NO2 Pollution Hotspots: Poland (Sentinel-5P, monthly)

## What this measures

The average amount of nitrogen dioxide (NO2) in the lower atmosphere each month over the most polluted tenth of Poland: its big cities, industrial areas and large power plants. It is measured from space by the TROPOMI instrument on the Copernicus Sentinel-5P satellite, in 10^15 molecules per square centimetre (the NO2 between the ground and roughly 10 km up).

## Why it matters

NO2 comes mostly from burning fuel in power plants, factories, trucks and cars, and lasts only hours, so it changes as soon as activity changes. Averaging only the hotspots keeps the clean background of a large country from drowning out what industry and traffic do. In April 2020, during the first pandemic lockdowns, the hotspot averages for the EU, India and Japan were 22% to 28% below April 2019.

## How to read it

Higher means more NO2 where people and industry are. NO2 has a strong seasonal cycle. In winter the sun is weaker, so NO2 breaks down more slowly, and heating adds emissions: over cities and industry, winter months often read two to three times as high as summer ones. Compare a month with the same month in earlier years, never with the month before. Wind, rain and cloud move single months a lot, so read several months together. The 12-month change removes the season.

## What it does not say

It is a column of gas, not a count of emissions: weather and chemistry change it as well as fuel burning, and it cannot say which source the NO2 came from. Cleaner engines and power plants lower it over the years even when output grows, so read changes over months, not decades, as activity. The satellite record is one instrument, but its processing changed over time (processor versions 2.4 to 2.9). KNMI advises using data from version 2.6 (November 2023 to September 2024) with care, because of a cloud-algorithm issue later rolled back, and notes that version 2.9.1 (from 22 November 2025) may show a small step in the record.

## How it's built

Each month KNMI averages the TROPOMI observations that pass its quality screening on a grid of 0.125° cells (about 14 km). Where no observation passes in a month, usually because of persistent cloud or snow and ice cover, the cell has no value. Palanor leaves those cells out and publishes a month only when cells with a value cover at least half of the area; in China's far north-east some winter months are missing for this reason. The hotspots are fixed: the 263 grid cells of Poland with the highest average NO2 in 2019 (the last full year before the pandemic, cells with at least nine monthly values), the top tenth of its cells. Keeping the same cells keeps the series comparable. Palanor takes their area-weighted mean each month. Boundaries from Natural Earth. History from May 2018.

## Source

Contains modified Copernicus Sentinel data (2018–2026), processed by KNMI: monthly mean tropospheric NO2 from TROPOMI on Sentinel-5P, TEMIS service (temis.nl). Boundaries: Natural Earth. Palanor calculation. Refreshed by /api/cron/no2-sync.