample collection, and radiosondes that were originally intended for small UAVs and weather balloons. Fully configured, the OPC and radiosonde payload weighs less than 600 grams and can operate for the duration of the flight. Throughout these flights, we sampled a horizontal range of up to 340 km and a vertical extent of 23 km for over 10 hours. Profiles along the ascent and descent of the heliotrope reflect the different atmospheric conditions and environments along the flight path, sunrise versus sunset and mountainous-urban versus agricultural. At float, we observe consistently low concentrations of aerosols across all size bins. Two of these campaigns took place during the first wave of restrictions for COVID-19 in New Mexico. With continued monitoring, we should be able to assess whether the dramatic change in aerosol concentrations due to anthropogenic sources during COVID-19 restrictions impacted the stratospheric aerosol burden over the Southwestern United States.