Rooftop sensors on U.S. embassies are warning the world about ‘crazy bad’ air pollution

In October 2010, as heavy smog hung over Beijing, the U.S. embassy's Twitter feed said its rooftop pollution sensor had detected “crazy bad” levels of hazardous microparticles. So-called PM2.5 had shot up to about 550 micrograms per cubic meter—a level to which programmers had given the sardonic label because they thought it would never be reached. The undiplomatic language ruffled feathers in Beijing; embassy staff apologized, deleted the tweet, and replaced the label with “beyond index.”

Yet that incident and others like it spurred public complaints that eventually elicited more aggressive efforts to tame the pollution. By now, rooftop sensors like those that drew attention to Beijing's pollution sprout from 26 diplomatic posts in 16 countries. Their immediate goal is to protect the health of U.S. diplomats. But they are raising concerns about air pollution from Sarajevo to New Delhi and supplying data to research efforts. The “little-air-monitor-that-could,” as physicist and former U.S. diplomat David Roberts calls it, has become a worldwide watchdog.

The first Beijing sensor, installed a decade ago, was meant to provide warnings of bad-air days to U.S. citizens. The fine particles it measures, mostly residues of coal burning and vehicle emissions, are linked to respiratory and heart disease. Third party apps made the readings widely available to Chinese who wanted to monitor PM2.5—and got little information from their own government.

China had been struggling to rein in air pollution ever since the late 1990s, after Beijing won the bid for the 2008 Olympics. But officials were embarrassed by the slow progress that the sensor readings highlighted, especially when the U.S. data undercut the Beijing government's rosy pronouncements of “blue sky” days, when pollution supposedly fell. Officials demanded that the embassy stop releasing the data. “We said we couldn't, since the data regarded the health of U.S. citizens,” recalls Gary Locke, who served as ambassador to China from 2011 to 2014.

Beijing officials also challenged the usefulness of the U.S. data, pointing out, for instance, that the embassy sensor was in only one location and therefore could be giving an incomplete picture. In response, the embassy teamed up with U.S. Environmental Protection Agency (EPA) scientists to validate the results. “Since we were being criticized, we wanted to make sure we were doing it correctly,” says Erica Thomas, a Department of State official who led air monitoring at the embassy in Beijing from 2010 to 2014.

Tensions came to a head in late 2011, when Beijing's smog got so bad that its airport canceled hundreds of flights. Based on monitoring of larger particles, municipal authorities insisted that the air was only “slightly polluted”—provoking ridicule from bloggers citing U.S. data. “It was the straw that broke the camel's back,” says Angel Hsu of Yale University, an expert on pollution in China. Days later, China proposed new air quality standards that included PM2.5 for the first time. It now runs the biggest PM2.5 monitoring system in the world.

Winters of discontent In 2016, as in previous years, the U.S. embassy's sensor in Bejing tracked a surge in PM2.5 late in the year. 0 50 100 150 Average PM2.5* (micrograms per cubic meter) Jan. Mar. May Jul. Sep. Nov. 0 50 100 Data: abcdefg hijkl mnop qrstu vwxyz 1234 56789 Unhealthy for sensitive groups Unhealthy Air quality category * Data are not fully verified or validated.

The Department of State, too, expanded its PM2.5 monitoring efforts, first within China and then around the world, and is using the sensors for research as well. Drawing on PM2.5 data from five diplomatic posts in China, a 2015 study in Atmospheric Environment revealed previously unknown variations in PM2.5 levels; for instance, in Beijing the particles tended to peak around midnight and bottom out in spring, because of weather patterns. And the U.S. embassy in Sarajevo is now testing cheaper sensors with an aim of deploying them “in different pollution environments,” says Caroline D'Angelo, who runs the Department of State's monitoring network in Washington, D.C.

Findings are radiating into other disciplines. During a stint at the U.S. consulate in São Paulo, Brazil, Tommy Flynn, a program manager with the South Carolina Department of Health and Environmental Control, is providing technical assistance on the monitors. But he has also learned from the consulate's practice of distributing filter masks to high-risk groups during ultra–high-pollution events. That's a lesson, he says, that could be applied in South Carolina during forest fires. And scientists from NASA and the World Bank are using embassy PM2.5 data, now posted on EPA's AirNOW monitoring website, to groundtruth satellite measurements of pollution, filling in gaps in the global map of reliable air quality measurements.

The U.S. sensors continue to serve as a diplomatic cudgel, as well. In India, where air pollution in New Delhi and other cities has skyrocketed, “the data has flowed from the embassy to the media and then created outcry,” says Christa Hasenkopf, a former Department of State official who now runs the air pollution nonprofit OpenAQ in Washington, D.C. As Locke says, the monitoring program “has taken on a life its own.”