In mid-October 2013, cold air heralding the advent of winter swept down from Siberia into northern China. The timing of this cold wave was particularly bad for the air quality in the country, as it coincided with the season when many farms were burning agricultural wastes after harvesting. The cold conditions caused a significant increase in the emissions from China’s numerous coal-burning power plants, as a result of an increase in the need for heat.
In the northeastern city of Harbin, city officials were responsible for turning on the city’s heating system, powered by coal, for its residences and businesses. In combination with smoke from agricultural burning and calm weather conditions as cold air settled over the eastern part of the country, air pollution dramatically intensified, leading to what some have termed an “air-pocalypse.” Schools and airports had to be shut down. Residents of Harbin reported that visibility conditions were so poor that buses got lost on their regular routes, and it was unsafe to cross streets because car headlights could not be seen. The Air Quality Index (AQI) measured in Harbin rose to its maximum possible measured value several times, and the concentration of small particulates (PM2.5), which are dangerous to human health, exceeded 1,000 micrograms per cubic meter of air—more than 40 times higher than the recommended maximum set by the World Health Organization.
One of the most common satellite measurements related to air quality is Aerosol Optical Depth (AOD), a quantity that indicates the amount of aerosol particles in the atmosphere. The Moderate Resolution Imaging Spectroradiometer (MODIS) provides daily observations of AOD over much of the Earth’s surface. AOD data from MODIS on NASA’s Aqua satellite were used in the Giovanni data analysis system to create a series of running 2-day averages from October 15 through 19 over eastern China.