There are many houses in Poland that are using coal heating, and unfortunately a lot of people burn there their thrash. Kraków is surrounded by smaller towns and villages, where single family houses are common. To make things even worse, Kraków is in a basin, which makes the air flow even more difficult. If you add there years of city mismanagement when it comes to air flow, you land in such a situation

From this source: https://www.iqair.com/mx-en/newsroom/krakow-among-top-10-mos...

"Krakow’s pollution stems from a mix of local and regional sources. A primary culprit is domestic heating, the burning of coal and wood in older, inefficient household boilers and stoves remains widespread in the Małopolska region (1).

Car traffic also adds nitrogen oxides and fine particulates, exacerbated by an ageing vehicle fleet. Topography and meteorology worsen the problem, Krakow sits in a basin-like region prone to temperature inversions and limited ventilation, allowing pollutants to accumulate.

Additionally, emissions drift in from surrounding municipalities and industrial zones, making regional coordination crucial to air quality. Despite a solid-fuel ban in the city since 2019 and the replacement of many coal boilers, compliance is uneven and some residents still use banned fuel."

I think it’s topology (concave) + widespread poor heating methods in the agglomeration + a very bad day + inefficient combustion engines.

I’d maybe include accurate measurements. The government isn’t trying to hide that and doesn’t have the means to, and highly quality sensors are widespread.

Despite government incentives and regulations some people burn garbage in stows. It's a local cultural thing and the state seemingly is powerless to do anything about it despite being the 20th economy in the world.

It’s in the valley and because Polish state is kinda weak they cannot enforce nearby villages to stop burning garbage to heat their homes.

There isn't much wind there at all so the pollution can't escape. I'm not saying this isn't the residents' fault, but it isn't entirely the residents' fault.

I'd suspect just small amount of datapoints with maybe bias for people installing air sensors because that particular area's air quality is bad for whatever reason (near to road, neighbour have old coal boiler etc.)

Coal and cars?

Looks like it clears up quite quickly.

It's almost as if slowing down the transition away from coal for political and social reasons is not such a great idea.

Assuming a large contributing factor is all the coal plants now running to sustain Germany's independence from nuclear? Berlin's air quality has also tanked a lot since the energy crisis started.