Air pollution in China is blocking light from the sun and reducing solar energy output, particularly in the northern and eastern parts of the country.

The issue is worst in the winter, when — according to research from Princeton University — air pollution in these regions blocks about 20 percent of sunlight from reaching solar panel arrays, on average.

That makes air pollution’s wintertime effect on solar energy production as significant as that of clouds, which have long been considered the main impediment to solar energy production.

The study shows that in the most polluted areas of northern and eastern China, aerosol pollution is reducing the potential for solar electricity generation by as much as 1.5 kilowatt-hours per square meter per day, or up to 35 percent.

Other researchers have recognized that aerosols, which include sulfate, nitrate, black carbon particulates and brown organic compounds, are contributing to solar dimming over large parts of China.

“Developing countries with severe air pollution that are rapidly expanding solar power, such as China and India, often neglect the role of aerosols in their planning, but it can be an important factor to consider,” says Xiaoyuan Li, a Ph.D. candidate in Princeton’s Department of Civil and Environmental Engineering and the study’s lead author.

To calculate how much of the sun’s radiation is reaching solar arrays on the ground, the scientists used a solar photovoltaic performance model, combined with satellite data from NASA instruments that measure irradiance from the sun and analyze aerosol components and clouds in the atmosphere.

They conducted nine separate analyses, which spanned 2003 to 2014 and covered all of China, to compare the impact of aerosols compared to clouds on solar power generation with and without technology that tracks the sun as it moves across the sky.

Li says the study’s findings should further spur countries like China and India to cut aerosol emissions so they reduce pollution and thereby increase their solar electricity generation more rapidly, in addition to the already known health benefits.

The findings can also help determine where to build new solar arrays. Aerosol pollution in China is concentrated in industrialized, urbanized regions. Meanwhile, remote, less populated areas have cleaner air. If research can quantify how much air pollution is reducing solar power output, policymakers can weigh the costs of transmitting electricity from cleaner regions to dirtier ones against the benefits of producing more power by building arrays where more sunlight reaches the ground.

The study, “Reduction of Solar Photovoltaic Resources Due to Air Pollution in China,” by Xiaoyuan Li, Fabian Wagner, Wei Peng, Junnan Yang and Denise L. Mauzerall, first appeared online October 23 in the Proceedings of the National Academy of Sciences.