Seoul has 40 air quality monitoring stations, 40 of which also measure PM2.5. You can read the current levels starting from the station nearest to you.
Seoul air depends less on what is emitted than on where you measure it. There are no large industrial complexes in the middle of the city, but there are an enormous number of cars — so at the very same moment, a spot on a main road and a street inside a residential block can read noticeably differently.
Seoul is ringed by mountains — Bukhansan, Dobongsan, Gwanaksan — so it is not flat ground. On windy days the terrain hardly matters. But when a winter high-pressure system settles in and the wind dies down, what the city emits has nowhere to go and simply stays.
It gets worse when the ground cools overnight and an inversion layer forms. Normally air gets colder with height, so warm air rises and carries pollutants up and away. Under an inversion the upper air is the warmer one, and it sits over the city like a lid. That is why some winter mornings read unusually high.
When the sun warms the ground later in the day, the lid lifts and the numbers come down. A day that starts "unhealthy" and turns "moderate" by afternoon is usually this and nothing else. Worth remembering when you plan when to go out — the morning reading is not the whole day.
The biggest source in Seoul is the road. Not just exhaust: tyres and brakes shed dust as they wear, and dust that has settled on the road gets thrown back into the air by passing traffic. Those last two do not go away when cars go electric, which is part of why roadside levels in Seoul are so stubborn.
That is also why Seoul has a higher share of roadside stations than other regions. Reading the main-road value and the residential value separately is what makes the number match your actual day.
Winter adds heating. And when the wind comes from the west, an inflow from outside the country lands on top of all this — if that coincides with stagnant air, high readings do not end in a day, they run for several. The stretches when the capital region sits at "unhealthy" for days on end are usually this combination.
Winter (December to March) is the worst. The season when heating emissions rise, the season when air stagnates, and the season when inflow from outside is frequent all land on exactly the same months. Any one of them is manageable; together they push the numbers up sharply.
Spring adds Asian dust on top. Its particles are coarse, so PM10 spikes first — that is the tell. If PM2.5 is ordinary but PM10 alone is unusually high, suspect Asian dust.
Summer brings frequent rain and southerly wind, and particulates fall to their lowest of the year. In their place comes ozone, as the sunlight strengthens. Even on a "good" particulate day, it is worth checking ozone separately from early afternoon into the evening.
Readings vary quite a bit between stations even within the same region — roadside or residential, close to an industrial area or not. Below are the stations in Seoul grouped by district.
Station and district names are shown in Korean, exactly as the monitoring agency publishes them.
| District | Count | Stations |
|---|---|---|
| 강남구 | 2 | 강남구, 도산대로 |
| 강동구 | 2 | 강동구, 천호대로 |
| 강서구 | 2 | 강서구, 공항대로 |
| 금천구 | 2 | 금천구, 시흥대로 |
| 노원구 | 2 | 노원구, 화랑로 |
| 동대문구 | 2 | 동대문구, 홍릉로 |
| 동작구 | 2 | 동작구, 동작대로 중앙차로 |
| 마포구 | 2 | 마포구, 신촌로 |
| 서초구 | 2 | 강남대로, 서초구 |
| 성동구 | 2 | 강변북로, 성동구 |
| 성북구 | 2 | 성북구, 정릉로 |
| 영등포구 | 2 | 영등포구, 영등포로 |
| 용산구 | 2 | 용산구, 한강대로 |
| 종로구 | 2 | 종로, 종로구 |
| 중구 | 2 | 중구, 청계천로 |
| 강북구 | 1 | 강북구 |
| 관악구 | 1 | 관악구 |
| 광진구 | 1 | 광진구 |
| 구로구 | 1 | 구로구 |
| 도봉구 | 1 | 도봉구 |
| 서대문구 | 1 | 서대문구 |
| 송파구 | 1 | 송파구 |
| 양천구 | 1 | 양천구 |
| 은평구 | 1 | 은평구 |
| 중랑구 | 1 | 중랑구 |
Stations are grouped into networks according to what they were built to watch.
What each level means and what to do about it is in the air quality scale. If you need to pick a mask, see choosing a mask; if indoor air is on your mind, see indoor air.
Some days a station goes quiet because of maintenance or a network fault. When that happens, just swipe to the next one — Seoul has 40, so there is almost always another nearby.
Choose by how far your home or office sits from a main road. In a building on a large road, the roadside network will match what you feel; inside a residential block, the urban ambient network will. Many districts in Seoul have both, so you can pick.
They belong to different networks. Jongno-gu, for example, has an urban ambient station and a separate roadside station. The roadside one reading higher is normal, not a fault.
The inversion broke. The lid of air formed by the ground cooling overnight disappears when the sun comes up, and the pollutants disperse upward. It is a common within-the-day swing in winter.