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MODIS observes progressive development of air pollution crisis in China

Cold conditions in October led to extremely unhealthy air quality in northern cities

MODIS observes progressive development of air pollution crisis in China

Aerosol Optical Depth over eastern China measured by MODIS-Aqua, averaged over the period October 18-19, 2013. During this period, crisis levels of air pollution were reported in the northern city of Harbin.

MODIS observes progressive development of air pollution crisis in China

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 (Fig. 1). The location of Harbin is shown in each figure. Many cities in northeastern China, including the capital city of Beijing and nearby Tianjin, also suffered from chronic air quality problems due to China’s reliance on coal for energy generation aggravated by smoke from agricultural burning (Fig. 2).  The location of these cities on China's coastal plains is a factor in their air quality problem.

[Also see update at the end of the article, extending the visualization series to October 23.]

2-day running average time-series of Harbin Smog Event, color palette

 Harbin smog event, October 2013, grey scale
Figure 1.   Series of 2-day averages of MODIS-Aqua AOD over northeastern China, shown with a color palette (top) and in grey scale (bottom).   The images show the dramatic increase in AOD as air quality worsened during the October 15-19, 2013 period. 2-day averages are used to provide an increased amount of data, reducing missing data from gaps between instrument scanning swaths or due to clouds.  Some colored areas may thus only display data that were acquired on a single day.  White areas are indicate missing data, which can also be caused by brightly reflective ground.   Click on either image to view it full-size. 
Map of eastern China     Eastern China physical map
Figure 2.  Left: Map of eastern China showing the location of major cities.  (Map courtesy of Zachary Mitchell, Colby College).  Right: Physical geography of eastern China.  (Map courtesy of Wikimedia).  Beijing, Tianjin, and Harbin are located on China's coastal plains, to the east of mountain ranges and land at higher elevations.  Click on the right image to view it full-size.


Giovanni was also used to create a time-series of MODIS-Aqua AOD for a one-degree area around Harbin (Fig. 3). This figure shows that AOD rose to higher levels earlier in October than were observed during the October 15-19 air quality crisis period. A possible explanation for this is that smoke from agricultural burning was detected above the city by MODIS in early October, when ground-level pollution was not yet that bad; the latter pollution only became extremely serious when the coal-burning plant emissions increased. These observations indicate the importance of combining surface data collection with remote sensing data to fully characterize events and circumstances that affect human health.  

[Also see update at the end of the article, extending the visualization series to October 23.]


A true-color MODIS-Aqua image of central China acquired on October 3, 2013 shows the numerous fires adding smoke and soot to the atmosphere over China (Fig. 4).   During this period, the smoke and smog appeared to concentrate above the central coastal plains, a process influenced by the regional topography. This pollution-laden air was then periodically transported to the northeastern coastal plains by wind and weather systems.  A mixture of this smoke and emissions from coal-burning power plants coalesced in Harbin when temperatures dropped, creating the dangerous air quality conditions in the city.

MODIS AOD time-series, Harbin, China, September-October 2013 
Figure 3.   Time-series of MODIS-Aqua AOD for a one-degree area centered on Harbin, China. This time-series displays higher AOD values early in October, prior to the onset of the air quality crisis in the city.  MODIS measures aerosol optical depth in the atmosphere and can more readily detect aerosols at altitude than near the ground – thus, the high values early in October may be due to smoke aloft in the atmosphere from agricultural burning.  The steady increase in AOD after October 15 coincides with the air quality crisis in Harbin. 
MODIS true-color image of China, October 3, 2013 
Figure 4.  True-color MODIS image of central China, acquired October 3, 2013.  Each red dot indicates a fire detected by the fire-sensitive infrared radiation band of MODIS.  The Yellow Sea is visible at top right.  Smoke from the fires is visible as a greyish haze as it accumulates on the central coastal plain.  Click on the image to view it full-size. 
Because of the delay in receiving MODIS data into Giovanni, when this article was first written only data through October 19 was available.  The pollution crisis in Harbin actually worsened in subsquent days.  Another set of 2-day running averages and an extended time series are shown below, for the period October 20-23 (Fig. 5 and Fig. 6).  It appears that clouds and smog may have caused conditions that led to missing data on some of these days (white areas).  The extended time-series for the same area as used for Figure 3 also shows missing data on some of the days, but on October 20 data was acquired, indicating the highest AOD value (~2.2) for this event (Fig. 6).  
Harbin smog event, update series, color palette
Figure 5. Series of 2-day averages of MODIS-Aqua AOD over northeastern China, for the period October 19-23, 2013.  The location of Harbin is shown in each image.  Click to view full size.
Harbin smog event, time series, extended to October 23
Figure 6. Time-series of MODIS-Aqua AOD for a one-degree area centered on Harbin, China, for the period September 1 - October 23, 2013.  The peak AOD values of ~2.2 occurred on October 20.

Additional reading: 
“Smog Shuts Down Harbin” (NASA Earth Observatory)
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Last updated: Oct 29, 2013 02:56 PM ET