Uncovering the secrets of the stratosphere

To reduce that uncertainty, we need data: not only from satellites or surface stations, but detailed, in-situ measurements that map aerosol layers up into the stratosphere. However, research aircraft campaigns are costly, carbon-intensive and limited in both geographical and vertical reach, making it difficult to monitor the far reaches of the upper atmosphere where aerosols may have their greatest influence.

Helping to meet that challenge is the University of Hertfordshire’s Particle Instruments and Diagnostics Group, led by Professor Chris Stopford. The team develops advanced tools for detecting, monitoring and characterising airborne particles across atmospheric science, environmental monitoring, defence, and public health.

They have spent years advancing optical particle detection, a technique that measures how airborne particles scatter laser light to reveal their size, shape and concentration. That work led to a novel solution: the Universal Cloud and Aerosol Sounding System (UCASS).

“We started with a simple question,” says Professor Stopford. “Could we build a high-precision aerosol detector that was light enough and affordable enough to fly on a weather balloon? The answer turned out to be yes.”

Weighing just 130 grams, the UCASS is a featherweight, low-cost optical particle counter capable of precisely measuring aerosol concentrations and droplet sizes. Unlike traditional instruments with built-in airflow systems, UCASS has an open design that draws in ambient air, cutting weight and complexity.

Crucially, it’s the first particle counter specifically designed to function as a dropsonde: a compact sensor system released from aircraft or high-altitude balloons that uses a parachute to drift steadily through the atmosphere, collecting data on the way down. UCASS can also be used as an ‘upsonde’, rising with a balloon or drone to capture data on the way up.

Together, these modes enable high-resolution measurements of aerosol, ice crystal, and droplet concentrations at different altitudes, filling key data gaps that aircraft instruments alone cannot reach. These in-situ data are essential to validate the data received from satellites, which in turn are used extensively for forecasting climate and weather.

Now, UCASS is taking on a new role through a partnership funded by the UK’s Advanced Research and Invention Agency (ARIA). Its £56.8m Exploring Climate Cooling programme aims to rigorously assess whether any proposed climate cooling approaches could ever be feasible, scalable, or safe.

As part of this programme, UK aerospace startup Voltitude – founded by former Airbus engineers –is leading Project StratoGuard, an effort to develop lightweight, navigable micro-balloons capable of operating in the stratosphere for up to 30 days, gathering aerosol data across the globe. At the heart of their sensing system is the UCASS.

Voltitude’s vision is to monitor natural and human-made events that mimic geoengineering scenarios, like volcanic eruptions, aircraft contrails, or Saharan dust storms.

“There’s no substitute for decarbonisation,” says Professor Stopford, “but if society ever reaches a point where climate interventions are seriously considered, we must first understand the risks. The instruments and systems tested through this ARIA project could give us data to model those risks accurately.”

The collaboration reflects a recurring theme in the work of Professor Stopford’s group: turning fundamental research into practical impact. His group’s earlier optical particle counter technologies have already been integrated into thousands of air quality monitoring systems worldwide.

ARIA was created to back high-risk, high-reward science – research that may seem speculative today but could transform our understanding tomorrow.