Detecting climate change using aerosols

Researchers analyzed long-term aerosol satellite observation big data focusing on the Pacific Ocean downwind of China. Using a newly developed metric that considered aerosols as tracers, they detected altered atmospheric transport patterns associated with climate change. They observed that the distance of transboundary air pollution moving east from China had shortened. Thus, long-term satellite-based Earth observations are crucial for early climate change detection and accurate evaluation of this trend.

Climate change is one of the most significant environmental challenges of present times, leading to extreme weather events, including droughts, forest fires, and floods. The primary driver for climate change is the release of greenhouse gases into the atmosphere due to human activities, which trap heat and raise Earth's temperature. Aerosols (such as particulate matter, PM2.5) not only affect public health but also influence the Earth's climate by absorbing and scattering sunlight and altering cloud properties. Although future climate change predictions are being reported, it is possible that the impacts of climate change could be more severe than predicted. Therefore, it is necessary to detect climate change accurately and as early as possible.

Building on these insights, a research team from Japan, led by Professor Hitoshi Irie from the Center for Environmental Remote Sensing at Chiba University, utilized long-term observational data to study the effect of climate change on transboundary air pollution in the downwind area of China by using aerosols. They utilized a completely unique perspective on how aerosols impact climate and developed a new metric to detect climate change by considering aerosols as tracers.

"The significance of this study lies in the fact that most of its results are derived from observational data. In natural sciences focused on Earth studies, the ultimate goal is to piece together highly accurate data obtained from observations to quantitatively understand the processes occurring on Earth and to pursue immutable truths. Therefore, the more observational data we have, the better. With the continued Earth observations by Japan's major Earth observation satellites (such as the GCOM series, GOSAT series, Himawari series, and ALOS series), we aim to complement these efforts with numerical simulations and data science methodologies to achieve a safe and secure global environment that mitigates the impacts of the climate crisis." explains Prof. Irie.

The research team included Ms. Ying Cai from the Graduate School of Science and Engineering, Chiba University, Dr. Alessandro Damiani from the Center for Climate Change Adaptation, National Institute for Environmental Studies, Dr. Syuichi Itahashi and Professor Toshihiko Takemura from the Research Institute for Applied Mechanics, Kyushu University, and Dr. Pradeep Khatri from Faculty of Science and Engineering, Soka University. Their study was made available online on May 23, 2024, and published in Science of The Total Environment on August 20, 2024.

China is a major contributor to air pollution in East Asia. The downwind area of China analyzed in this study is a unique open ocean area with minimal human interference yet an important zone of transboundary air pollution pathways, making it an ideal location for studying meteorological variations due to climate change.

In their study, the researchers analyzed aerosol optical depth (AOD) datasets derived from satellites, reanalysis datasets, and numerical simulations focused on the Pacific Ocean in the downwind area of China, over 19 years from 2003 to 2021. AOD, a measure of the amount of sunlight blocked by aerosols, is a key factor is analyzing aerosols and their impact on climate change.

The researchers developed a new metric called RAOD which utilized the potential of aerosols as tracers to evaluate the impact of climate change on transboundary air pollution pathways. Using RAOD the researchers were able to quantify significant temporal variations in aerosol transport. They discovered that long-term changes in RAOD due to climate change were outweighed by larger year-to-year variations in the meteorological field. Moreover, seasonal trends showed that aerosols moved west to east during spring and winter, and northward in summer. They concluded that the probability of aerosols from China to be transported far eastward was low, highlighting a shift in transboundary pollution pathways due to global warming. In this study the authors successfully detected climate change using long-term satellite observational data, in contrast to most existing studies that tracked transboundary air pollution using model simulations.

"These results suggest that RAOD is a valuable metric for quantifying the long-term changes in transboundary air pollution pathways due to climate change. These results are particularly significant because most of them are derived from observational data," says Prof. Irie, highlighting the importance of the study. Sharing the future implications of their study he concludes, "The effects of climate change could be more severe than currently predicted. This study will help verify climate change predictions from an unconventional perspective of 'aerosol observation,' enabling a more accurate understanding of climate change progression and implementation of rational countermeasures."

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Air pollution hides increases in rainfall

We know that greenhouse gas emissions like carbon dioxide should increase rainfall. The emissions heat the atmosphere, causing a one-two punch: warmer oceans make it easier for water to evaporate, and warmer air can hold more water vapor, meaning more moisture is available to fall as rain. But for much of the 20th century, that increase in precipitation didn't clearly show up in the data.

A new study led by researchers at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) finds that the expected increase in rain has been largely offset by the drying effect of aerosols -- emissions like sulfur dioxide that are produced by burning fossil fuels, and commonly thought of as air pollution or smog.

The research is published today in the journal Nature Communications.

"This is the first time that we can really understand what's causing extreme rainfall to change within the continental U.S.," said Mark Risser, a research scientist at Berkeley Lab and one of the lead authors for the study.

He noted that until the 1970s, the expected increases to extreme rainfall were offset by aerosols.

But the Clean Air Act caused a drastic reduction in air pollution in the United States.

"The aerosol masking was turned off quite suddenly. That means rainfall might ramp up much more quickly than we would have otherwise predicted."

Traditional climate models have struggled to confidently predict the human impact on rainfall at scales smaller than a continent -- and that regional level is precisely where most climate change adaptations and mitigations take place.

By using a new method and relying heavily on measurements from rain gauges from 1900 to 2020, researchers were able to more robustly determine how human activities have influenced rainfall in the United States.

"Prior to our study, the Intergovernmental Panel on Climate Change [IPCC] had concluded that the evidence was mixed and inconclusive for changes in U.S. precipitation due to global warming," said Bill Collins, associate laboratory director for the Earth and Environmental Sciences Area at Berkeley Lab and co-lead author on the study.

"We have now provided conclusive evidence for higher rainfall and also helped explain why past studies assessed by the IPCC reached conflicting conclusions."

Specifically, the study isolates how greenhouse gas and aerosol emissions affect both average and extreme rainfall.

Researchers confirmed that increased greenhouse gas emissions, which quickly disperse over the whole planet, cause an increase in rainfall.

The impact from aerosols is more nuanced. Over the long term, aerosols cool the planet, which causes a drying effect.

But they also have a faster, more local response. That fast impact depends on the season, with aerosols generally reducing rainfall in the winter and spring, and amplifying it in summer and fall over much of the United States.

"The seasonality piece is really important," Risser said. "For rainfall, the nature of climate change depends on what season you're talking about, since different kinds of weather systems create precipitation in different parts of the year."

Some of the conflicting studies looking at precipitation trends of the last century can be explained by how the effect of aerosols offsets the effect of greenhouse gases, and how models and simulations factor in these two driving forces.

The researchers noted that tracking aerosols and incorporating them more fully into models and simulations will be important for improving the predictions used for infrastructure design and water resource management.

The United States has already seen examples of recent increases in extreme precipitation, with several intense, record-setting storms in the past few years.

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Blowing snow contributes to Arctic warming

When it comes to global warming trends, the Arctic is a troubling outlier. The Arctic warms nearly four times faster than the global average, and aerosols play an important role in that warming. Scientists have long known that pollutants from other regions can accumulate in the Arctic atmosphere where they alter atmospheric chemistry, absorb sunlight, and affect local weather patterns, leading to localized warming that melts ice and snow. Sea salt particles dominate aerosol mass concentration, but their production mechanisms and impact on Arctic climate have remained unclear.

Atmospheric scientists led by Jian Wang, director of the Center for Aerosol Science and Engineering (CASE) and professor of energy, environmental & chemical engineering in the McKelvey School of Engineering at Washington University in St. Louis, investigated the production and impact of sea salt aerosols on Arctic warming. Their results, published Sept. 4 in Nature Geoscience, revealed abundant fine sea salt aerosol production from blowing snow in the central Arctic, increasing particle concentration and cloud formation.

"Over the past few decades, scientists have identified 'Arctic haze' as the primary source of aerosols in the Arctic during winter and spring. This haze results from the long-range transport of pollutants," said Xianda Gong, first author on the study and a former postdoctoral researcher in Wang's lab. "However, our study reveals that local blowing snow, which produces sea salt particles, contributes a more substantial fraction to the total aerosol population in the central Arctic."

Wang's team analyzed data collected by the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC). Such observations are difficult to obtain -- the MOSAiC expedition entailed international collaboration and freezing an icebreaker into the central Arctic ice pack to drift with the sea ice for an entire year -- but essential to understanding the full picture of atmospheric conditions in the Arctic.

"The MOSAiC expedition let us observe how aerosols and clouds evolve over the course of a year and led to this discovery," Wang said. "Sea salt particles in the Arctic atmosphere aren't surprising, since there are ocean waves breaking that will generate sea salt aerosols. But we expect those particles from the ocean to be pretty large and not very abundant. We found sea salt particles that were much smaller and in higher concentration than expected when there was blowing snow under strong wind conditions," Wang said.

In the central Arctic, the coldest winter nights are the clearest, when heat from Earth can escape into space unimpeded. Under a cozy blanket of clouds, though, longwave radiation gets trapped and contributes to warming, so any process that leads to increased cloud formation and lingering cloudiness also boosts surface temperatures. Small aerosol particles, including those fine sea salt aerosols produced by blowing snow that Wang's team discovered, turn out to be very good for cloud formation.

"These sea salt particles can act as cloud condensation nuclei, leading to cloud formation," Gong said. "Considering the absence of sunlight in the winter and spring Arctic, these clouds have the capacity to trap surface long-wave radiation, thereby significantly warming the Arctic surface."

Though scientists had not observed this phenomenon before, fine sea salt aerosols from blowing snow have always been part of the Arctic climate system. With this observational confirmation and systematic study, which revealed that sea salt particles produced from blowing snow account for about 30% of total aerosol particles, climate models can now be updated to include the effects of these fine particles.

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Hot chemistry quickly transforms aromatic molecules into harmful aerosols

Joint research groups at Tampere University, University of Helsinki, Lund University and Pi-Numerics, Salzburg, have established key early steps in the conversion of aromatic molecules, a major constituent of traffic and other urban volatile emissions, into aerosol. Their findings increase understanding of the chemical processes that degrade urban air quality and influence climate change.

Many aromatic molecules are carcinogenic and have negative impacts on health. Their primary source is exhaust fumes from motor vehicles. Aromatics can form aerosol particles when they collide in the atmosphere with the hydroxyl radical, a molecule colloquially dubbed "atmospheric detergent" due to its acute propensity to react chemically. When breathed in, aerosol particles can lead to a myriad of chronic health issues and even death. These particles also affect Earth's climate by reflecting sun light and increasing the formation of clouds.

Despite their importance to the urban environment, details of the reaction processes that form aerosol from aromatics have until now remained unresolved.

The group of researchers used a combination of quantum mechanics, targeted experiments, and modeling, to establish the early steps in the reaction process of toluene, one of the most abundant aromatic molecules.

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Cloud study demystifies impact of aerosols

Aerosol particles in the atmosphere have a bigger impact on cloud cover -- but less effect on cloud brightness -- than previously thought, new research shows.

Aerosols are tiny particles suspended in the atmosphere, and they play a key role in the formation of clouds.

With aerosols increasing due to human activities, numerous assessments by the Intergovernmental Panel on Climate Change (IPCC) have suggested they could have an important impact on climate change because clouds reflect sunlight and therefore keep temperatures cooler.

However, this cooling impact of aerosols on clouds is difficult to measure, and this has led to significant uncertainty climate change projections.

The new study -- led by the University of Exeter, with national and international academic partners and the UK's Met Office -- used the 2014 Icelandic volcano eruption to investigate this.

"This massive aerosol plume in an otherwise near-pristine environment provided an ideal natural experiment to quantify cloud responses to aerosol changes, namely the aerosol's fingerprint on clouds" said lead author Dr Ying Chen.

"Our analysis shows that aerosols from the eruption increased cloud cover by approximately 10%.

"Based on these findings, we can see that more than 60% of the climate cooling effect of cloud-aerosol interactions is caused by increased cloud cover.

"Volcanic aerosols also brightened clouds by reducing water droplet size, but this had a significantly smaller impact than cloud-cover changes in reflecting solar radiation."

Previous models and observations suggested this brightening accounted for the majority of the cooling caused by cloud-aerosol interactions.

Water droplets usually form in the atmosphere around aerosol particles, so a higher concentration of these particles makes it easier for cloud droplets to form.

However, as these cloud droplets are smaller and more numerous, the resulting clouds can hold more water before rainfall occurs -- so, more aerosols in the atmosphere can lead to more cloud cover but less rain.

The study used satellite data and computer learning to study cloud cover and brightness.

It used 20 years of satellite cloud images from two different satellite platforms from the region to compare the periods before and after the volcano eruption.

The findings will provide observational evidence of aerosols' climate impacts to improve the models used by scientists to predict climate change.

Jim Haywood, Professor of Atmospheric Science at the University of Exeter and part of the Global Systems Institute, and a Met Office Research Fellow, said: "Our earlier work had showed that model simulations could be used to disentangle the relative contribution of aerosol-cloud-climate impacts and potentially confounding meteorological variability.

"This work is radically different as it does not rely on models; it uses state-of-the-art machine learning techniques applied to satellite observations to simulate what the cloud would look like in the absence of the aerosols.

"Clear differences are observed between the predicted and observed cloud properties which can be used to assess aerosol-cloud-climate impacts."

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Older wildfire smoke plumes can affect climate

Aerosols carried in wildfire smoke plumes that are hundreds of hours old can still affect climate, according to a study out of the University of California, Davis.

The research, published in the journal Environmental Science and Technology, suggests that wildfire emissions even 10 days old can affect the properties of aerosols -- suspended liquid or particles that are key to cloud formation.

Research in aerosols and particulate matter pollution related to wildfires has most often focused on the early hours of smoke plumes, not several days later after smoke has traveled to other areas.

Enhancing modeling

This research helps fill in a knowledge gap and can inform future predictions about the climate and atmospheric effects of wildfire over the lifetime of aerosols, particularly in rural or pristine areas with relatively clean air, said Qi Zhang, an environmental toxicology professor and lead author of the study.

"These parameters are really useful for atmospheric and chemical models," she said. "It's a really important component to solving the effects on climate. To capture those characteristics is super critical."

Zhang, Ph.D. student Ryan Farley and others spent time in 2019 at the Mount Bachelor Observatory atop a volcanic mountain in Oregon. That year was relatively calm in terms of wildfire, but smoke plumes and aerosols were still observed. Some were at least 10 days old and came from as close as Northern California and as far as Siberia, Russia.

The properties and chemical composition of aerosols can do a number of things: scatter or absorb solar radiation affecting temperature, seed clouds to produce rain or snow, or change the reflectivity of clouds -- all of which affect climate.

Aerosol properties change with age

Scientists found that particulate matter concentrations were low, but oxidized organic aerosols from burning biomass -- such as trees, grasses and shrubs -- were detected throughout the samples.

The aerosols, which have a life cycle of about two weeks, were larger in aged samples compared to those found shortly after a fire starts.

"The properties of the smoke determine the effects on the climate," Zhang said. "The really aged aerosols can behave very differently than the fresh ones. You want to capture these aerosols over the lifetime to properly account for the effects."

Aerosols in the background

Older aerosols produced by wildfires can be present but not obvious and still affect climate.

"It's not something you just notice but it's in the background," she said.

Knowing that information becomes ever more important as "biomass burning has become more and more frequent," Zhang said.

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New geochemical study confirms cause of end-Permian mass extinction event

The most severe mass extinction event in the past 540 million years eliminated more than 90 percent of Earth's marine species and 75 percent of terrestrial species. Although scientists had previously hypothesized that the end-Permian mass extinction, which took place 251 million years ago, was triggered by voluminous volcanic eruptions in a region of what is now Siberia, they were not able to explain the mechanism by which the eruptions resulted in the extinction of so many different species, both in the oceans and on land.

Associate professor Laura Wasylenki of Northern Arizona University's School of Earth and Sustainability and Department of Chemistry and Biochemistry is co-author on a new paper in Nature Communications entitled, "Nickel isotopes link Siberian Traps aerosol particles to the end-Permian mass extinction," in collaboration with Chinese, Canadian and Swiss scientists. The paper presents the results of nickel isotope analyses performed in Wasylenki's lab on Late Permian sedimentary rocks collected in Arctic Canada. The samples have the lightest nickel isotope ratios ever measured in sedimentary rocks, and the only plausible explanation is that the nickel was sourced from the volcanic terrain, very likely carried by aerosol particles and deposited in the ocean, where it dramatically changed the chemistry of seawater and severely disrupted the marine ecosystem.

"The study results provide strong evidence that nickel-rich particles were aerosolized and dispersed widely, both through the atmosphere and into the ocean," Wasylenki said. "Nickel is an essential trace metal for many organisms, but an increase in nickel abundance would have driven an unusual surge in productivity of methanogens, microorganisms that produce methane gas. Increased methane would have been tremendously harmful to all oxygen-dependent life."

"Our data provide a direct link between global dispersion of Ni-rich aerosols, ocean chemistry changes and the mass extinction event," Wasylenki said. "The data also demonstrate that environmental degradation likely began well before the extinction event -- perhaps starting as early as 300,000 years before then. Prior to this study, the connection between Siberian Traps flood basalt volcanism, marine anoxia and mass extinction was rather vague, but now we have evidence of a specific kill mechanism. This finding demonstrates the power of nickel isotope analyses, which are relatively new, to solve long-standing problems in the geosciences."

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Californian smoke drifted as far as Europe in 2020 and caused heavy clouding of sun

The smoke from the extreme forest fires on the US West Coast in September 2020 travelled over many thousands of kilometres to Central Europe, where it continued to affect the atmosphere for days afterwards. A comparison of ground and satellite measurements now shows: The forest fire aerosol disturbed the free troposphere over Leipzig in Germany as never before. An evaluation by an international research team led by the Leibniz Institute for Tropospheric Research (TROPOS) revealed an extraordinary optical thickness on 11 September 2020, which attenuated sunlight by a third. The study, published in Geophysical Research Letters, is the first publication to show that ESA's novel Aeolus satellite can not only reliably measure global wind profiles but also aerosols in the atmosphere as it was shown by comparing Aeolus measurements with lidar measurements from the ground. The Centre National de Recherches Météorologiques (CNRM) of the University of Toulouse, the German Aerospace Center (DLR) and the European Space Agency (ESA) were involved in the study.

Since August 2018, a new type of research satellite has been orbiting the Earth, named after a Greek wind god -- Aeolus. The aim of Aeolus is to actively measure wind from space and thus improve weather forecasting. On board of this satellite of the European Space Agency (ESA) is the "Atmospheric Laser Doppler Instrument" (ALADIN), a high-performance laser. ALADIN is the first instrument in space that can actively measure vertical profiles of wind speed. It uses the principle of a light radar (short: Lidar from "LIght Detection And Ranging"). A signal is emitted and the reflection provides information about location and distance. The Doppler effect is then used to measure the wind speed at different heights in the atmosphere. To validate the laser measurements in space, they are compared with laser measurements from the ground. Several research groups from Germany are involved in this effort within the framework of the EVAA initiative (Experimental Validation and Assimilation of Aeolus observations). TROPOS, for example, measures with its lidar devices every Friday evening and Sunday morning when the Aeolus satellite flies over Leipzig. The data from ground and space can then be compared. On 11 September 2020, this resulted in the rare constellation that the extraordinary plume of smoke from the Californian forest fires could be measured over Leipzig simultaneously from ground and from space.

"Using revolutionary laser technology, Aeolus is currently the only satellite in the world that can measure profiles of horizontal wind speed as well as the backscatter and extinction of aerosols and clouds independently. The satellite thus provides valuable information on the radiative properties of these smoke aerosols," emphasises Dr Sebastian Bley of TROPOS, who has been involved in the Aeolus project at the European Space Agency's (ESA) ESRIN research centre for the past three years. "It is expected that this unique configuration will contribute to improved predictions of such global smoke dispersion but also of weather in general."

In September 2020, the heat from the extreme forest fires on the US West Coast transported the smoke to high altitudes. Once high up, it was then transported with the jet stream across North America and the Atlantic to Europe. In Leipzig, Germany, the smoke layer appeared at an altitude of around 12 kilometres on the morning of 11.09.2020 and sank to an altitude of around 5 kilometres in the course of the day. This is shown by the data from the PollyXT lidar at TROPOS. Lidar measurements in Leipzig confirmed the strong attenuation of the direct sunlight on this Friday: "It was -- measured by the Aerosol Optical Thickness (AOT) -- the strongest influence of forest fire aerosol on the free troposphere above Leipzig ever observed since the beginning of regular lidar observations in 1997," reports Dr Holger Baars from TROPOS, "The free troposphere is the region of the atmosphere in which the weather takes place but the direct influence from the ground is low. We were able to estimate an average mass concentration of forest fire aerosol of 8 micrograms per cubic metre between 4 and 11 km altitude. At the peak it was even 22 micrograms per cubic metre -- that's quite remarkable for these altitudes." Saturday and Sunday were hazy days despite cloudless skies. The UV index of the Federal Office for Radiation Protection (BfS), among others, also showed how strongly the smoke layers dampened solar radiation in Saxony: the TROPOS station in Melpitz near Torgau registered about a quarter less UV radiation at noon on 12 September 20 than would have been possible under clear sky. The unusual state of the atmosphere was particularly striking at sunset with a distinctive milky-yellow light.

The researchers were able to confirm the origin of the smoke using a computer model: The backward simulation proves that the air masses that arrived at noon on 11 September at an altitude of 8.5 km above Leipzig originated from the west coast of North America, where intense fires took place days before. The frequency and intensity of fires in California continued to increase during the first week of September as satellite images show. Slightly weaker fires were observed in Oregon, Washington and Montana. "Due to the prevailing winds, the travel time of the smoke from the US West Coast to Europe was only about 3 to 4 days. The air masses even made the approximately 3000 kilometres across the Atlantic Ocean between Newfoundland and Ireland at high speed in only one day (9 September)," explains Martin Radenz from TROPOS.

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Trace gases from ocean are source of particles accelerating Antarctic climate change

Scientists exploring the drivers of Antarctic climate change have discovered a new and more efficient pathway for the creation of natural aerosols and clouds which contribute significantly to temperature increases.

The Antarctic Peninsula has shown some of the largest global increases in near-surface air temperature over the last 50 years, but experts have struggled to predict temperatures because little was known about how natural aerosols and clouds affect the amount of sunlight absorbed by the Earth and energy radiated back into space.

Studying data from seas around the Peninsula, experts have discovered that most new particles are formed in air masses arriving from the partially ice-covered Weddell Sea -- a significant source of the sulphur gases and alkylamines responsible for 'seeding' the particles.

A new study shows that increased concentrations of sulphuric acid and alkylamines are essential for the formation of new particles around the northern Antarctic Peninsula. High concentrations of other acids and oxygenated organics coincided with high levels of sulphuric acid, but by themselves did not lead to measurable particle formation and growth.

An international team of researchers from the University of Birmingham; Institute of Marine Science, Barcelona, Spain; and King Abdulaziz University, Jeddah, Saudi Arabia studied summertime open ocean and coastal new particle formation in the region, based on data from ship and land stations, and today published its findings in Nature Geoscience.

The researchers revealed that the newly discovered pathway is more efficient than the ion-induced sulphuric acid-ammonia pathway previously observed in Antarctica and can occur rapidly under neutral conditions.

Study co-author Roy Harrison OBE, Professor of Environmental Health at the University of Birmingham, commented: "New particle formation is globally one of the major sources of aerosol particles and cloud condensation nuclei. This previously overlooked pathway to natural aerosol formation could prove a key tool in predicting the future climate of polar regions.

"The key to unlocking Antarctica's climate change lies in examining particles created in the atmosphere by the chemical reaction of gases. These particles start tiny and grow bigger, becoming cloud condensation nuclei leading to more reflective clouds which direct outgoing terrestrial radiation back to earth and warm the lower atmosphere."

New particle formation is globally one of the major sources of aerosol particles and cloud condensation nuclei. Existing research suggests that natural aerosols contribute disproportionately to global warming, whilst sulphuric acid is thought to be responsible for most aerosol seeding observed in the atmosphere.

The research team identified numerous sulphuric acid-amine cluster peaks during new particle formation events -- providing evidence that alkylamines provided the basis for sulphuric acid nucleation.

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Flushing a public toilet? Don't linger, because aerosolized droplets do

 Flushing a toilet can generate large quantities of microbe-containing aerosols depending on the design, water pressure or flushing power of the toilet. A variety of pathogens are usually found in stagnant water as well as in urine, feces and vomit. When dispersed widely through aerosolization, these pathogens can cause Ebola, norovirus that results in violent food poisoning, as well as COVID-19 caused by SARS-CoV-2.

Respiratory droplets are the most prominent source of transmission for COVID-19, however, alternative routes may exist given the discovery of small numbers of viable viruses in urine and stool samples. Public restrooms are especially cause for concern for transmitting COVID-19 because they are relatively confined, experience heavy foot traffic and may not have adequate ventilation.

A team of scientists from Florida Atlantic University's College of Engineering and Computer Science once again put physics of fluids to the test to investigate droplets generated from flushing a toilet and a urinal in a public restroom under normal ventilation conditions. To measure the droplets, they used a particle counter placed at various heights of the toilet and urinal to capture the size and number of droplets generated upon flushing.

Results of the study, published in the journal Physics of Fluids, demonstrate how public restrooms could serve as hotbeds for airborne disease transmission, especially if they do not have adequate ventilation or if toilets do not have a lid or cover. Most public restrooms in the United States often are not equipped with toilet seat lids and urinals are not covered.

For the study, researchers obtained data from three different scenarios: toilet flushing; covered toilet flushing and urinal flushing. They examined the data to determine the increase in aerosol concentration, the behavior of droplets of different sizes, how high the droplets rose, and the impact of covering the toilet. Ambient aerosol levels were measured before and after conducting the experiments.

"After about three hours of tests involving more than 100 flushes, we found a substantial increase in the measured aerosol levels in the ambient environment with the total number of droplets generated in each flushing test ranging up to the tens of thousands," said Siddhartha Verma, Ph.D., co-author and an assistant professor in FAU's Department of Ocean and Mechanical Engineering. "Both the toilet and urinal generated large quantities of droplets smaller than 3 micrometers in size, posing a significant transmission risk if they contain infectious microorganisms. Due to their small size, these droplets can remain suspended for a long time."

The droplets were detected at heights of up to 5 feet for 20 seconds or longer after initiating the flush. Researchers detected a smaller number of droplets in the air when the toilet was flushed with a closed lid, although not by much, suggesting that aerosolized droplets escaped through small gaps between the cover and the seat.

"The significant accumulation of flush-generated aerosolized droplets over time suggests that the ventilation system was not effective in removing them from the enclosed space even though there was no perceptible lack of airflow within the restroom," said Masoud Jahandar Lashaki, Ph.D., co-author and an assistant professor in FAU's Department of Civil, Environmental and Geomatics Engineering. "Over the long-term, these aerosols could rise up with updrafts created by the ventilation system or by people moving around in the restroom."

There was a 69.5 percent increase in measured levels for particles sized 0.3 to 0.5 micrometers, a 209 percent increase for particles sized 0.5 to 1 micrometers, and a 50 percent increase for particles sized 1 to 3 micrometers. Apart from the smallest aerosols, comparatively larger aerosols also pose a risk in poorly ventilated areas even though they experience stronger gravitational settling. They often undergo rapid evaporation in the ambient environment and the resulting decreases in size and mass, or the eventual formation of droplet nuclei, can allow microbes to remain suspended for several hours.

"The study suggests that incorporation of adequate ventilation in the design and operation of public spaces would help prevent aerosol accumulation in high occupancy areas such as public restrooms," said Manhar Dhanak, Ph.D., co-author, chair of FAU's Department of Ocean and Mechanical Engineering, and professor and director of SeaTech. "The good news is that it may not always be necessary to overhaul the entire system, since most buildings are designed to certain codes. It might just be a matter of redirecting the airflow based on the restroom's layout."

During the 300-second sampling, the toilet and urinal were flushed manually five different times at the 30-, 90-, 150-, 210-, and 270-second mark, with the flushing handle held down for five consecutive seconds. The restroom was deep cleaned and closed 24 hours prior to conducting the experiments, with the ventilation system operating normally. The temperature and relative humidity within the restroom were 21 degrees Celsius (69.8 degrees Fahrenheit) and 52 percent, respectively.

"Aerosolized droplets play a central role in the transmission of various infectious diseases including COVID-19, and this latest research by our team of scientists provides additional evidence to support the risk of infection transmission in confined and poorly ventilated spaces," said Stella Batalama, Ph.D., dean of the College of Engineering and Computer Science.

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Organic crystals' ice-forming superpowers

 At the heart of clouds are ice crystals. And at the heart of ice crystals, often, are aerosol particles -- dust in the atmosphere onto which ice can form more easily than in the open air.

It's a bit mysterious how this happens, though, because ice crystals are orderly structures of molecules, while aerosols are often disorganized chunks. New research by Valeria Molinero, distinguished professor of chemistry, and Atanu K. Metya, now at the Indian Institute of Technology Patna, shows how crystals of organic molecules, a common component of aerosols, can get the job done.

The story is more than that, though -- it's a throwback to Cold War-era cloud seeding research and an investigation into a peculiar memory effect that sees ice form more readily on these crystals the second time around.

The research, funded by the Air Force Office of Scientific Research, is published in the Journal of the American Chemical Society.

Throwback to cloud seeding

Molinero's research is focused on how ice forms, particularly the process of nucleation, which is the beginning of ice crystal formation. Under the right conditions, water molecules can nucleate ice on their own. But often some other material, called a nucleant, can help the process along.

After several studies on the ways that proteins can help form ice, Molinero and Metya turned their attention to organic ice nucleants (as used here, "organic" means organic compounds containing carbon) because they are similar to the ice-producing proteins and are found in airborne aerosols.

But a review of the scientific literature found that the papers discussing ice nucleation by organic compounds came from the 1950s and 1960s, with very little follow-up work after that until very recently.

"That made me really curious," Molinero says, "because there is a lot of interest now on organic aerosols and whether and how they promote the formation of ice in clouds, but all this new literature seemed dissociated from these early fundamental studies of organic ice nucleants."

Additional research revealed that the early work on organic ice nucleants was related to the study of cloud seeding, a post-war line of research into how particles (primarily silver iodide) could be introduced into the atmosphere to encourage cloud formation and precipitation. Scientists explored the properties of organic compounds as ice nucleants to see if they might be cost-effective alternatives to silver iodide.

But cloud seeding research collapsed in the 1970s after political pressures and fears of weather modification led to a ban on the practice in warfare. Funding and interest in organic ice nucleants dried up until recently, when climate research spurred a renewed interest in the chemistry of ice formation in the atmosphere.

"There has been a growing interest in ice nucleation by organic aerosols in the last few years, but no connection to these old studies on organic crystals," Molinero says. "So, I thought it was time to "rescue" them into the modern literature."

Going all classic

Phloroglucinol is one of the organic nucleants studied in the mid-20th century. It showed promise for controlling fog, but less for cloud seeding. Molinero and Metya revisited phloroglucinol as it proved potent at ice nucleation in the lab.

One question to answer is whether phloroglucinol nucleates ice through classical or non-classical processes. When ice nucleates on its own, without any surfaces or other molecules, the only hurdle to overcome is forming a stable crystallite of ice (only about 500 molecules in size under some conditions) that other molecules can build on to grow an ice crystal. That's classical nucleation.

Non-classical nucleation, involving a nucleant surface, occurs when a layer of water molecules assembles on the surface on which other water molecules can organize into a crystal lattice. The hurdle to overcome in non-classical nucleation is the formation of the monolayer.

Which applies to phloroglucinol? In the 1960s, researcher L.F. Evans concluded that it was non-classical. "I am still amazed he was able to deduce the existence of a monolayer and infer the mechanism was non-classical from experiments of freezing as a function of temperature alone!" Molinero says. But Molinero and Metya, using molecular simulations of how ice forms, found that it's more complicated.

"We find that the step that really decides whether water transforms to ice or not is not the formation of the monolayer but the growth of an ice crystallite on top," Molinero says. "That makes ice formation by organics classical although no less fascinating."

Holding on to memories of ice

The researchers also used their simulation methods to investigate an interesting memory effect previously observed with organic and other nucleants. When ice is formed, melted and formed again using these nucleants, the second round of crystallization is more effective than the first. It's assumed that the ice melts completely between crystallizations, and researchers have posed several potential explanations.

Molinero and Metya found that the memory effect isn't due to the ice changing the nucleant surface, nor to the monolayer of water persisting on the nucleant surface after melting. Instead, their simulations supported an explanation where crevices in the nucleant can hold on to small amounts of ice that melt at higher temperatures than the rest of the ice in the experiment. If these crevices are adjacent to one of the nucleant crystal surfaces that's good at forming ice, then it's off to the races when the second round of freezing begins.

Something in the air

Other mysteries still remain -- the mid-century studies of organic crystals found that at high pressures, around 1500 times atmospheric pressure, that the crystals are as efficient at organizing water molecules into ice as an ice crystal itself. Why? That's the focus of Molinero's next experiments.

More immediately, though, phloroglucinol is a naturally-occurring compound in the atmosphere, so anything that researchers can learn about it and other organic nucleants can help explain the ability of aerosols to nucleate ice and regulate the formation of clouds and precipitation.

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