Spatio-temporal assessment of PM2.5 and PM10 pollution in core and peripheral municipalities of the Kathmandu valley using regulatory and low-cost sensor data

The air we breathe is a complex mixture of invisible gases and tiny solid particles. Among these, the most dangerous are fine specks of dust and soot, small enough to slip deep into our lungs and even enter our bloodstream. Scientists call these particulate matter. When these particles are less than 2.5 micrometers wide, they are labeled as PM2.5; when they are slightly larger, up to 10 micrometers, they are PM10. While these measurements might sound technical, the reality is simple: the more of these particles floating in the air, the greater the risk to human health, causing everything from coughing and asthma to serious heart and lung diseases. In many parts of the world, especially in rapidly growing cities nestled in valleys, the air becomes thick with these pollutants. The challenge for scientists is not just to measure the air in one spot, but to understand how pollution moves and changes across a whole city, from the busy center to the quieter edges, and how the shape of the land and the weather trap or clear these harmful specks.

In the Kathmandu Valley of Nepal, a bowl-shaped basin surrounded by mountains, the air quality has become a severe public health crisis. The very shape of this valley, with its steep walls and flat floor, acts like a lid on a pot, trapping air and preventing it from flowing away. This natural trap is especially effective during the winter, when cold air settles near the ground and stops the wind from mixing the atmosphere. For years, researchers have known that the city center is polluted, but they lacked a clear picture of how the air quality differs in the surrounding towns and how the pollution shifts from season to season. Traditional air monitoring stations are expensive and require significant maintenance, so they are often few and far between, leaving large gaps in our knowledge. To fill these gaps, a team of researchers from Tribhuvan University combined data from official government monitors with a new network of affordable, low-cost sensors. By placing these sensors in both the dense urban core and the rapidly developing peripheral municipalities, they created a detailed, high-resolution map of the valley's air.

The study, which analyzed daily air quality data from 2023 to 2024, revealed a stark and persistent problem. The air in the city center is consistently worse than in the surrounding areas. On average, the urban core saw PM2.5 concentrations of 40.71 micrograms per cubic meter, while the peripheral areas measured 27.22 micrograms per cubic meter. This difference is not just a small fluctuation; it represents a significant inequality in exposure. During the dry seasons, specifically the winter and the period before the monsoon rains, the pollution in the city center can be up to 1.6 times higher than in the outskirts. The researchers found that the city center is a hotspot where local sources, such as heavy traffic, construction dust, and industrial emissions, pile up on top of the background pollution that affects the entire valley. In contrast, the outer areas, while still polluted, benefit from slightly better air mixing and fewer concentrated local sources.

The timing of the pollution tells an equally important story. The air is cleanest during the monsoon season, when heavy rains wash the dust and soot out of the sky. However, even during this wettest time, the city center still struggles to meet safety guidelines. As the rains stop and the dry season begins, the air quality deteriorates rapidly. The worst months are January and April, when the combination of cold, stagnant air and dry conditions allows pollutants to accumulate to dangerous levels. During these dry periods, the city center exceeds the World Health Organization's safety limits on nearly every single day. The study showed that in the winter and pre-monsoon seasons, the city center had zero days where the air met the strict international safety standards. This is a far cry from the occasional clean day; it is a state of chronic, daily exposure to hazardous air.

One of the most revealing findings was how the type of pollution changes depending on where you are. In the city center, the air is filled with a mix of fine particles and coarser dust, likely stirred up by vehicles and construction work. In the peripheral areas, the pollution is dominated by finer particles, which often come from burning biomass for cooking or heating. The researchers also looked at whether pollution patterns changed between weekdays and weekends. They found almost no difference. The air remained just as polluted on Saturdays and Sundays as it did on Mondays. This suggests that the main drivers of pollution in the valley are not just daily commuter traffic, which might drop on weekends, but continuous sources like brick kilns and industrial operations that run seven days a week. These facilities keep the background level of pollution high, regardless of the day.

The study also investigated where the pollution comes from during the worst episodes. By tracking the path of air masses, the researchers discovered that during extreme pollution events, winds often blow from the west, carrying dust and smoke from the vast plains of India across the border and into the Kathmandu Valley. This means that the city's air quality is not just a local issue but is heavily influenced by regional weather patterns and emissions from neighboring countries. When the wind blows from the west, it brings a fresh load of pollutants that gets trapped in the valley's bowl, compounding the local emissions. This transboundary transport explains why pollution spikes can happen even when local conditions seem manageable.

The researchers concluded that relying on a few official monitoring stations is not enough to understand the true nature of air pollution in a complex city like Kathmandu. The use of low-cost sensors proved essential in capturing the sharp differences in air quality between the city center and the outskirts, revealing a level of detail that was previously missing. The data shows that while the entire valley suffers from poor air quality, the burden is not shared equally. The people living in the urban core face a much higher risk, breathing air that is significantly more toxic than their neighbors in the peripheral towns. The study emphasizes that managing this crisis requires understanding these local differences and the regional forces that drive them. Without addressing both the local sources, such as traffic and brick kilns, and the regional transport of pollutants, the air in Kathmandu will likely remain unsafe for the foreseeable future. The findings provide a clear, evidence-based foundation for creating targeted strategies to protect public health, ensuring that air quality management reaches not just the city center, but the entire valley.