AQI levels and principal pollutant for 161 cities with quasi-complete data over the course of 2014. The dataset
provided through the MEP website was originally missing several days in February, but the data was added and collected for our graphic on January 26, 2015.
How are pollution levels calculated in China?
China’s MEP measures airborne pollution using its Air Quality Index (AQI)
. The AQI is based on the concentration levels of six major atmospheric pollutants: sulphur dioxide (SO2
), nitrogen dioxide (NO2
), suspended particulates smaller than 10 microns in diameter (PM10), carbon monoxide (CO), ozone (O3
), and suspended particulates smaller than 2.5 microns in diameter (PM2.5). The index is employed at monitoring stations in 367 cities across the nation.
The agency measures and assigns an individual air quality score (IAQI) to each of the six pollutants over a period of one, eight, or 24 hours. A city’s finial AQI is the highest of those six scores with that particular pollutant being the city’s major pollutant. When the index is lower than 50, the ministry does not name the major pollutant.
In the desert
Whereas the worst day of the year for most cities appears during the winter, as demand for electricity rises for heating, the story is slightly different for desert cities of the northwest. On April 24, a severe sandstorm swept the provinces of Xinjiang and Gansu, causing the AQI to rise to its maximum level. Between April 23 and May 5, the AQI reached its maximum level of 500 for 11 out of 13 days.
It’s better in the south
AQI levels in the south of China are consistently better. Sitting at the bottom of the charts are Haikou and Sanya, the main cities in the island province of Hainan.
Big cities’ smog
How did China’s most populated cities fare in comparison to each other? Here are the twenty top cities by urban area population (2010 census).
Good days, bad days
For these same large cities, how does it look like when you group the AQI by level?
It turns out that while Beijing does not have the highest median, it does have more days with moderate (orange-coloured) or higher levels of air pollution. At the same time, it registered 28 days of excellent air, more than than many cities in this listing.
As the main AQI reading is based on the level of the principal pollutant of the day, these graphics clearly show that PM2.5 remains China’s biggest threat. However, O3 consistently appears in the warm months of the year.