From Aerosols to Ancient Cities: Reflections from the 12th International Aerosol Conference in Xi’an, China
This summer, I had the opportunity to travel to Xi'an, China to attend the Twelfth International Aerosol Conference, one of the major international gatherings for researchers working across aerosol science and atmospheric processes.
For me, the conference was an exciting opportunity to present two aspects of our work at the Centre for Climate Repair: my research into potential pathways for scaling Marine Cloud Brightening (MCB) and the Centre's wider work on developing seawater aerosol generation technologies. It was also a chance to meet researchers from across the world working on everything from atmospheric pollution and aerosol-cloud interactions to particle measurements, modelling and climate intervention.
But the trip was about much more than the conference itself.
Getting to Xi'an
I travelled to Xi'an from Beijing by high-speed rail. The journey covered around 1,200 kilometres in just over four hours, with trains reaching speeds of up to 350 km/h.
The experience was remarkably comfortable and convenient. Rather than flying between two major cities, I could simply walk onto a train in central Beijing and arrive in central Xi'an a few hours later. The affordability and extensive coverage of China's high-speed rail network make it an impressive example of how electrified transport can provide a practical alternative to domestic air travel.
China now has by far the world's largest high-speed rail network, with more than 45,000 kilometres of high-speed railway in operation. The rapid electrification of transport more broadly, including the widespread adoption of electric vehicles, was also striking throughout my trip.
From Xi'an station, I took the metro to the conference centre for just 77 yuan, around 77 pence.
It was a good introduction to a city where moving around without a car was remarkably easy.
Aerosols, climate and a call to action
The conference opened with a plenary lecture from James Hansen, whose 1988 testimony before the US Congress played a pivotal role in bringing the risks of human-caused global warming to public attention.
His talk provided a sobering reminder of just how important aerosols are to our understanding of the climate systems.
Aerosols influence the climate directly by scattering and absorbing sunlight, but they also interact with clouds in complex ways. These processes remain an important source of uncertainty in our understanding of the climate system and therefore in projections of future warming.
For me, it was a powerful reminder that even as we work towards potential climate interventions, there remains an enormous amount we need to understand about the fundamental physics and chemistry of the atmosphere.
It was also a call to action for the aerosol community: to continue improving our understanding of these processes and to ensure that the knowledge we develop ultimately contributes to better decisions for society.
Over the week, the conference covered an enormous range of topics, including atmospheric aerosols and pollution, aerosol measurement technologies, particle formation, modelling methodologies and aerosol-cloud interactions.
There was also a dedicated session on Solar Radiation Modification (SRM), chaired by our collaborator Professor Zoran Ristovski from Queensland University of Technology in Australia.
Yashasvi Raj with Professor Zoran Ristovski
Much of the SRM session focused on Stratospheric Aerosol Injection (SAI) and associated modelling studies. I noticed that relatively few researchers in the room were familiar with Marine Cloud Brightening, which made the session an especially valuable opportunity to introduce the work being carried out at the Centre for Climate Repair.
What is Marine Cloud Brightening?
Our PhD research at the Centre investigates the engineering feasibility and scalability of Marine Cloud Brightening as a potential regional climate intervention.
The basic concept is relatively simple: increase the reflectivity, or albedo, of existing low marine clouds by increasing the number of cloud condensation nuclei (CCN) available in the marine boundary layer.
Our approach is to atomise seawater into vast numbers of submicron droplets. As the droplets evaporate, they leave behind tiny sea-salt particles. If these particles are transported into suitable cloud layers, they can act as additional CCN, potentially increasing the number of cloud droplets and therefore the optical thickness and reflectivity of the cloud.
The scale required is enormous. We are ultimately interested in systems capable of producing on the order of 1015 aerosol particles per second, while achieving an appropriate particle-size distribution.
That immediately turns MCB into an engineering challenge.
The question is not simply can we produce sea-water aerosol? It is whether we can produce the right particles, at an enormous rate, continuously, reliably and with sufficiently low energy consumption to make deployment practical.
A real world system would also need to operate in a harsh marine environment, where corrosion, nozzle fouling and blockage present significant engineering challenges.
These constraints make conventional atomisation technologies difficult to scale and motivate us to investigate alternative approaches.
Taking MCB beyond the laboratory
My presentation focused on one of the questions that has increasingly shaped my PhD research: how could we scale MCB deployment beyond individual laboratory-scale aerosol generators?
In particular, I presented my work investigating air lubrication systems (ALS) as a potential platform for large-scale aerosol generation.
Air lubrication is already being explored by the maritime industry as a way of reducing hydrodynamic drag. Large quantities of air are injected beneath ships to create a layer of bubbles along the hull, reducing frictional resistance and potentially lowering fuel consumption.
This raises an interesting question: could technologies already being deployed across the maritime sector provide part of the infrastructure needed for future MCB systems?
Rather than designing an entirely new fleet of specialised vessels, could we potentially integrate aerosol generation with existing maritime technologies?
This is still an open research question, but exploring these kinds of deployment pathways is becoming increasingly important as we consider how laboratory-style climate engineering concepts could ever be translated into meaningful real-world systems.
The poster generated some particularly interesting conversations about the broader question of where climate interventions might be deployed and why.
One question that stayed with me concerned the difference in economic incentives between climate-vulnerable regions. Places such as the Great Barrier Reef have enormous tourism and ecological value, creating strong economic incentives to protect them from climate impacts. But how do we protect regions that may face equally severe climate risks without the same economic resources or incentives?
That is not simply an engineering question. It is a question about equity, governance and who gets to benefit from climate intervention technologies.
Building new collaborations
One of the most rewarding parts of the conference was the opportunity to connect with researchers outside the immediate MCB community.
My presentation led to conversations about potential collaborations with researchers at institutions including the University of Venice and Stockholm University. I also had the unexpected pleasure of running into friends and colleagues I had met previously at the 22nd International Conference on Nucleation and Atmospheric Aerosols in Vienna.
The conference exhibition was equally fascinating. Dozens of companies showcased technologies for aerosol classification, particle sizing and sampling across applications ranging from air pollution and black carbon to viral transmission and sea-spray aerosols.
It was striking to see how many different scientific questions ultimately depend on similar fundamental technologies.
Whether the particle being measured is a pollutant, a biological aerosol, a sea-spray particle or a potential climate intervention aerosol, researchers often face the same fundamental questions: What particles are being produced? How large are they? How many are there? Where do they go? And how do we measure them accurately?
Seeing this breadth of applications helped put our own work into a much wider scientific context.
Exploring Xi'an
Of course, I couldn't spend a week in Xi'an without exploring the city.
One of my favourite ways to get around was by bicycle. Xi'an has an extensive network of inexpensive rental bikes and I could cycle around the city for less than 1 yuan per journey.
The dedicated cycling infrastructure was particularly impressive. Many cycleways were physically separated from traffic, sometimes with trees or barriers between bicycles and cars. Despite noticing that helmet use was relatively uncommon, I felt remarkably safe cycling around the city.
It was also a fantastic way to experience Xi'an at street level.
Xi'an has played a hugely important role in Chinese history and was the eastern terminus of the ancient Silk Road. The city's history as a crossroads of cultures is still visible today, particularly in the Muslim Quarter.
I spent an evening wandering through the markets and sampling local food, including roujiamo, a Chinese-style meat-filled flatbread 'hamburger', as well as a variety of traditional kebabs, fresh-pressed pomegranate juice and other street food.
One evening, I also accidentally stumbled upon an enormous water and light show in front of the Giant Wild Goose Pagoda. I had no idea what I had walked into but the scale of the spectacle was incredible.
The conference excursions provided another opportunity to experience the history and natural heritage of the region. We visited the world-famous Terracotta Warriors, as well as a wildlife sanctuary showcasing four species associated with the Qinling Mountains: the takin, giant panda, crested ibis and golden snub-nosed monkey.
It was a remarkable contrast to spend the morning discussing aerosol physics and climate intervention and the afternoon standing among thousands of ancient terracotta soldiers.
Leaving with new ideas
The Twelfth International Aerosol Conference left me with a renewed appreciation for just how broad the field of aerosol science really is.
Marine Cloud Brightening sits at the intersection of atmospheric physics, fluid mechanics, aerosol science, engineering, climate science and, increasingly, questions around deployment and governance. Conferences like this provide an opportunity to step outside that narrow intersection and see how our challenges relate to much wider scientific communities.
Perhaps most importantly, I left Xi'an feeling hopeful.
I was incredibly grateful for the welcome I received throughout my time in China. People I had never met before were remarkably generous, often offering small gifts, helping me navigate unfamiliar places and patiently communicating with me despite my very limited ability to speak Chinese.
The warmth and generosity I experienced made the trip particularly memorable.
I returned to Cambridge with new research ideas, potential collaborations and a reinvigorated sense of purpose as I work towards completing my PhD. There are still enormous scientific and engineering challenges ahead but conversations at conferences like this reinforce how much can be achieved when researchers from different disciplines and countries come together.
I am very grateful to the Centre for Climate Repair for supporting my attendance at the conference. I hope the connections and ideas developed in Xi'an will help us push our research forward and contribute, in however small a way, to advancing our understanding of how aerosol science and engineering might help address some of the most difficult challenges posed by climate change.
And hopefully, it won't be my last trip to China!