Monitoring Air Pollution Dynamics of NO2 and SO2 in Sri Lanka Using Sentinel-5P from 2019 to 2024

Air pollution is a silent, invisible force that shapes the health of cities and the stability of ecosystems, yet in many parts of the world, we lack the tools to see it clearly. For decades, scientists have relied on ground-based stations to measure the air we breathe, but these physical monitors are expensive to build and maintain, leaving vast regions of the globe without data. In recent years, a new perspective has emerged from space. Satellites equipped with sensitive instruments can now peer through the atmosphere to detect specific gases, such as nitrogen dioxide and sulfur dioxide, which are released by cars, factories, and power plants. These gases are not just local nuisances; they are precursors to acid rain and fine particles that harm human lungs. Understanding where these pollutants gather and how they move is essential for protecting public health, but doing so in countries with few ground monitors has been a significant challenge.

A team of researchers from Sri Lanka has turned this challenge into an opportunity by using a six-year window of satellite data to map the air quality across their entire island nation. Between 2019 and 2024, they utilized observations from the Sentinel-5P satellite, a spacecraft orbiting Earth that carries an instrument capable of measuring atmospheric gases every single day. By combining these satellite images with geographic information systems, a type of software that organizes data by location, the team created a detailed, month-by-month picture of nitrogen dioxide and sulfur dioxide levels across all twenty-five administrative districts of Sri Lanka. Their work reveals a story of how human activity and weather patterns interact to shape the air we breathe, offering a low-cost blueprint for monitoring pollution in regions where ground stations are scarce.

The study uncovered a clear and persistent pattern for nitrogen dioxide, a gas primarily produced by vehicle exhaust and industrial combustion. The highest concentrations were found along the western coastal corridor, a densely populated strip of land that includes the capital city of Colombo and its neighboring districts of Gampaha and Kalutara. Here, the satellite data showed levels consistently reaching between 3 and 5 parts per billion, a measure of how many molecules of the gas exist in a given volume of air. This corridor is the heart of Sri Lanka's urbanization and traffic, and the data confirms that the density of cars and industry is the primary driver of the pollution. In contrast, the northern and eastern parts of the island, which are less developed and have fewer vehicles, showed much lower levels, often dropping to near zero.

The researchers also tracked how the seasons change the air. Sri Lanka experiences two major monsoon seasons, periods of heavy rain that sweep across the island. The data showed that these rains act as a natural cleanser. During the southwest monsoon, which runs from May to September, the nitrogen dioxide levels in the western and southern regions dropped significantly, often falling into the lowest categories. The rain washes the pollutants out of the atmosphere and the wind helps to disperse them. Conversely, during the dry months from January to April, the pollutants tend to linger, especially in the urban centers where they are trapped by lower wind speeds.

A particularly striking moment in the data occurred in 2020, during the global pandemic. When the government imposed strict movement restrictions to stop the spread of the virus, the satellite images captured a dramatic and immediate drop in nitrogen dioxide. In April and May of that year, the high-pollution zones around Colombo and Kalutara, which usually showed the highest levels, fell to the lowest category, dropping below 1 part per billion. This sharp decline provided undeniable proof that the pollution was coming from human activity rather than natural sources. However, the effect was temporary; as restrictions eased and life returned to normal by September, the pollution levels rose again, mirroring the return of traffic and industrial work.

Sulfur dioxide told a different story. While nitrogen dioxide spread out along roads and cities, sulfur dioxide, which is largely produced by burning fossil fuels like coal and oil, clustered tightly around specific industrial sites. The researchers found distinct hotspots near thermal power plants in the Puttalam district and industrial zones in Kalutara. Unlike the widespread urban haze of nitrogen dioxide, sulfur dioxide appeared as concentrated plumes tied directly to these point sources. The levels of this gas were generally lower than nitrogen dioxide, mostly staying between 0 and 2 parts per billion, and while they did fluctuate slightly with the seasons, they remained tied to the operations of the power plants and factories rather than the daily rhythm of city traffic.

The study also compared two different types of satellite data products to see which was better for different tasks. One product, called Near Real-Time, is processed quickly and is useful for spotting sudden changes, such as the sudden drop in pollution during the pandemic lockdown. The other, called Offline, undergoes more thorough processing and provides a smoother, more reliable picture for studying long-term trends over several years. The researchers found that while the quick data was good for immediate events, the processed data was superior for understanding the slow, steady changes in air quality over the six-year period.

Looking at the long-term trends from 2019 to 2024, the researchers noted that while the pandemic caused a brief dip, nitrogen dioxide levels in urban areas have been slowly creeping back up, suggesting that vehicle emissions and urban expansion are continuing to outpace any emission reductions. Sulfur dioxide levels remained relatively stable but showed signs of increasing in the industrial districts, hinting at either rising industrial output or a reduced ability of the atmosphere to clear the air. The study also pointed out that in the central highlands, where the terrain is mountainous, pollutants sometimes get trapped in valleys during dry seasons, creating localized pockets of higher pollution that are distinct from the coastal patterns.

This research demonstrates that free satellite data can serve as a powerful, cost-effective tool for nations that lack extensive ground monitoring networks. By mapping the air quality across the entire island, the study provides a baseline that policymakers can use to make informed decisions. The findings suggest that to improve air quality, Sri Lanka needs to focus on reducing vehicle emissions in the western corridor and managing industrial output in the power-generating districts. The maps generated by this study offer a clear visual guide for where to place new monitoring stations, where to enforce stricter regulations, and how to plan for a future where the air remains safe to breathe. Ultimately, the work proves that even without a dense network of ground sensors, we can now see the invisible clouds of pollution and understand the forces that move them.