Spatiotemporal Assessment of Ambient Air Quality in Selected Urban, Tourist and Industrial Regions of Himachal Pradesh, India: A Case Study

The Invisible Weather Report

Imagine the air around us isn't just empty space, but a giant, invisible soup. Sometimes this soup is clear and refreshing, like a mountain breeze. Other times, it gets thick with tiny, invisible specks of dust, smoke, and gas that we can't see but can feel in our lungs. Scientists call this "air pollution," and they have a special scoreboard called the Air Quality Index (AQI) to tell us how "yucky" the soup is at any given moment. Think of the AQI like a weather report for your lungs: a low number means it's a great day to run outside, while a high number means you might want to stay inside or wear a mask.

The main ingredients in this soup that scientists worry about most are PM2.5 and PM10. These aren't names of people, but sizes of tiny particles. PM10 are like coarse sand grains that get stuck in your nose and throat, while PM2.5 are so small—like microscopic glitter—that they can sneak deep into your lungs and even jump into your bloodstream. To understand how bad the air is, scientists look at how these particles mix with other gases like carbon monoxide or ozone. The big question is: does the air get dirtier because of heavy traffic, factories, or just because we live in a valley where the wind can't blow the smog away? This is the mystery this paper tries to solve.

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The Great Air Quality Race: Himachal Pradesh

In this study, two researchers, Om Raj and Indramani Dhada, decided to play detective with the air in Himachal Pradesh, a beautiful state in the Indian Himalayas. They didn't just look at one spot; they picked five different towns that represent five different "personalities" of the region:

• Baddi: The industrial powerhouse, packed with factories.
• Solan: A busy, growing city with lots of schools and traffic.
• Shimla: The capital city, famous for tourists and its ridge-top location.
• Kullu: A valley town that acts as a commercial hub for travelers.
• Manali: A high-altitude tourist town, deep in the mountains with very little industry.

They watched these towns for exactly 30 days, from June 25 to July 24, 2026. Instead of setting up their own machines, they used the daily reports already published by the public platform AQI.in. It was like checking the daily scores of five different teams in a league, but the teams were towns and the score was how clean their air was.

The Scoreboard: Who Won and Who Lost?

When the researchers added up the scores, the results were as clear as a mountain stream. The towns split into two distinct teams: the "Dirty Dozen" (low elevation, heavy industry/traffic) and the "Clean Crew" (high elevation, tourism).

1. Baddi (The Industrial Heavyweight): This town had the worst air. Its average AQI was 109.1. That means on 19 out of 30 days, the air quality was bad enough to be considered "unhealthy" (crossing the 100 mark). On its worst day, the AQI hit 149.
2. Solan (The Busy City): Coming in second, Solan had an average AQI of 95.7. It crossed the "unhealthy" line on 13 days.
3. Kullu, Shimla, and Manali (The Mountain Cleaners): These three were in a league of their own. Kullu averaged 51.8, Shimla 47.6, and Manali took the gold medal with the cleanest air at 37.4. None of these three hill towns ever reached an AQI of 100, even for a single day.

The difference was staggering. The air in Baddi was nearly three times dirtier than the air in Manali. The researchers found that elevation and land use were the main reasons. Baddi sits low in the foothills where factories and heavy trucks live, trapping the pollution. Manali sits high up in the mountains where the wind blows freely and there are no factories.

The "Smoking Gun": What Makes the Air Dirty?

The researchers wanted to know why the AQI went up and down. They looked at the ingredients: PM2.5, PM10, Carbon Monoxide (CO), Sulfur Dioxide (SO2), Nitrogen Dioxide (NO2), and Ozone (O3).

They found a "smoking gun" that was hard to miss: Particulate Matter (PM2.5).

• The AQI score moved almost perfectly in sync with PM2.5 levels. The connection was so strong it had a correlation score of 0.995 (where 1.0 is a perfect match).
• In Baddi, the average PM2.5 was 39.0 µg m⁻³, while in Manali, it was just 9.5 µg m⁻³.
• The study showed that if you know the PM2.5 level, you basically know the AQI level. The other gases, like SO2 and NO2, were barely there or didn't seem to drive the main score.

Interestingly, they found a weird twist with Carbon Monoxide (CO). Usually, you'd think more cars mean more CO and more pollution. But in this study, the towns with the most CO (Manali and Kullu) actually had the cleanest air overall. Why? Because Manali is a narrow, high valley where traffic gets stuck, trapping CO, but because there are no factories, the dangerous dust (PM2.5) is low. In Baddi, the factories and trucks create so much dust that it overwhelms the CO levels. The math showed a negative link between CO and the AQI, but the authors explained this wasn't because CO "cleaned" the air; it was just a geographic quirk of where the different towns sat.

The Monsoon Effect

The study took place during the monsoon season (June–July). You might expect rain to wash everything clean, and it did help. All the towns showed a pattern where the air got cleaner in mid-July after a period of rain. However, even with the rain, Baddi stayed dirty. While the other towns dipped below 60, Baddi had a second spike of pollution in mid-July, hitting 149 on July 19. This suggested that while rain helps, local sources like factories and freight trucks in Baddi are so strong they keep pumping out pollution regardless of the weather.

What This Means (And What It Doesn't)

The authors are careful to say this is a case study, not a final verdict on the whole state. They used data from just 30 days in the summer. They admit that winter might look very different, as cold weather and heating can trap pollution even tighter. They also didn't measure the air themselves; they used public data, so they can't guarantee the machines were perfectly calibrated.

However, the findings are a strong signal. They prove that in Himachal Pradesh, where you live matters more than the season when it comes to air quality. If you are in an industrial valley like Baddi, the air is consistently dirtier than in a tourist hill town like Manali. The study confirms that PM2.5 is the main villain driving the Air Quality Index in these regions.

This work gives the state a "baseline"—a starting point. It shows that public data can be used to compare very different places (factories vs. mountains) and helps officials see that cleaning up the air in Baddi requires a different strategy than in Shimla. The authors suggest that the next step is to watch these towns for a whole year and figure out exactly which factories or roads are the biggest culprits, so they can finally clear the air.