Flights have been delayed and schoolchildren have been told to stay at home as air pollution in Shanghai has reached hazardous levels.
According to monitoring from the US embassy in Shanghai, concentrations of PM2.5 (fine particulate matter – see our China air pollution Q&A here) have reached more than 500 micrograms per cubic metre today (6 December).
The embassy categorises exposure to more than 500 over 24 hours as ‘beyond index’, and 301-500 as ‘hazardous’, which means vulnerable people including those with heart or lung conditions, the elderly and children should stay indoors and everyone should avoid all physical activity outdoors.
This episode in Shanghai echoes off-the-chart smog in the northern Chinese city of Harbin in October, when air pollution also surpassed the 500 microgram per cubic metre mark.
Coal burning, which increases in the winter months as the weather worsens, is the primary source of fine particle air pollution in Shanghai according to two studies sampling the air in Shanghai.
Coal burning rise
In a 2010 study, sulphate and nitrate – which are a result of coal combustion and can react in complex ways in the atmosphere to create air pollution - were found to be the dominant chemicals in PM2.5, and accounted for 80% of the particles. A similar 2008 study published in the Journal of Hazardous Materials found "the major source of PM2.5 in Shanghai was not traffic-derived emissions, but the stationary industrial contribution emitted from coal use."
Compared to other sectors power plants contribute 60-70% of sulfate and 35-40% of nitrate to the total PM2.5 mass in Shanghai, according to a modelling analysis published in Atmospheric Environment in 2012.
Also, 41% of carbons that make up PM2.5 in Shanghai are from coal – these primary sources of PM2.5 react with the nitrate and sulphate gases in complex ways to form fine particulate.
But burning coal around Shanghai also has an effect, as pollution travels in low-lying air masses that flow into the city.
The map below shows the air mass movement between 1-6 December in red.
The heat map is made up of tonnes of SO2 (sulphate dioxide) emitted by power plants and industry per year – an indicator of coal combustion (orange is the biggest amounts). The air mass trajectories, coupled with the emissions data, point to the heavy coal-burning regions of Jiangsu, Anhui, Shandong and Henan as the main regional sources.
(Map source: Greenpeace analyst Lauri Myllyvirta using US National Oceanic and Atmospheric Administration HYSPLIT modelling.)
Pollution from Hebei, Shandong, Anhui and Jiangsu were discovered to be the main source of PM10 (particulate matter) transported to Shanghai in winter, according to a 2012 paper published in Science for the Total Environment.
This regional transport effect has a significant impact on air quality in Shanghai, with one 2011 study attributing its contribution to air pollution events in the city in 2007 as 34 to 64%.