A Tale of Two Waters
Discover how atmospheric waters reveal the complex story of urban pollution in one of the world's most populated cities
Every winter, something dramatic unfolds in Delhi's atmosphere. As temperatures drop, the air transforms into a thick, milky soup that blankets the city in a surreal haze. Flights get canceled, trains slow to a crawl, and the iconic India Gate disappears from view.
But this phenomenon is more than just a meteorological nuisance—it's a massive natural laboratory where fascinating chemistry occurs right above our heads. While residents complain about the inconvenience, scientists are collecting samples of fog and dew, discovering that these everyday occurrences tell a compelling story about air pollution, environmental health, and even potential water sources for this parched region.
What makes Delhi's atmospheric waters particularly intriguing is how differently fog and dew behave chemically, despite both being formed from atmospheric water vapor. Through meticulous collection and analysis, researchers have uncovered that these two types of atmospheric water capture different chemical signatures of pollution, interact differently with airborne particles, and ultimately play distinct roles in Delhi's environmental ecosystem.
Delhi's winter fog can reduce visibility to less than 50 meters, causing major disruptions to transportation and daily life.
Understanding fog and dew chemistry helps scientists track pollution sources and develop better air quality management strategies.
Fog is essentially a cloud at ground level, consisting of tiny water droplets suspended in the air that reduce visibility to less than 1 kilometer. It forms when air becomes saturated—unable to hold all its water vapor—through either cooling or adding moisture.
In Delhi, radiation fog is most common, developing on calm, clear nights when the ground loses heat rapidly, cooling the air immediately above it 1 .
Dew represents water that condenses directly onto surfaces like leaves, cars, and specially designed collectors. It forms when objects cool down (through radiative heat loss) to temperatures below the dew point of the surrounding air, causing water vapor to condense onto these surfaces 7 .
Unlike fog, dew doesn't significantly impair visibility.
| Characteristic | Fog | Dew |
|---|---|---|
| Formation Process | Air saturation and suspension | Surface condensation |
| Visibility Impact | Reduces to <1 km | Minimal impact |
| Collection Method | Active strand collectors | Passive condensers |
| Droplet Size | 1-20 micrometers | Varies by surface |
| Primary Period | Early morning hours | Late night to early morning |
The Indo-Gangetic Plain, where Delhi is situated, provides ideal conditions for both fog and dew formation during winter months. Several factors combine to create this unique environment:
To compare fog and dew chemistry directly, researchers designed a comprehensive sampling campaign during the winter months of 2014-2015 3 . The methodology was rigorous:
Researchers gathered 24 fog samples and 19 dew samples from the Jawaharlal Nehru University campus in New Delhi, representing a relatively cleaner urban environment that still receives pollutants from surrounding areas 3 .
Fog samples were collected at both ground level and at a height of 15 meters on a rooftop to account for potential variations at different elevations 3 .
Once collected, the samples underwent detailed chemical analysis to unravel their composition:
Researchers immediately measured the acidity or alkalinity of each sample using precision pH meters, providing insights into the overall chemical character 3 .
This sophisticated technique allowed scientists to separate and quantify various soluble inorganic ions present in the samples, including anions like sulfate, nitrate, and chloride, as well as cations like ammonium, calcium, and sodium 3 .
Strict protocols ensured that the samples weren't contaminated during collection, storage, or analysis, preserving their natural chemical signatures for accurate interpretation.
Dew samples were consistently alkaline with an average pH of 6.26, while fog samples were more acidic, especially those collected at rooftop level (pH 5.38) 3 .
For context, natural rainwater has a pH of approximately 5.6, meaning the dew was notably more alkaline than expected.
This reversal in the predominance of ammonium versus calcium ions pointed to different chemical processes and pollution sources influencing the two types of atmospheric water.
| Ion Type | Fog (Order of Dominance) | Dew (Order of Dominance) |
|---|---|---|
| Cations | NH₄⺠> Ca²⺠> Mg²âº~K⺠> Na⺠| Ca²⺠> NH₄⺠> Na⺠> K⺠> Mg²⺠|
| Anions | SO₄²⻠> NO₃â»~Clâ» > HCO₃⻠> Fâ» > NOâ‚‚â» | SO₄²â»~HCO₃⻠> Clâ» > NOâ‚‚â» > NO₃⻠> Fâ» |
One of the most intriguing discoveries was the reversed ratio of nitrite to nitrate in fog versus dew:
This finding suggests that the alkaline nature of dew might promote the transfer of gaseous nitrogen oxides and base-catalyzed transformation to nitrous acid (HONO), which then dissolves as nitrite 3 . Since HONO is a significant source of hydroxyl radicals (the "detergents" of the atmosphere), this discovery has important implications for understanding Delhi's atmospheric chemistry.
| Cation | Neutralizing Factor in Fog | Neutralizing Factor in Dew |
|---|---|---|
| Ammonium (NHâ‚„âº) | Highest | Medium |
| Calcium (Ca²âº) | Medium | Highest |
| Magnesium (Mg²âº) | Lowest | Lowest |
The high ammonium content in fog points to sources like agricultural activities (from fertilizer use and livestock) and fossil fuel combustion 3 .
The elevated calcium levels in dew suggest significant influence from resuspended crustal materials—dust from roads, construction sites, and bare soil 3 .
The presence of organic acids in both fog and dew indicates contributions from vehicular emissions, biomass burning, and the atmospheric oxidation of volatile organic compounds 3 .
The higher sulfate concentrations in both fog and dew (SO₄²â»/NO₃⻠ratio of 2.2 in fog and 4.18 in dew) suggest that sulfur-containing pollutants—likely from coal combustion and industrial sources—play a more significant role in acidity formation than nitrogen oxides 3 .
Essential Research Tools for Atmospheric Water Chemistry
| Tool/Technique | Function | Application in Delhi Study |
|---|---|---|
| Caltech Active Strand Cloud Collector | Captures fog droplets by impaction on mesh strands | Collection of fog samples for chemical analysis |
| Dew Condensers | Standardized surfaces for dew formation | Controlled collection of dew samples |
| Ion Chromatograph | Separates and quantifies ionic species | Analysis of anions and cations in samples |
| Automatic Weather Sensors | Monitors meteorological parameters | Correlation of chemistry with weather conditions |
| pH Meter | Measures acidity/alkalinity | Determining overall chemical character of samples |
The comparative study of fog and dew chemistry in New Delhi reveals much more than different chemical compositions—it highlights the complex interplay between pollution, meteorology, and atmospheric processes in an urban environment. The findings demonstrate that:
These insights extend beyond academic interest. Understanding the chemistry of atmospheric waters can inform air quality management strategies and reveal how pollutants are processed and removed from the air. Furthermore, with water scarcity being a critical issue in many regions, including parts of India, studies of dew chemistry contribute to assessing its potential as an alternative water source in arid and semi-arid environments 7 .
The next time you walk through a foggy morning in Delhi or notice dew glistening on a leaf, remember that you're witnessing more than just a weather phenomenon—you're seeing a dynamic chemical laboratory where the story of the city's air pollution is being written in tiny water droplets.
Understanding fog and dew chemistry helps scientists:
While focused on Delhi, these findings have implications for other megacities facing similar air quality challenges, particularly those in developing regions with rapid urbanization and industrialization.