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Pollution in Eastern China Cuts Light, Useful Rainfall

New research shows that air pollution in eastern China has reduced the amount of light rainfall over the past 50 years and decreased by 23% the number of days of light rain in the eastern half of the country. The results suggest that bad air quality might be affecting the country’s ability to raise crops as well as contributing to health and environmental problems.

The study links for the first time high levels of pollutants in the air with conditions that prevent the light kind of rainfall critical for agriculture. Led by atmospheric scientist Yun Qian at the Department of Energy’s Pacific Northwest National Laboratory, the study appears August 15 in the Journal of Geophysical Research-Atmospheres.

People have long wondered if there was a connection, but this is the first time we’ve observed it from long-term data. Besides the health effects, acid rain and other problems that pollution creates, this work suggests that reducing air pollution might help ease the drought in north China.

—Yun Qian

Over the last 50 years, the southern part of eastern China has seen increased amounts of total rainfall per year. The northern half has seen less rain and more droughts. But light rainfall that sustains crops has decreased everywhere.

Previous work has shown that pollution can interfere with light rain above oceans, so the team suspected pollution might have something to do with the changes over land. Light rain ranges from drizzles to 10 millimeters of accumulation per day and sustains agriculture. (Compared to heavy rain that causes floods, loss of light rain has serious consequences for crops.)

While the light rains have diminished, pollution has increased dramatically in China in the last half of the 20th century. For example, while China&rsuqo;s population rose two and a half times in size, the emissions of sulfur from fossil fuel burning outpaced that considerably, rising nine times.

The team charted trends in rainfall from 1956 to 2005 in eastern China, which has 162 weather stations with complete data collected over the entire 50 years. From this data, the team determined that both the north and south regions of eastern China had fewer days of light rain—those getting 10 millimeters per day or less—at the end of the 50-year timespan. The south lost more days—8.1 days per decade—than the north did, at 6.9 days per decade. However, the drought-rattled north lost a greater percentage of its rainy days, about 25% compared to the south’s 21%.

To probe what caused the loss of rainfall, the team looked at how much water the atmosphere contained and where the water vapor traveled. Most parts of eastern China saw no significant change in the amount of water held by the atmosphere, even though light rains decreased. In addition, where the atmosphere transported water vapor didn’t coincide with light rain frequency.

These results suggested that changes in large-scale movement of water could not account for the loss of the precipitation. Some of pollution’s aerosols can seed clouds or form raindrops, depending on their size, composition and the conditions in which they find themselves. Because these skills likely contribute to rainfall patterns, the researchers explored the aerosols in more depth.

Cloud droplets form around aerosols, so the team determined the concentration of cloud droplets over China. They found higher concentrations of droplets when more aerosols were present. But more droplets mean that each cloud droplet is smaller.

This result suggested that aerosols create smaller water droplets, which in turn have a harder time forming rainclouds. The team verified this with computer models of pristine, moderately polluted or heavily polluted skies. In the most heavily polluted simulation, rain fell at significantly lower frequencies than in the pristine conditions.

An examination of the cloud and rain drops showed that these water drops in polluted cases are up to 50% smaller than in clean skies. The smaller size impedes the formation of rain clouds and the falling of rain.

Qian said the next step in their research is to examine new data from the DOE’s Atmospheric Radiation Measurement Climate Research Facility in the central eastern Chinese city of Shouxian. The data was collected from April to December of 2008.

This work is important because modeling studies of individual cases of pollution's effect on convective clouds have shown varying results, depending on the environmental conditions. The ARM data collected at Shouxian should provide more detailed measurements of both aerosols and clouds to enable us to quantify the impacts of aerosols on precipitation under different atmospheric and pollution conditions.

—Ruby Leung, co-author

The work was supported by the Office of Biological and Environmental Research within the DOE Office of Science under a bilateral agreement on regional climate research with the China Ministry of Science and Technology.


  • Yun Qian, Daoyi Gong, Jiwen Fan, L. Ruby Leung, Ralf Bennartz, Deliang Chen, and Weiguo Wang (2009) Heavy pollution suppresses light rain in China: observations and modeling. J. Geophys. Res., Vol. 114, D00K02, doi: 10.1029/2008JD011575



So, maybe IPCC and clan should be spending a little more time addressing the climate problems arising from real pollution than on demonizing a plant fertilizer.


The problem isn't fertiliser, it's too much fertiliser. It's the problem with CO2, but also with all the chemical fertilisers used by agricultural activity. Nobody is demonising CO2, just the too heavy concentration of it.

Henry Gibson

If no humans existed on the earth, even very high levels of CO2 would not be a problem. ..HG..


Life on earth does just fine at 1500-2000ppm - and has done for millennia.

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