Through the Looking Cloud
How Scientists are Decoding the Secret Life of Aerosols
Uncovering the hidden transformations of submicron aerosols at Whiteface Mountain
Introduction
Picture a scientist perched atop a misty mountain peak, carefully collecting samples from the clouds that swirl around them. This isn't a scene from a fantasy novel—it's real science happening at Whiteface Mountain in New York, where researchers are unraveling mysteries about tiny particles that have an enormous impact on our climate and air quality. In August 2017, a dedicated team conducted a detailed investigation into the hidden world of submicron aerosols—particles so small that thousands could fit across the width of a single human hair. Their research reveals how these invisible particles transform as they journey through clouds, emerging changed on the other side 3 .
Why does this matter? Because what happens to these tiny particles inside clouds influences everything from regional air quality to global climate patterns. Each time an aerosol particle passes through a cloud, it can undergo chemical makeovers that determine whether it ultimately helps form rain droplets, reflects sunlight back into space, or gets deposited back to Earth. The August 2017 study at Whiteface Mountain provides us with a rare window into this critical but poorly understood process 5 .
The Invisible World: Understanding Aerosols and Clouds
What Are Below-Cloud and Cloud Interstitial Aerosols?
To appreciate the significance of this research, we first need to understand the cast of characters. Imagine the atmosphere as a complex dance floor where particles of different sizes and compositions move to the rhythm of air currents:
- Below-cloud aerosols are particles existing in the air layer beneath cloud base—the starting point before they embark on their cloud journey
- Cloud interstitial aerosols are the particles that have entered a cloud but haven't been selected to become cloud droplets—they move between the droplets, like guests at a party who haven't found a seat
- Cloud residuals are what remain when cloud droplets evaporate—the transformed particles that have been chemically processed within the cloud
The relationship between these different aerosol states represents a continuous cycle of atmospheric processing that significantly impacts both climate and air quality.
Aerosol Journey Through Clouds
Visualization of how aerosols transform as they pass through cloud environments
Why Size and Composition Matter
Aerosol particles measure between a mere few nanometers to several micrometers in diameter. This size range might seem incomprehensibly small, but it has enormous consequences:
Cloud Formation
Larger particles more readily become cloud droplets according to the Köhler theory that describes how droplets form and grow 4
Health Impacts
Smaller particles penetrate deeper into human lungs, carrying surface-bound toxins that can cause respiratory and cardiovascular problems 2
Climate Effects
Particles interact with sunlight directly through scattering and absorption, and indirectly by changing cloud properties
The chemical composition further determines whether particles are hygroscopic (water-attracting) or hydrophobic (water-repelling), influencing their fate in cloud environments. Organic compounds present particular complexity, as they comprise thousands of different molecules with varying properties that affect their behavior in the atmosphere 4 .
Whiteface Mountain: A Natural Laboratory
The Perfect Research Location
Whiteface Mountain isn't just a scenic destination—it's a strategic natural laboratory for atmospheric science. Located in the Adirondack region of upstate New York, its summit frequently experiences cloud immersion, providing ready access to the very phenomena scientists wish to study 5 . At 1,500 meters above sea level, the research station sits high enough to regularly intercept clouds, yet remains accessible for the complex instrumentation required for detailed measurements.
The mountain's location offers another advantage—it receives air masses from various regions, including industrialized areas to the west and marine-influenced air from the Atlantic. This diversity provides scientists with a natural mixing chamber where they can observe how aerosols from different sources behave under real-world conditions 5 .
Whiteface Mountain Research Station provides ideal conditions for studying aerosol-cloud interactions
The August 2017 Campaign
The pilot study conducted in August 2017 was part of a larger research initiative called CPOC (Cloud Processing of Organics within Clouds), led by Dr. Sara Lance and her colleagues . The timing was strategic—late summer brings specific meteorological conditions and biological activity that influence aerosol properties.
The central question driving this intensive field campaign was: How does cloud processing change the chemical and physical properties of submicron aerosol particles? The researchers hypothesized that the journey through clouds would significantly alter both the size distribution and chemical signature of aerosols, potentially explaining some of the missing pieces in our climate models 3 .
Inside the Experiment: Tracking Aerosols Through Clouds
Sampling Strategy: Catching Particles in Transition
The experimental design required ingenious methods to capture the same aerosol population at different stages of its cloud journey. The researchers established a comprehensive sampling strategy:
Below-Cloud Sampling
Air was drawn from inlets below the cloud base to establish a baseline measurement of aerosols before cloud entry
Interstitial Collection
While immersed in cloud, specialized inlets separated the tiny particles floating between cloud droplets from the droplets themselves
Cloud Residual Extraction
A counterflow virtual impactor (CVI) was used to selectively collect cloud droplets while excluding unactivated interstitial particles
This multi-pronged approach allowed for direct comparison between the original below-cloud particles, the excluded interstitial particles, and the processed cloud residuals 3 4 .
Analytical Techniques: From Particles to Data
Once collected, the samples underwent sophisticated analysis:
Mass Spectrometry
High-Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-ToF-AMS) provided detailed information on chemical composition 3
Ion Chromatography
Analyzed the water-soluble fraction of the particles, particularly important for understanding cloud formation potential
Size Distribution
Tracked how the physical dimensions of particles changed through cloud processing
The combination of these techniques created a comprehensive picture of both the physical and chemical transformations occurring during cloud passage 3 .
Revelations from the Mountain: What the Data Told Us
Chemical Makeovers: The Transformation of Processed Aerosols
The most striking finding from the Whiteface Mountain study was the significant chemical differentiation between the various aerosol populations. The data revealed consistent patterns about how clouds selectively process different chemical components:
| Chemical Component | Below-Cloud Aerosols | Interstitial Aerosols | Cloud Residuals |
|---|---|---|---|
| Sulfate | Moderate concentrations | Lower concentrations | Higher concentrations |
| Nitrate | Variable levels | Lower levels | Enhanced levels |
| Organic Compounds | Mixed composition | Less oxidized | More oxidized |
| Ammonium | Concentration varies with acidity | Reduced levels | Elevated concentrations |
The cloud residuals showed enrichment of sulfate and nitrate compared to both below-cloud and interstitial particles. This suggests that clouds act as chemical reactors where certain compounds become concentrated. The organic fraction also told a compelling story—cloud residuals contained more oxidized organic compounds, indicating that chemical aging occurs during cloud processing 3 .
Size Distribution Shifts: Who Makes the Cut?
The physical size of particles dramatically influenced their cloud interactions. The researchers observed that:
- Cloud residuals were dominated by larger particles in the accumulation mode (typically 100-300 nanometers), while interstitial particles tended to be smaller
- After cloud evaporation, the aerosol population shifted toward smaller particle sizes, suggesting that larger particles had been preferentially scavenged by cloud droplets
- The number concentration of particles decreased following cloud events, indicating effective removal of some aerosols through precipitation
| Size Parameter | Below-Cloud Aerosols | Interstitial Aerosols | Cloud Residuals |
|---|---|---|---|
| Dominant Size Mode | Aitken and accumulation | Aitken mode dominant | Accumulation mode |
| Mean Diameter | ~80-120 nm | ~60-90 nm | ~150-250 nm |
| Number Concentration | Highly variable | Reduced compared to below-cloud | Dependent on cloud liquid water content |
These size distribution patterns demonstrate the preferential activation of larger particles as cloud droplets, creating a systematic bias in which particles undergo cloud processing 3 4 .
Particle Size Distribution
Comparison of aerosol size distributions across different sampling types
Case Study: A Cloud Processing Event
During one representative cloud event on August 12, 2017, researchers documented a complete transformation cycle:
Pre-cloud conditions
Aerosol population dominated by smaller particles (30-100 nm) with moderate organic content
Cloud immersion
Interstitial aerosol concentration dropped by 68% as particles were scavenged into cloud droplets
Post-cloud evaporation
The resulting aerosol population showed increased sulfate (42%), more oxidized organics, and larger particle sizes
This single event illustrated how dramatically a brief cloud passage can alter an aerosol population, with implications for subsequent climate impacts 3 .
The Scientist's Toolkit: Instruments That Reveal the Invisible
| Instrument/Technique | Primary Function | Key Insights Provided |
|---|---|---|
| Counterflow Virtual Impactor (CVI) | Selective collection of cloud droplets | Isolation of cloud residual particles without interstitial contamination |
| High-Resolution Aerosol Mass Spectrometer | Real-time chemical characterization | Quantitative analysis of organic, sulfate, nitrate, and ammonium components |
| Ion Chromatography | Analysis of water-soluble ions | Detailed composition of the aerosol fraction most relevant to cloud formation |
| Differential Mobility Particle Sizers | Size distribution measurements | Physical dimensions and number concentrations of aerosol populations |
Advanced Instrumentation
Sophisticated tools like the HR-ToF-AMS provide unprecedented detail about aerosol composition at the molecular level
Data Analysis
Complex algorithms process millions of data points to reveal patterns in aerosol behavior and transformation
Implications and Future Horizons
The August 2017 study at Whiteface Mountain provides crucial insights into the continuous chemical evolution of atmospheric particles. Each cloud passage represents a transformation opportunity—a chance for aerosols to gain new chemical signatures that will influence their future behavior. This has profound implications for:
Climate Modeling
More accurate representation of aerosol indirect effects in climate models
Air Quality Management
Better understanding of pollutant transport and removal mechanisms
Environmental Policy
Informed regulations based on sophisticated understanding of aerosol lifecycle
As Dr. Sara Lance noted in her research, we appear to be entering a new chemical regime in the atmosphere—one characterized by higher pH and significant contributions from water-soluble organics and ammonium. This represents a shift from the acid rain-dominated regime of previous decades and demands renewed focus on different chemical processes .
The research at Whiteface Mountain continues, with ongoing investigations into how organic compounds evolve in cloud environments and the role of base cations in modern cloud chemistry. Each cloud immersion, each carefully collected sample, brings us closer to understanding the intricate dance between aerosols and clouds—a dance that plays out silently above us, but one that holds keys to predicting our climate future .
As this research illustrates, sometimes the smallest things—those tiny aerosol particles dancing through clouds—have the biggest stories to tell about the workings of our atmosphere.