Exploring the complex relationship between hydrocarbons and sulfur compounds in combustion particles and their impact on cloud formation and climate.
Imagine a microscopic particle of soot—black, carbon-rich, and utterly hydrophobic—spewing from an engine exhaust into the atmosphere. Left to its own devices, this particle would resist water, failing to perform one of atmosphere's most critical functions: forming cloud droplets. Now imagine that same particle undergoing a remarkable transformation, becoming encased in a thin film of sulfuric acid, suddenly capable of attracting water molecules and blossoming into a full-fledged cloud droplet. This alchemical process lies at the heart of one of atmospheric science's most fascinating stories—how the chemical dance between hydrocarbons and sulfur-containing compounds in combustion particles determines their ability to form clouds, with profound implications for our climate.
The interplay between these two classes of compounds creates what scientists call a "hydrocarbon-sulfur paradox". On one hand, sulfur-containing compounds can enhance the cloud-forming potential of particles by making them more water-absorbent. On the other hand, certain hydrocarbons can stubbornly resist interaction with water, effectively shutting down the cloud-forming process altogether.
Understanding this delicate balance has become crucial in predicting how human activities—from maritime shipping to power generation—influence cloud formation, precipitation patterns, and ultimately, global climate.
Typical size range of Cloud Condensation Nuclei (CCN)
Smaller than the width of a human hair
Every cloud droplet in our atmosphere forms around a microscopic heart—a solid or liquid particle called a Cloud Condensation Nucleus (CCN). These tiny specks of matter, typically measuring between 0.001 and 0.1 micrometers (about 100 times smaller than the width of a human hair), provide a surface for water vapor to condense upon. Without CCN, clouds as we know them couldn't form—water vapor would need impossibly high levels of supersaturation to spontaneously form droplets.
Not all particles are created equal when it comes to cloud-forming abilities. Their effectiveness depends on three key properties:
The relationship between droplet size and saturation ratio according to Köhler theory
The relationship between these factors is elegantly captured by Köhler Theory, a mathematical model developed by meteorologist Hilding Köhler in the 1930s that predicts at what point a particle will activate into a cloud droplet. The theory balances two competing effects: the Kelvin effect (which makes small droplets evaporate due to surface tension) and the solute effect (which helps droplets grow due to dissolved substances). When the solute effect wins, the particle successfully becomes a cloud droplet.
A critical concept in CCN science is the "mixing state"—how different chemical compounds are arranged within aerosol populations. Imagine a collection of particles fresh from combustion:
Each particle has a different composition—some are purely carbonaceous, others are sulfates, and others might be mineral dust.
Each individual particle contains a blend of components—carbon cores coated with sulfate shells, for instance.
This distinction matters tremendously because an internally mixed particle containing both hydrophobic (water-repelling) and hydrophilic (water-attracting) components can behave completely differently than its externally mixed counterparts. Through atmospheric aging processes, particles can transition from external to internal mixtures, fundamentally changing their cloud-forming properties 1 .
| Fuel Type | Sulfur Content | Hydrocarbon Type | CCN Activity |
|---|---|---|---|
| High-sulfur ship fuel | High | Mostly non-volatile OC | Moderate (enhanced by sulfuric acid coating) |
| Low-sulfur diesel | Low | Non-volatile OC | Low (highly hydrophobic) |
| Biomass burning | Variable | Mixed solubility OC | Variable (depends on combustion conditions) |
| Gas turbine (high temp) | Variable | Low non-volatile OC | Higher (more soluble components) |
Table 1: How Fuel Composition Affects CCN Activity
To truly understand the hydrocarbon-sulfur paradox, scientists designed the European PartEmis project (Measurement and prediction of emissions of aerosols and gaseous precursors from gas turbine engines). This comprehensive study aimed to characterize and quantify exhaust emissions from a gas turbine engine under different operating conditions and fuel sulfur concentrations 3 .
The researchers faced a complex challenge: combustion emissions contain hundreds of different chemical compounds constantly evolving through atmospheric processes. Previous studies had shown that sulfur coatings could enhance CCN activation, and that organic compounds could potentially hinder it, but no one had systematically tested how these components interacted within the same particles under controlled conditions that simulated atmospheric aging.
The PartEmis team employed an impressive array of instruments and techniques to get a complete picture of particle behavior:
Operated a gas turbine engine under different combustion temperatures and with fuels containing varying sulfur concentrations.
Used multiple techniques to determine particle composition including online chemical monitoring and thermal stability measurements.
Particles were exposed to controlled supersaturations to determine at what point they would activate into cloud droplets.
The experimental data was fed into Köhler theory-based models to test predictive capabilities against actual behavior 3 .
This multi-faceted approach allowed the team to directly link chemical composition with CCN activity—a crucial step in understanding the underlying mechanisms.
The PartEmis experiments revealed several fascinating insights:
When sulfur dioxide in the exhaust oxidized to form sulfuric acid, this acidic coating dramatically increased particles' hygroscopicity. Even a tiny coating comprising just 0.1 to 3 volume-percent of water-soluble matter could increase the fraction of CCN-activated particles from ≤10⁻⁴ to approximately 10⁻² at a water vapor saturation ratio of 1.006 3 . That's an improvement of two orders of magnitude!
Particles containing a large fraction of non-volatile organic compounds grew significantly less at high relative humidity than particles with lower organic content. The organic material was essentially "partially compensating" for the sulfur-related enhancement of CCN activation 3 .
The transition from externally mixed particles to internally mixed particles could be tracked through changes in CCN activation curves, with internal mixing generally promoting better activation 1 .
Critical supersaturation required for CCN activation across different fuel types
Impact of different hydrocarbon types on CCN activation potential
| Experimental Condition | Critical Supersaturation (%) | Activation Diameter (nm) | Key Controlling Factor |
|---|---|---|---|
| High sulfur fuel, low non-volatile OC | 0.4-0.6 | ~80 | Sulfuric acid coating dominance |
| High sulfur fuel, high non-volatile OC | 0.7-1.0 | ~120 | Organic inhibition effect |
| Low sulfur fuel, high non-volatile OC | >1.2 | >200 | Extreme hydrophobicity |
| After atmospheric aging | 0.3-0.5 | ~60 | Internal mixing formation |
Table 2: CCN Activation Results from PartEmis Experiments
| Organic Compound Type | Solubility Characteristics | Impact on CCN Activity | Typical Sources |
|---|---|---|---|
| Strong surfactants | Surface-active, low critical micelle concentration | Can enhance activation via surface tension lowering | Sea spray, certain organics |
| Weak surfactants | Moderately soluble, some surface activity | Minor enhancement possible | Partially oxidized organics |
| Non-volatile, insoluble OC | Hydrophobic, resistant to evaporation | Significantly inhibits activation | High-temperature combustion |
| Water-soluble OC | Readily dissolves in water | Moderate enhancement, similar to inorganics | Low-temperature combustion, aged aerosols |
Table 3: Hydrocarbon Solubility and CCN Impacts
Understanding the hydrocarbon-sulfur-CCN relationship requires specialized equipment. Here are the workhorse instruments behind these discoveries:
The cornerstone instrument that exposes particles to controlled supersaturations and counts how many activate into droplets.
Provides real-time measurement of chemical composition, allowing researchers to correlate composition with CCN activity.
Measures how particles grow under high humidity—a key indicator of their potential CCN activity.
Specifically measures black carbon concentrations, helping quantify the hydrophobic component.
Heats aerosol samples to remove volatile components, helping distinguish between volatile and non-volatile organic compounds.
The hydrocarbon-sulfur dynamic extends far beyond laboratory curiosity—it has real-world consequences for climate prediction and environmental policy.
International regulations have progressively limited the sulfur content in ship fuels, reducing it from 3.5% to 0.5% in 2020. While this has clear benefits for reducing acid rain and respiratory problems, its effect on cloud formation is complex. Research has shown that switching to low-sulfur fuels produces highly hydrophobic particles that resist cloud formation, potentially reducing cloud cover in shipping lanes 6 .
Similarly, wet scrubbing technology—which removes sulfur from exhaust—can have opposing effects depending on implementation. When used with high-sulfur fuels, scrubbing can increase CCN activity, but the combination of low-sulfur fuels and scrubbing can result in emissions that are particularly poor at forming clouds 6 .
The hydrocarbon-sulfur interplay also plays out in wildfire and agricultural burning emissions. Biomass burning produces complex mixtures of organic compounds with varying solubility. The CCN activity of these particles depends heavily on the type of biomass burned and combustion conditions. For instance, laboratory studies show that complete CCN activation occurs at different supersaturations for different fuel sources: cow dung cake (0.3% SS), wood burning (0.4% SS), and coal burning (0.7% SS) 2 .
Today's scientists approach these questions with increasingly sophisticated tools. Modern CCN research includes bulk-surface partitioning models that account for how surfactants concentrate on droplet surfaces, surface tension modifications to classical Köhler theory, and advanced chemical speciation using techniques like Scanning Transmission X-ray Microscopy (STXM) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy 5 6 .
Changes in CCN activation potential following sulfur content regulations in shipping fuels
The story of how hydrocarbons and sulfur-containing compounds influence cloud formation continues to unfold, with new chapters being written as combustion technologies evolve and research methods advance. What began as a simple narrative—"sulfur helps, hydrocarbons hinder"—has matured into a sophisticated understanding of competing effects, mixing states, and atmospheric processing.
What emerges clearly is that we cannot judge a particle's cloud-forming potential by its source alone. A soot particle that begins as hopelessly hydrophobic may become an efficient cloud droplet nucleus through atmospheric processing that adds sulfate coatings. Conversely, a particle rich in sulfur may see its cloud-forming potential hampered by a dominant fraction of non-volatile organic material.
As we modify our fuel compositions and pollution control technologies—reducing sulfur here, altering combustion conditions there—we're unconsciously engineering the cloud-forming properties of our atmosphere. Understanding the hydrocarbon-sulfur tango ensures that these engineering efforts lead to clearer skies and a more predictable climate future.
The next time you watch a cloud drift across the sky, consider the microscopic drama unfolding within—the dance between hydrocarbon and sulfur that helped bring that cloud into being, connecting human industry to the very fabric of our atmosphere.