Why does the smog strike Beijing even when the city is closed down?

With streets empty of cars and most people yet to return to work after coronavirus quarantines and lockdowns, the one problem Beijing shouldn’t have faced in the past weeks is smog, or that’s how it seems to people living in the city. Indeed, on the national level, China’s had the lowest air pollution levels on record in the past weeks. However, Beijing has been notably smoggy, experiencing three air pollution episodes since the start of the national Chinese New Year holiday and the coronavirus response. What’s going on?

A now-deleted but widely reposted article on WeChat captured the sentiment well: “Coronavirus shatters several years of research on the causes of smog!” The article lists eight different sources of air pollution and claims that each of them have been eliminated either by earlier policies or by the virus containment measures:

• Everyone’s at home and there are no cars on the street, so the smog can’t have anything to do with cars
• All the factories are shut , so the smog can’t have anything to do with industry
• All construction sites have stopped work, so the smog can’t have anything to do with construction dust
• Coal stoves have been replaced by gas

…and so on. You get the point.

I was a bit mystified as well – until I started digging into satellite imagery and industry operating surveys. I found that most steel plants have in fact been running full steam in the past weeks. See e.g. this large coal power plant and several steel mills in Tangshan, a city neighboring Beijing with the largest concentration of steel industry anywhere in the world – every white puff is a plume from a large furnace or boiler:

Image credit: Planet Labs

Industry surveys found that blast furnace operating rates were the same as at the same time last year. Coal power plant operating rates were down by about one third – but this does not always mean lower pollutant emissions, as emission control performance suffers when plants are running at low load.

Oil consumption is also reduced, but the reduction is nowhere near 100%. Refinery throughput, a proxy of oil demand, is down by 30%. Cars within the inner city are a small fraction of China’s total transport oil demand, which is dominated by freight.

Overall, in a widely published analysis that came out last week, I estimated that China’s fossil fuel consumption is about 25% lower than at the same time last year, due to the effect of the virus. So we should not expect air pollution levels to fall by 100%, rather by about 1/4.

And this is indeed what has happened on the national level:

National average PM2.5 levels have dropped like a stone in the past weeks, with the Yangtze River Delta (YRD) region around Shanghai seeing particularly dramatic reductions.

However, Beijing air pollution has been worse than at any point during last winter. The larger region around Beijing has seen better air quality, but much less pronounced than other parts of the country There are two reasons for this:

• The two largest sources of air pollution in the Beijing region are steel industry and the heating of buildings. This contrasts with the regions that have seen the largest air quality improvements, where power plants, transport and smaller industries are more prominent. These two sectors are the least affected by the virus – buildings need to be heated even when cities and villages are locked down, and might even be heated more as people spend all of their time indoors. The steel industry is loath to shut furnaces as shutdown and startup incur a lot of costs. So the industry has kept on producing, resulting in record-high stockpiles.
• Beijing’s geography makes its pollution levels very sensitive to wind directions – enormous industrial clusters and high population densities to the south and east mean high pollution when wind comes from those directions, while wind from grasslands to the north and west brings clean air. So bad luck in terms of wind direction and speed can have a major impact on week-to-week pollution levels.

In short, the smog episodes in Beijing have been a combination of continued industrial and residential emissions and unfavorable weather patterns. Without the reduction in emissions that has happened due to the virus containment measures, these episodes would simply have been even worse.