How relative humidity and particle size distribution create the haze that blankets one of China's most dynamic megacities
Look at a skyline of a modern megacity like Guangzhou on a typical day. The towering skyscrapers don't stand out with crisp clarity but instead seem to blend into a milky, whitish haze. This phenomenon isn't just an aesthetic issue; it's a visible sign of an invisible battle in the air we breathe. This haze, which affects everything from public health to urban climate, is caused by aerosol light extinction—a scientific term for how tiny particles in the air scatter and absorb light, effectively "extinguishing" it from our view.
For years, the blame was placed squarely on pollutant mass. But scientists in Guangzhou have been piecing together a more complex puzzle, discovering that two other hidden players are just as important: the size distribution of the particles and a surprisingly powerful influencer, relative humidity . Understanding this relationship is crucial, not just for Guangzhou, but for any city striving for a clearer, healthier sky.
To understand Guangzhou's haze, we need to meet the main actors in this atmospheric drama.
These are tiny solid or liquid particles suspended in the air. In a city like Guangzhou, they come from vehicle exhaust, construction dust, industrial emissions, and even chemical reactions in the atmosphere . They are the primary culprits behind light extinction.
This is the combined effect of scattering (light being deflected in different directions) and absorption (light energy being taken in by particles). Together, they reduce visibility by preventing light from traveling straight through the atmosphere.
This is the amount of water vapor present in the air relative to the maximum it can hold. RH is a master manipulator. Many aerosols are hygroscopic, meaning they absorb water vapor. As humidity increases, these particles act like tiny sponges, swelling in size .
A particle's ability to scatter light is dramatically dependent on its size. A particle swollen with water can scatter far more light than the same particle when it's dry. This simple physical principle explains why haze often worsens on humid days, even without additional pollution emissions.
The most critical discovery in urban air quality is the concept of hygroscopic growth. It's not just that wet air looks hazier (like fog); it's that the pollutant particles themselves are physically changing.
Small, dry particles scatter limited light
Good visibility
Swollen particles scatter much more light
Reduced visibility, increased haze
Here's a simple analogy: Imagine a handful of small, dry marble-sized balls. If you shine a light through them, some light gets through. Now, imagine each marble absorbs water and swells to the size of a ping-pong ball. The same number of particles now presents a much larger surface area, deflecting and scattering vastly more light, creating a much denser "veil."
In Guangzhou, which has a humid subtropical climate, this effect is magnified. The air is often full of water vapor, providing endless fuel for these aerosol particles to grow, leading to severe haze episodes even when the initial mass of pollutants hasn't increased .
To crack the code of Guangzhou's haze, scientists set up a sophisticated monitoring station—a "supersite" equipped with state-of-the-art instruments. Let's walk through a typical analysis.
The experiment was designed to continuously track the relationship between particle properties, humidity, and light extinction.
A Scanning Mobility Particle Sizer (SMPS) was used to count and measure particles across a wide range of sizes, from a few nanometers to several hundred nanometers. This provided a real-time "census" of the particle population in the air .
An instrument called a Humidified Tandem Differential Mobility Analyzer (HTDMA) took a sample of particles and exposed them to controlled, elevated humidity (e.g., 90% RH). It then measured how much those particles grew in size.
The extinction coefficient (a direct measure of how much light is being lost per meter of air) was measured using instruments like transmissometers or calculated based on particle chemistry and size data.
Over months of continuous operation, researchers correlated spikes in light extinction with changes in relative humidity and shifts in the particle size distribution .
The data revealed a clear and powerful narrative about how humidity transforms the urban atmosphere.
Assumes similar initial particle mass and number.
| Parameter | Dry Day (40% RH) | Humid Day (80% RH) | Change |
|---|---|---|---|
| Typical Particle Diameter | 100 nm | 250 nm | +150% |
| Particle Volume | 0.52 µm³ | 8.18 µm³ | +1473% |
| Calculated Extinction Coefficient | 300 Mm⁻¹ | 800 Mm⁻¹ | +167% |
| Visual Range (Approx.) | 13 km | 5 km | -62% |
Analysis: This table shows the dramatic "invisible" transformation. Even without more particles being emitted, humidity alone can cause a severe drop in visibility by making existing particles larger and more effective at scattering light .
Analysis: This confirms that the "accumulation mode" particles (which include sulfates, nitrates, and organic matter) are the dominant villains in the haze story. These are the particles most susceptible to hygroscopic growth .
Analysis: This explains why the haze forms. The atmosphere in Guangzhou is often rich in sulfate and nitrate particles, which have a high growth factor. When these are present, even a modest rise in humidity can trigger a significant haze event .
What does it take to study an invisible veil? Here are the key tools used in this research.
The "Particle Census Tool." It classifies particles by electrical mobility and counts them, providing a detailed number size distribution.
The "Growth Chamber." It exposes particles to controlled humidity to measure exactly how much they swell, quantifying their hygroscopicity.
The "Black Carbon Detective." It specifically measures light absorption by black carbon (soot), a key component of the extinction coefficient.
The "Scattering Gauge." It directly measures the amount of light scattered by the aerosol sample in a chamber .
The research from Guangzhou paints a clear picture: combating urban haze is not just about reducing the total weight of pollutants. It's a more nuanced battle. We must pay critical attention to the type of particles we emit—specifically targeting those hygroscopic sulfates and nitrates that act as water-activated "haze amplifiers"—especially during periods of high humidity.
This knowledge empowers policymakers to create more sophisticated and effective air quality management strategies. By understanding the intricate dance between particle size and relative humidity, we can better predict severe haze events and take targeted actions. The quest to lift the invisible veil over our cities continues, but with every experiment, the path to a clearer, brighter sky becomes more visible.