Clouds may look simple from the ground, but the science behind how they form is surprisingly complex. Tiny particles suspended in the atmosphere act as seeds for cloud droplets, influencing rainfall, cloud lifetime, and how much sunlight clouds reflect back into space.
Study: Removal of trace gases can both increase and decrease cloud droplet formation. Image Credit: OlegRi/Shutterstock.com
Many of these particles come from natural marine sources, but human activities also contribute aerosols through continental pollution and ship emissions. When additional particles enter shallow, low-lying clouds, they can increase the number of cloud droplets, brighten the clouds, and reduce precipitation efficiency. These effects make aerosol pollution an important, but still uncertain, part of the climate system.
A recent study published in Science Advances suggests that trace gases surrounding atmospheric particles may play a much larger role in this process than previously understood. By examining how the removal of selected gases affected particles’ ability to attract water, researchers found that changes in atmospheric chemistry could make the particles either more or less likely to initiate cloud droplets.
The findings highlight an unexpectedly strong connection between atmospheric particles and the gases around them. They also raise questions about whether conventional measurements based on dried aerosol samples fully capture how mixtures of marine aerosol and transported urban pollution behave in humid coastal air.
The Aerosol-to-CCN Link
In shallow, low-lying liquid-water clouds, droplet number concentrations are highly sensitive to the addition of submicrometer-sized cloud condensation nuclei (CCN). These are airborne particles onto which water vapor can condense, allowing cloud droplets to form.
Adding CCN to these clouds generally increases droplet number concentration, cloud lifetime, cloud fraction, and cloud albedo, which is the proportion of incoming sunlight reflected by a surface. At the same time, it can reduce precipitation efficiency.
Particles associated with human activity, including those from continental pollution and ship emissions, have already changed cloud reflectivity. This effect has offset some of the warming caused by carbon dioxide, although its magnitude remains highly uncertain.
Understanding the relationship between aerosols and CCN is therefore essential for determining how air pollution affects clouds, rainfall, and climate. This requires researchers to reconcile measurements of aerosol size and composition with predicted and observed CCN behavior.
Researchers typically dry an aerosol stream before measuring particle size and chemical composition because water associated with the particles is difficult to quantify at higher relative humidity. However, drying can also remove semivolatile compounds associated with dissolved gases, gases adsorbed onto particle surfaces, and the particles’ aqueous phase.
The loss of these compounds can change the supersaturation required for a particle to activate into a cloud droplet. As a result, measurements of dried particles may not fully represent the behavior of aerosols in the humid conditions where clouds develop.
Click here to download a free PDF copy of this page
Role of Dissolved and Surface-Active Species
The critical supersaturation needed for droplet formation is controlled partly by the concentration of dissolved compounds inside the droplet and its surface tension. Organic compounds and sulfates are among the major dissolved species found in aqueous aerosols.
Water-soluble inorganic gases, including nitric acid, can also enter growing droplets and increase their dissolved content. Nitric acid may additionally help dissolve compounds that were already present in the particle but were only sparingly soluble.
Organic trace gases such as acetaldehyde and methylglyoxal can adsorb at the boundary between air and water, reducing droplet surface tension. During the upward movement and cooling of an air parcel, semivolatile organic vapors can also condense and dissolve into droplets, adding to their solute content.
These mechanisms are generally expected to reduce the supersaturation required for aerosol particles to form cloud droplets. However, most previous evidence for the influence of trace gases has come from laboratory experiments rather than field observations.
The new study tested whether removing gas-phase semivolatile compounds, followed by reequilibration between the particles and surrounding air, would alter the supersaturation needed to activate ambient particles.
Measuring Coastal Aerosols
From February to August 2023, the researchers conducted size-resolved CCN measurements near the summit of Mt. Soledad in San Diego, United States. Located a few kilometers from the Pacific coast, the site receives air influenced by both offshore marine sources and pollution transported from the Los Angeles metropolitan area.
Transport from Los Angeles typically takes between 0.5 and 1.5 days. During that journey, persistent stratocumulus clouds over the Pacific Ocean can substantially process the polluted air. The site therefore offered an opportunity to investigate aerosols under environmentally relevant coastal conditions where marine air and transported urban pollution interact.
The researchers connected a scanning mobility particle spectrometer to a continuous-flow CCN counter and a condensation particle counter. This allowed them to measure particle size distributions and the size-resolved hygroscopicity parameter, known as κ. This parameter represents how readily particles take up water and become active as CCN.
During alternating scans, particles leaving the differential mobility analyzer were passed through a denuder. The device removed more than 99% of several groups of gas-phase compounds, including alkanes, weakly functionalized acids, aldehydes, and alcohols containing fewer than 10 carbon atoms.
Comparing denuded and undenuded samples allowed the researchers to assess how removing these gases, and allowing the aerosol to re-equilibrate, changed the particles’ potential to form cloud droplets.
Trace Gases Altered Cloud-Forming Potential
Removing the organic trace gases changed the CCN-derived hygroscopicity parameter by as much as 50%. The relative difference between undenuded and denuded particle hygroscopicity ranged from approximately minus 25% to plus 25%.
Crucially, the effect did not move in only one direction. Passing the samples through the denuder made particles either more or less active as CCN. The direction of the response depended on the relative humidity of the sample and the concentration of trace gases.
The effect was weakest in early spring and strongest in late spring and summer. This seasonal pattern indicated that aerosol hydration and the composition of the surrounding gas phase helped determine how particles responded when the trace gases were removed.
The researchers attributed the changes to rapid exchanges between the particle and gas phases. Removing compounds from the gas phase may have caused substances associated with the particles to volatilize. In other cases, changes in surface adsorption may have affected the particles’ ability to take up water.
The scale and speed of the response revealed a stronger coupling between atmospheric gases and particles than expected. It also showed that treating an aerosol as an isolated particle may overlook chemical exchanges that influence its environmental behavior.
Among the bulk particle-composition variables examined, ammonium, nitrate, and chloride fractions were statistically significant at the 5% confidence level. Twenty-two trace gas compounds, primarily organic acids, were also significant at that level.
Cross-correlation between gas-phase compounds suggested that several substances could jointly influence the observed changes. Lactic acid produced the largest increase in the coefficient of determination, keto-butyric acid was among the most abundant organic acids, and cyanoacetic acid had the steepest negative slope.
All statistically significant trace gases had negative slopes, indicating that higher concentrations were associated with a decrease in the relative hygroscopicity difference. Regression analysis confirmed this negative relationship. However, the researchers could not identify a single compound or mechanism that fully explained the response.
Implications for Pollution and Climate Research
The findings suggest that methods based on dried aerosol samples may not always capture how pollution particles behave in the atmosphere. Drying can alter semivolatile material associated with the particles, while removing surrounding gases can trigger rapid chemical repartitioning between the gas and particle phases.
This matters because CCN activity helps determine the number and size of droplets in a cloud. These properties can affect cloud brightness, persistence, and precipitation efficiency. If conventional measurements overlook the influence of trace gases, estimates of the effects of pollution on coastal clouds and climate may also carry an unrecognized source of uncertainty.
Previous modeling studies cited by the researchers indicate the possible scale of this effect. A change of plus or minus 50% in the hygroscopicity of organic aerosol could alter CCN concentrations by up to 40%. Separate calculations suggest that a 20% change in hygroscopicity could produce at least a 10% change in droplet number concentration under certain stratus-cloud conditions.
These estimates do not mean the field measurements caused equivalent changes in regional clouds. The wider atmospheric effect will depend on which gases are responsible, where they occur, their concentrations, and the humidity and composition of the surrounding air.
The study was also conducted at one coastal location, and some of the observed responses remain incompletely explained. Further field measurements will be needed to determine whether similar gas-particle interactions occur in other polluted, marine, and continental environments.
Nevertheless, the results demonstrate that removing trace gases can either increase or decrease the potential for cloud droplet formation. Accounting for the atmosphere as an interacting mixture of gases, particles, and water could provide a more accurate picture of how air pollution influences clouds, precipitation, and climate.
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.
Source:
Ravichandran, E. et al. (2026). Removal of trace gases can both increase and decrease cloud droplet formation. Science Advances ,12(3), eadx0960. DOI: 10.1126/sciadv.adx0960, https://www.science.org/doi/10.1126/sciadv.adx0960